Single Photon Interference and the Hong-Ou-Mandel (HOM) Effect in an Interferometer

Question: Reading on the Hong-Ou-Mandel (HOM)  effect, I came to wonder how exactly we can be certain that interference that occurs in apparatuses such as a Michelson interferometer and Mach-Zehnder interferometer is truly an occurrence of single-photon interference rather than two-photon interference as seen in the HOM effect. So, my question is: how is it that we can be confident that the interference in such interferometers are truly single photon interactions or two photon interactions?
Comment: Isn't HOM explicitly about two photon interactions, i.e. when two photons, with similar waveforms, are incident on the beamsplitter? In this case all you need to do is detect two clicks and count them
Answer: In all cases can you reduce the intensity to make it extremely unlikely that you have more than 2 photons in the HOM case, and more than one photon in the single-photon case.  Moreover, in both cases the actual number of photons detected is the number expected in a run.
If you look at the data of the original HOM experiment, the count rate in 10mins is quite low (not even 1000 pairs).  Indeed at the bottom of the dip the rate is 100ish coincidence events over 600 seconds, which is extremely low.  A modern version have near 0 rates so you know you never detect two photons in different detectors.
Of course the theory could be wrong at every step, i.e. if could be that the modelling of the sources is wrong, the modelling of the detectors is wrong, and the modelling of the interferometer is wrong, but these are checked independently so the probability that something is misunderstood when the pieces are put together is minuscule: it is simpler to believe the current theoretical explanation than concoct some convoluted alternative.
The same holds for the single photon interference experiments.
Finally, these have been repeated multiple times: HOM is now used for benchmarking sources.  You can be pretty sure that, if there was a bug or an inconsistency somewhere, it would be known by now.  In other words, the weight of experimental evidence is now overwhelmingly in favour of the "traditional" explanations.
Answer: So far I have seen no experimental explanation so I think this entry is also somewhat valuable.
Here are some excerpts from a presentation of the Max Planck Institute (MPI) about two photon interference:
https://www.mpq.mpg.de/5020845/0508b_two-photon_interference.pdf
A classical mach-zender interferometer would be similar to that of a quantum eraser:

You have a laser, a beam splitter that splits the beam power 50%/50%, a distance between the beams exactly the same on both arms (top and straight if you follow the beam path after the first beam splitter), then another beam splitter that is imaged onto a screen in the background. The quantum eraser is a different idea altogether so I will not go too deep into how it works, but here what is important is that when all the polarization filters are set to 0°, you can clearly see the classical interference of both arms in both screens.

Here you can see the setup originally suggested by HOM in their seminal paper, where essentially you need a "subpicosecond photon wave packet" for the experiment to work. This means that the laser is pulsed at a couple femtoseconds, in order to achieve the aforementioned photon packets. The interference then is related to the displacement of the beam splitter:

HOM do note that the apparent influence of the choice of position of the detectors and its relation to the counts as seen before:

"This conclusion is related to the fact that in quantum mechanics we cannot associate an objective physical reality with the two photons that is independent of the measurement we choose to make. The phenomenon involves the same violation of local realism that was recently tested in the experiments of Aspect and his collaborators, and was first discussed by Einstein, Podolsky, and Rosen"

In the framework that HOM establish the certainty is correlated to the choice of detectors and distance of the beam splitters, as they play a pivotal role in the measurements that you obtain.
There are more involved ways to obtain something close to "single photons", but you cannot directly (at least not in the sources where I looked it up) a single photon that interferes with another single photon. You can get a very close approximation and infere what happens with relation to the setup, but single photons are difficult to maneuver and a hot research topic.
You can see here such setup:

The best way to consistenly know whether you have a photon or a wave is by having the 2 detectors (going back to my explanation of the MZI from Thorlabs, the screens are simply "visual" detectors):
https://www.qutools.com/files/quED/worksheets/qutools_HBT.pdf
Qutools gives a very succint image to explain the phenomenon:

Once light is indivisible in more fundamental units (or photons), the particles will travel through one path ONLY. You can tell if this occurs through the second order coherence function which is zero (the second order coherence function is smaller than the first order coherence function) for single photons. All other light sources have a value of exactly one (1) because there will always be an event where, for example, both detectors have a coincidence measurement (e.g. both detectors measure something, even random light from the room). At the point where there are no coincidences between detectors, a single photon has been detected.
Answer: We can tune both arms of an MZ interferometer so that no photons go thru. So now we have 0 photon interference!
Photons will not travel the arms if the path length between mirrors is lambda x n + 0.5 lambda. ( n is an integer).
Per Dirac Feynman and many others each photon behaves on its own. It is the EM field that controls/guides the photons.  The EM field sees all in the apparatus.
Answer: In our current era of technology, this is pretty straightforward.

Two Photon Interference: As you mention the HOM effect, I have the original 1987 paper they presented - still relevant today.  Two photon interference requires indistinguishable photons, in this case produced from parametric down conversion (PDC).  The key elements are that the range of the overlap of the photons is extremely small (femtosecond regime), and follows theory.  The dip is clearly a result of destructive interference of the photon pair at the beam splitter.  You know it is 2 photons because there are clicks at both detectors.  To put it in perspective, the theoretical resolution of the overlap is on the order of 500-1000 nm or approx. 3 femtoseconds.  Actual was not quite that.  The photons were at a wavelength of about 752 nm.

Measurement of subpicosecond time intervals between two photons by interference

On the other hand: single photon interference can be detected using PDC pairs as well.  However, one is used as the the idler (A in the diagram) to herald the presence of the lone signal photon (D in the diagram) in the interferometer apparatus.  So there cannot be 2 photon interference, as there is no point of overlap at all in the MZI (i.e. there is just one photon entering).  The interference of the single photon between the 2 possible paths to D is constructive, while paths to the other output port cancel.

Using a Mach Zehnder Interferometer to observe Quantum Interference using a Single Photon Beam (a summary of Pearson & Jackson's paper "A hands-on introduction to single photons and quantum mechanics for undergraduates" which I could not locate other than as an abstract).
So in these cases, you know clearly whether you are observing single or double photon interference.


Mouse problem in draftsight

Question: I have installed draftsight (CAD alternative app to Autodesk one) for Ubuntu but I am not able to use the mouse in the model space.
The "arrow" is moving slowly and delayded than in other applications and than the other part of the application (menu, labels).
The problem can be that the app is a beta version?
Answer: I have learned of a partial solution from youtube comments and a few other locations. Assuming you are using a USB mouse unplug the mouse, start Draftsight, the plug the mouse in. The mouse wheel doesn't seem to work so zoomiing is a pain but the cursor is at least usable.
[Z] [Enter] [E] [Enter] to zoom to the limits of the drawing
[Z] [Enter] [W] [Enter] to draw a zoom window

Mouse problem in draftsight


What's the dinstinction between these intensifiers, such as very, quite, rather, pretty, extremely, etc.?

Question: Word choice and usage:
I'm always curious about the the usage of intensifiers, such as very, quite, rather, pretty, extremely, etc.. I also remember my teacher told there was a turn of tone when using these words and some of them had a passive emotion several years ago. In these days, I see a website (https://www.ragan.com/Main/Articles/Quite_rather_and_pretty_The_differences_between_Br_44738.aspx). I become thoroughly disordered. So, I want to know the distinction between these intensifiers, and where and when I should use them. Does they have a positive or negative emotion.
Comment: Unfortunately (for those learning the language), such intensifiers are not rigidly defined, but can have subtly (or sometimes substantially) different meanings depending on context and "tone of voice".  Who is speaking also is a factor.  (A certain POTUS seems to apply entirely different meanings to many words.)
Answer: Since the words have strong similarities, we will find the differences in their connotations more than in their denotations (definitions or synonyms).
Let's first put these handy adjectives in order of intensity as commonly used. Note that all are neutral terms with the context conveying any positive or negative tone, as I hear them in the US.

Mildest to strongest: rather, pretty, quite, very, extremely

Rather is used as a mild, vague, and casual term: "It's rather good" means it is vaguely more than good, to some undefined degree. Say I want to compliment you but not be too personal: Your jacket is rather nice says that I am not overdoing my attentions.
Pretty is used as a vague, but stronger, casual term: "It's pretty good" means it is vaguely more than good, to a strong but undefined degree. Say I want to compliment you with a friendly remark: Your jacket is pretty nice says that I think you are dressed just right.
Quite is used as a strong but vague term in any setting: "It's quite good" means it is definitely more than good, to an undefined degree. Say I want to compliment you with a firm remark: Your jacket is quite nice says that I think your jacket is the right choice and better than just nice.
Very is used as a distinct term in any setting: "It's very good" means it is 'B+,' more than good, to a powerful and hard-to-beat degree. Say I want to compliment you with a decisive remark: Your jacket is very nice says that I can hardly find a better choice for you than that one.
Extremely is used as an intense term in any setting: "It's extremely good" means it is as good as it can get with nothing better. Say I want to compliment you with the most decisive remark: Your jacket is extremely nice says that I cannot find a better choice for you than that one.
Comment: Are these for Us English, UK English or another variety? I suspect that makes a difference, indeed that was rather the point of the article that Ethan was confused by. In my part of the English speaking world, Scotland, if you described someone's jacket *to their face* as 'quite nice' they would probably be offended at your damning with faint praise because here 'quite nice' would be taken to mean '**only** 'quite' nice, which is less than 'very nice' and possibly less than an unqualified 'nice'. Of course, all of *that* is dependent on tone and context..
Comment: @Spagirl I'm not sure about the Scots usage but in England saying _rather nice_ with the right sort of falling cadence conveys serious admiration! I think the OP's opened a can of worms with this question.
Answer: I quite disagree with the ranking of quite and very in the accepted answer. The basic meaning of quite is "totally or completely". Merriam Webster lists its meanings as:

1
  :  wholly, completely not quite finished
2
  :  to an extreme :  positively quite sure —often used as an intensifier with a, quite a swell guy quite a beauty
3
  :  to a considerable extent :  rather quite near.

The Oxford On-Line Dictionary has a similar definition:

1  To the utmost or most absolute extent or degree; absolutely; completely.
1.1 US  Very; really (used as an intensifier)
2 To a certain or fairly significant extent or degree; fairly.

Because we are just dealing with nuance rather than meaning, I would add that very is so overused that often it doesn't intensify to a (very) great degree.


3.15A and 4A fuses - can I swap?

Question: I have an old oscilloscope which doesn't work. I tried swapping the 3.15A/250VAC fuse with a 4A/250VAC fuse. The fuse immediately blew. My questions are: Did the fuse blow because it was the wrong fuse? Or did it blow because there's likely something else wrong with the oscilloscope? Must the fuse be exactly as specified? Thank you.
Comment: The amperage rating of a fuse is the maximum.  I would say something is wrong with the scope.  (Physics lab instrumentation repair question  ;)  )
Comment: Thank you @Qmechanic and Bill N. Yes, it looks like a problem with a blown capacitor and possibly some shorting. There may have been a previous attempt to fix it.
Comment: The rating on the fuse is the current at which the fuse will break. Therefore, if you put in a 4A fuse and it blew, then there was at least 4A flowing through that fuse. Fuses are always (supposed to be) low resistance, so if you had put in a 3A fuse, the current flow would have been the same and the 3A fuse would have blown too. Something else is wrong with the scope.
Comment: Many thanks, @DanielSank. I think there might be a short somewhere. Do you know what would happen if there was a short to ground in the circuit? Would that cause the fuse to blow?
Answer: First, replacing a fuse with one of a higher current rating is a bad idea.  In this case, assuming the two fuses were the same type (fast or slow blow), then there is definitely something wrong with your scope.  Since the scope is not meant to exceed 3.15 A, but actually exceeded 4 A, there is clearly something wrong.  By making it take more current before the power was shut off, you may have even damaged it further.
This is all assuming the two fuses were of the same type.  If the original was a slow blow, and the new a fast blow, then what you observed could still happen in normal operation.  Look at the old and new fuses carefully.  This isn't universal, but usually a fast blow fuse is just the melting link suspended between the ends.  A slow blow usually has the melting wire wrapped around a ceramic rod.
If the original was fast blow and you replaced it with a slow blow, then there is really something wrong with the scope.
Comment: I thought it was universal that 'slow-blow' fuses could be identified because they are marked with a `T` either before or after the current rating, on one of the end caps - e.g. `T1A` or `3.15A T`?
Comment: @nekomatic The __T__ designation for time delay is very common but not universal and may not be standard in countries where the latin characters are not used.
Answer: It is also possible that you have different types of fuse between the 3.15 A and the 4A. Fuse comes in two types: Slow-blow and regular. Slow blow fuses are used when there can be a short-term current surge in the system, like when you power on your oscilloscope. If the old 3.15A is a slow-blow and you put in a 4A regular, it will blow right when you turn it on, which is what you are seeing. I'd suggest looking at the 3.15A to see if it's a slow-blow kind or if it's a regular. Then you need to match the 4A to it. I'd bet that it's a slow-blow.
Comment: Hello Lost in Knowledge, yes, I used a time-delay 4A fuse. One difference is that the 4A fuse is glass, the 3.15A is ceramic. Would that make a difference?
Comment: Theoretically, there is a difference between a glass fuse and a ceramic fuse. A ceramic fuse typically withstands more abuse when it comes to surge current. Having said that, I have yet to see substantial differences in practice. In your case, your best bet is to get the same fuse as the original one and try it out with your scope.
Comment: In truth there are 3 types of fuse. Ultra-fast, Fast (the most common) and Slow. Ultra-fast are recommended for sensitive circuits (like CMOS devices) and when you`re designing you should pick a fuse with I2t lesser than the minimum I2t of your circuit. Fast are the ordinary ones, recommended for general use. Slow ones can withstand for a few seconds much larger currents than its specification, so then are recommended for inductive circuits, like motors. When picking a fuse, you should not look at only its nominal current, but the right type is equally important and sometimes maybe even more.
Answer: By changing the fuse to a higher current, you are changing the power which the oscilloscope can draw from the wall.  The power is
$$ P = IV $$
for current \$I\$, voltage \$V\$.  So by moving from a 3A to a 4A fuse you are increasing the power available inside the scope by about 30%.
You're unlikely to start a fire by switching to a 4A fuse, but I wouldn't go any higher without debugging the actual problem.
Comment: If the 4A fuse blows, then the device is drawing more than 4A and the 3A fuse will also blow.
Comment: Ok, thanks Rob. So in your opinion, is it likely that the fuse would still blow even if I used a 3.15A?
Answer: Just a comment.
If this is a T3 fuse, (small, glass or ceramic tube, measuring 20mm length x 5mm diameter) you are likely mis-reading the etched info on the silver end-cap.
Are reading is T3.15A250V, yes? The period/dot may be a separator that indicates you have a T3 fuse, with a 15A, 250V rating.  (I'm looking at a ceramic one I just removed from a power supply right now as I type this ... it's definitely a 15A fuse ... and is marked as such on the fuse holder in the power supply as well.)  Amps did not used to be marked with periods, per the old standard,
So that could explain why you are instantly blowing a 4A fuse. You probably need to draw somewhere near 5-10 Amps through that part of your device (likely the electron gun of the CRT on the oscilloscope). Beware though - we haven't eliminated the possibility of a fault. Something blew the original fuse ... be physical weakening due to age / impact or a true fault.
Long story short, read your manual (or Google for it) and verify what you need ... especially if you picked up the device second-hand. You never know what the previous owner installed correctly or incorrectly. Trust, but verify. ;>
PS: T shapes come in multiple sizes (eg: 1/4"d x 1-1/4"l) and in both glass (with metal link) and ceramic. In something like an oscilloscope (or anything with a CRT) I wouldn't be surprised to see a ceramic fuse to prevent the possibility of arcing in a very high current-flow failure.
Edited to correct something ... there are 3.15 Amp fuses are marked in A and not mAh ... something I had not previously seen. Thanks to Sam, for pointing that out.
Comment: Yeah, verify.  A 15A fuse is robust enough for 1.6kW in the USA and 3.2kW in 220V regions.  This does not sound like a suitable fuse for an oscilloscope.
Comment: Welcome John, but -1 from me; 3.15A 20x5mm fuses do exist, **marked with 3.15A**. I'm not disputing that you believe you have a 15A fuse ((I'm not there to see it) but as an example, [here](https://uk.rs-online.com/web/p/cartridge-fuses/5414690/) is a page from RS Components of a 3.15A fuse, and you can see the end cap making in the photo is 3.15A. (Just FYI, don't include your signature in an answer or question, it's against site rules. Thanks!)
Comment: This question is from 2015.

3.15A and 4A fuses - can I swap?


Including teaching statement in RA position application package

Question: It seems to me that it does not make sense to include my teaching statement in an application package for a research associate position (UK), which is focused on a specific research project (the only teaching-related activities there might be presentations, seminars or similar events). Could including teaching statement hurt or, vice versa, benefit the application and subsequent decisions?
Comment: What does the job description or application website ask for? Only submit what they require.
Comment: @BillBarth: Thank you. I realize that in such situation a teaching statement is not **needed**. The question is, however, whether it could be _beneficial_ (due to small teaching component - seminars, etc.) or clearly have a _negative_ effect. P.S. I already applied without including TS, but I'm still curious about this question.
Answer: I wouldn't hold it against anyone personally, but it could be bad or it could be good. If there are teaching or training opportunities as part of the job (like there are for people I hire), then it might be marginally helpful. It at least indicates an interest in the ancillary functions of the role. However, if it's a pure research position unattached to an organization with no possibility of a teaching role, a particularly unkind or paranoid sort might hold it against you since it could serve, to them, as an indication you'd prefer a professor/lecturer position, and that you are likely to not stick around very long if you can find one. This kind of stuff matters a lot more to some folks than it does to me personally. There's some legitimacy to this kind of thinking since the hiring process is time-consuming and expensive. You don't want to lose someone shortly after getting them on board.
My recommendation is to stick to the requested materials and no more. Tailor your application, CV, and letters to highlight the things that the position description wants.
Comment: Thank you for your nice answer (+1 and accepted) - the logic is clear to me and indeed makes sense. I'm glad that I applied without TS, intuitively following this logic or, likely, my gut feeling. However, I'm not too worried :-(, since my chances to be hired at several positions I'm currently applying for aren't very high, to put it politely. Several of such positions mention list of publications, which I currently don't have (besides dissertation). However, I decided to apply anyway, since I have some skills that they need, plus I don't want in the future to feel guilty that I haven't tried.


Practical differences between the different RL architectures

Question: I have tried many different RL architectures: DQN, PPO, Policy optimization, and for my specific problem they all failed in their basic setup. Eventually I discovered that my problem had too sparse/deceptive rewards and that they would never work. This made me wonder what the practical differences are between the architectures. In my view it looks like that the more recent architectures have better exploration/exploitation trade offs. And that the main problem with RL is not finding any good trajectories to learn from, but that all algorithms are pretty good at performing once it has found that good trajectory. So is the exploration/exploitation trade off the biggest difference on which most architectures differ on?
Comment: those are algorithms, not architectures, and they differ in the ability to learn... however, if your reward signal is a +1/-1 after 382739857 actions, it's veeery hard for all of them to learn, as there is no "supervision" signal to optimize for... you might want to look at keywords like "reward shaping" and "intrinsic curiosity"
Answer: In theory on-policy algorithms like PPO and actor-critic should perform better than off-policy algorithms such as DQN. The downside is that you will need to collect new data once the policy is updated, while with DQN you have a replay buffer that reuses experiences.
PPO also allows you to make several updates before collecting new data thus increasing the sample efficiency of policy gradient methods. But still DQN is way more efficient. So if your environment is slow, not parallelizable, or you cant even simulate it and have to use the real thing, then maybe use DQN.
Another major difference between DQN and direct policy optimization methdos (like PPO) is that you cannot use DQN with continuous action spaces. There is just no way to compute the maximum over the action space in the equation $Q(s_t,a_t) = r_{t+1} + \gamma \max_{a \in \mathcal{A}}Q(s_{t+1},a)$. Policy gradient methods have no problem with this.
Regarding exploration, there are a lot of great works that research this topic both for policy gradient methods and for DQN-style off-policy methods. I can't really say which line of research has better results.
Finally, regarding your problem, I wanted to say that if you implement the algorithms yourself there might be a great chance that you implemented something incorrectly. Especially in the case of PPO, which is known to have a lot of details not mentioned in the official paper. Using a correct implementation is very important for hard problems. You might be able to solve atari pendulum, but it will fail on harder tasks. Here is a nice suggestion for testing your implementation on increasingly harder environments: CartPole, LunarLander, Pong, Breakout.
You can also check out these two blogposts and compare your implementation of PPO:
https://iclr-blog-track.github.io/2022/03/25/ppo-implementation-details/
https://pi-tau.github.io/posts/actor-critic/


Simple Indesign cc script (I hope)

Question: I keep having to do the same ting over and over.  I get a text file with a 2-line header and the there is one list of names.  I need to keep the header full width of the page and then have the two groups in two columns.  The two groups don't always have the same number of people, but they are labels "Group 1" and "Group 2"  Currently I just have a previously used group of three frames and I just drag the two single column frames to the appropriate height, but there are so many of these it would be great to have something automated.  Like select the entire text block, make 3 frames all the same length, etc.  Any ideas?  I do vba and actions in PhotoShop all the time, but scripts elude me.
Comment: A image would supplement your explanationa and would go a long way forward describing what your after. I dont see why you would need a script for your explanation that table wouldn't do
Comment: If you don't know how to do it yourself, be careful with qualifying writing a script as "simple"  That said, I agree with @jooja: I'm having troubles following your description, so an image or two (before/after) would be useful. Pending that, writing up something might or might not be simple for me - I can't say yet.
Answer: Sounds like you may be doing some sort of text merge. The heading style can be set to "span" columns. That way you would need only one (two column) frame.

Simple Indesign cc script (I hope)


What is the age of Machu Picchu principal temple?

Question: Brian Foerster in this video tells a theory that Incas built their city on top of an older one, judging by the difference in the block material (granite), size, and the precision of their composition. Couldn't we verify this using scientific methods of age detection?
Comment: Note that we have very precise methods to date organic materials thanks to C14/ring trees/philogenetics, but not so for inorganic materials. The usual method for finding the age of a city is go to the burial mounds/waste pits and date the organic material there. It is because inorganic materials do not suffer many alterations, and those that we know of (erosion, etc.) are pretty variable.
Comment: When a man/animal lives, his C14/C12 proportion remains stable, when he dies C14 begins desintegrating and we can use the level of C14 left in relation to the original to know the time of his dead and birth (actually only his dead, but the estimates usually give enough time to live several lives). A block of granite also has radiactive elements decaying, but those have been decaying since the rock did solidify, regardless if it was buried in the ground of if it was part of a building.
Answer: Although opinions differ, the overwhelming academic view as things stand is that construction at Machu Picchu began around 1450. I don't think anyone is completely ruling out an earlier temple being built on the site before that, but the 'evidence' in the video is pretty scanty: it's well-established that precision stone-working had been in place in the Andes for hundreds of years before the Incas, for example at Tiwanaku in Bolivia (some pictures) or at sites like Sillustani near Lake Titicaca.
As for the different stone being used, the granite used as shown in the video was also used at demonstrably Inca sites such as Ollantaytambo, so again it's hard to see this as 'proof' of MP being built by a different civilization. Sorry, but there we are...
Comment: Strictly speaking, the presence of similar technology at other cities (including Andes settlements) doesn't rule out the theory that these temples were build before the cities. My question is about any real scientific measurements of the granite stones the temples are built from.
Answer: Not to mention many natives that knew of the area claimed it wasn't the Incas that built Machu Picchu, but hey what do savages know?
The best way to look at what happened in the past, is to look at what is happening today, still doesn't paint the full picture, though it helps.
How many low tech groups of people are making beautiful stone structures on mountain tops in this day? Well none we know of. So that implies someone of fairly evolved technology had gone up there and settled the plateau. Those people could have very well died off, then other people moved in.
If I discovered Machu Picchu, I'd move in too, just look at that view.
Comment: This answer would benefit from sources.
Comment: "The best way to look at what happened in the past, is to look at what is happening today" but we do the opposite here. We read the past sources to better understand what is happening now.


How my neighbour machines in the same network knows my interface ip?

Question: In my local machine if I set the IP address to an interface, how other machines know this and route the packets to me if anyone tries to send packets to my new IP? I am in a private network.
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
Answer: Simple answer: The machines don't know the route.
The IP address that you use must be in the range of the so-called "subnet" the machine belongs to.
This means that all machines that can be reached without using a router in between must have an IP address in the same range and all these machines need to know this range.
In an Ethernet network you need the MAC address of a machine to send data to that machine.
If a machine wants to send data to another machine, it will first send an ADP packet (used together with IPv4) or an NDP packet (IPv6) to all machines in the "subnet".
This ADP or NDP packet contains the IP address of the machine to which the data shall be sent. The machine having that IP address will send an answer back to the machine that wants to send the data.
The first machine now knows the MAC address of the second machine and can send data to the second machine.
In (very) old Ethernet installations data was always sent to all machines in the network. It was the task of the network card to filter out all data that was sent to machines with another MAC address.
In current Ethernet installations "switches" are used. When data is routed through a switch, the switch remembers the sender's MAC address and the "port" (the cable) on which the data was received. When a packet shall be sent to that computer later, the switch knows the route to this MAC address. When a switch does not know the route, it routes the data to all ports (which may mean: to all computers in the "subnet" - just like in old Ethernet installations).
If a router is between two computers, the computers are in different "subnets".
In this case the computers must know the (MAC or IP) address of the router which is used to reach the destination computer's "subnet".
In the case of an IPv4 home network the DHCP protocol is often used to tell all computers the IP address range of the home network (which is a subnet) and the "default router" (which is the router that must be used for all IP addresses outside that subnet). This information does not change if you change the IP address of your computer because the new IP address must be inside the IP address range of your home network.
Comment: @Martian tnx for your response, I make sense to me. I have one more doubt. After switch caches the mac address and the port, Incase if an IP address assigned to a different host, will switch drops the cache if yes, is it costly.
Comment: I am asking this because in my case single IP will be assigned to a different host from time to time but it is sure that one IP will be assigned to one Host at any given time.
Comment: @KarthikNedunchezhiyan A switch doesn't care about the IP address, it only uses the MAC address. The MAC address is inserted by the local router and whether it is accurate or not depends on the router's ARP caching. If an IP is reassigned on short notice, the new host should send a gratuitous ARP (GARP) to update all ARP caches.


Is electric current a scalar quantity?

Question: According to the definition of a Scalar quantity that i have read in class 9 is that ''those quantities which has only magnitude but no direction is known as a scalar quantity''.....But in class 10 i read that charges need to flow in a particular direction in order to form a electric current......From this argument we can conclude that a current has a specified directions which denies the definition of being a scalar quantity......
Answer: That definition of a vector quantity is a little too simple. It needs to not only have direction, but the directions need to add depending on the angles between them in a specific way to give an overall equivalent quantity.
Current in a circuit isn't really a vector quantity, it has direction but that is equivalent to just the sign of the current. You can have a positive current going in one direction and a negative current in the other - they will still add but not in a vector sense.
Perhaps there needs to be a 3rd term in between scalar and vector.
Comment: In the Maxwell equations, $\nabla\times\vec B = \mu_0 (\vec J + \epsilon_0 \partial_t \vec E)$, the current $\vec J$ is most certainly a vector quantity. I suspect your answer (and the question) is talking about currents in an electric circuit, but it might be worthwhile to mention that, in general, current is indeed a vector quantity.
Comment: @ACuriousMind - good point, I was think in terms of the question
Answer: Current is what is known as a pseudoscalar. This justification for this comes from the definition of current. Current, $I$, is defined as the net flow of charge per unit time through some surface. We define a vector field called the current density, $\vec{J}$, that describes the net flow of charge density at each point in space. It's related to the charge density ($\rho$, charge per unit volume), and mean velocity at each point, $\vec{v}$, by:
$$\vec{J} = \rho \vec{v}.$$
If $\vec{J}$ is constant, then the net time rate at which charge is flowing through some flat surface with area $A$ is given by:
$$I = A \hat{n} \cdot \vec{J},$$
where $\hat{n}$ is a vector that has length $1$ and is perpendicular to $A$ (it's a 'surface normal'). $A$ is flat, though, so the definition of $\hat{n}$ is ambiguous (it's perpendicular, but on which side of $A$?). The usual choice in lower level physics class is to just tell students to pick a direction and stick with it. At higher levels, $\hat{n}$ is defined by a cross product, in the case of a parallelogram, and an integral of cross products in general. That makes $\hat{n}$ a pseudovector. Since $\vec{J}$ is an ordinary vector, the result of the dot product must be a pseudo scalar.
Punchline: current density $\vec{J}$ is unambiguous because it is an ordinary vector, and $I$ depends on which direction you define to be positive flow for positive charges through the surface.
Answer: A point O where current 4A and 3A enter at an angle of 60° but output current is 7A where output current doesn't depend upon input current. So, current flows simple algebraic sum, due to this reason current is a scalar quantity.

Is electric current a scalar quantity?


Will US brokers accept me? I'm a citizen of Iran but a resident of Georgia

Question: Any citizens of Iran here who can share their experience?
Comment: Are you referring to the US State of Georgia or the country in Eastern Europe
Comment: While there are exceptions, generally as a citizen of Iran, you would be subject to a lot of financial sanctions which would effectively cut you off from anything like this but I am sure there are exceptions, I just can't think of any.
Comment: I'm referring to the country of course!
Comment: @GµårÐïåñ so what do you do then?
Comment: Just to be clear, to John's point, it is not as clear as of course, especially given you mentioned US Brokers, but I assumed it was the country because of the country you mentioned it was an assumption that happened to be right, not because it was obvious. That being said, not sure much you can do unfortunately unless someone like HSBC and alike who have been fined for "ignoring" the sanctions but I wouldn't consider those a real solution honestly. The points of the sanctions are exactly this, to frustrate people into compliance - whatever it is, I don't want a political conversation.
Answer: You should be able to open an account if you are a legal permanent resident/green card holder.
In fact, National Origin is a protected class, so discrimination solely based on your Iranian origin is illegal, and you should consult a lawyer if you are denied because of it. However, there are some things to consider:

Can you prove you are a legitimate resident of Georgia/legal permanent resident?
Are you on the OFAC sanctions list (here)?
Is the money you are trying to put in the broker US-sourced? Or at least NOT Iranian sourced? If it's from Iran, it would raise strict scrutiny. You would have to work with an OFAC compliant currency exchange broker in Iran to transfer the money. Citizens/residents are not allowed to have bank accounts in Iran either (Source). Again, if you are just trying to put money you made in the US into a US broker, I don't think there would be an issue.
Comment: @user113269: If you don't live in (and arent a citizen/legal resident of) the US, why would you want to invest with US brokers?  Aren't there brokers who do business in the country of Georgia?
Comment: I don't live in the US
Comment: My comment on National Origin is irrelevant then if you are not a resident of the US. A number of brokerages do allow foreign citizens to open brokerage accounts. Do you have legal identification papers for Georgia similar to a greencard? If so, then you can might can open a brokerage account with a brokerage that services Georgia. If all you have is an Iranian passport, then no, it's probably not legal for any US-domiciled brokerage to service you.
Comment: If they were a US resident even, this would not have been an issue, usually a reference to a blocked country implies they are outside the jurisdiction that would provide them the protective cover to partake but that's the fault of the poster not to make clear, doesn't mean your answer is wrong per se, just not relevant to what they were looking for.


Why should I roll the meat in flour for a ragout

Question: Most recipes for ragout call for rolling the meat in flour before adding, including one recipe whre you don't sear the meat but add it with the vegetables and everything else once the onions are done. What is the function of the flour, especially in the latter case?
Comment: feel free to add tags, I know little english cooking terminology so ...
Answer: Flour is a thickening agent, plain and simple. However adding it without cooking it first can lead to your dish having a raw flour taste, and you can get blobs of flour in it, so I would ignore the recipes which say coat with flour without cooking it first, or use cornstarch (cornflour) instead.
Alternatively you can make a roux with flour in a separate pan and add it in later. This is simply mixing equal weights of flour and butter and cooking it in a frying pan until it has the color you want. A lighter roux will add a paler color and a milder flavor, a darker roux adds a deeper color and richer flavor.
Comment: When you roll your meat in flour you aren't adding additional fat, when you  cook it separately you are adding fat. In meat dishes there's usually enough fat already, and rolling/searing your meat is less complex with less cleanup
Comment: so it doesnt matter wether I roll my meats in flour or add the flour some other way?
Comment: @GdD this is also how I make my gravy for biscuits and gravy. I'm the hot pan roll the sausage crumbles around in 5 table spoons of floor, one table spoon at a time letting it golden each layer, then dump in the prepared and spiced milk, stir constantly and let reduce to a nice perfect white biscuit gravy.
Answer: I don't think it's a thickening agent in this case, although it contributes a bit. Especially when you roll the meat in it, it is used for the Maillard reaction. It is a reaction between proteins and carb chains. The meat alone has lots of protein, but relatively little carbohydrates from the glucoproteins, and so it's not crisping optimally on its own.
When you add some flour to it, it crisps alone while the other vegetables are softening, it doesn't need a separate sear. Of course, you should take care to use the appropriate temperature - if it is so cold that the vegetables start stewing in their own liquid, the meat will get too wet to crisp too.
This is based on my understanding that you add the meat and all vegetables together and only later add the liquid. If you meant that you dump the liquid at the same time too, my guess is that the person who wrote down this recipe didn't realize what is happening and decided that it's easier to not wait with the liquid. Or that it is derived from a meatball recipe and the author again didn't realize that the rolling makes no sense. The thickening hypothesis is insofar shaky that there are better ways to add flour for thickening.
Comment: This recipe (one of Jamie Olivers) indeed calls for adding the meat at the same time as the liquid, but I find it plausible that the flour is a holdover from earlier iterations of the same recipe where the meat was seared separately.


prepared meals and storage time in fridge before delivering them to clients

Question: my daughter prepares meals for her small catering business. After she prepares them and stores them in plastic containers she stores them in our fridge which is taking up too mush space. They foods are cooked and vary from pasta to vegetables to seafood and other meats. Sometimes she does not deliver these prepared meals until the 2nd or 3rd days how soon must she deliver them to her clients before they spoil.
Comment: Why would anyone want to eat 2-3 day old anything from a caterer? Is this a standard practice that I'm not aware of?
Comment: It seems your real problem is the space this food takes up in your fridge. That's not something we can help you with.
Comment: It's very likely a violation of food safety rules for her to store them in your refrigerator anyway. Commercial food preparation and storage is normally required (IME) to be **completely separate** from a household kitchen, even if the person using both is the same.
Comment: This might not be a duplicate: sure, the OP is asking how long it's safe as if that's the whole question, but this is in a catering context not a home cooking context, and as other comments and the answer show, that changes the question.
Answer: In catering, you do not use your home fridge because your family opens and closes it so many times that the temperature drops and the food in it that is cooked does not stay at a steady temperature.
For example, you said she may prepare seafood. That is a Big NO NO.  The seafood should be prepared and served within hours.
Think about it like this:

You prepare it.
You store it 2-3 days.
You deliver it or it gets picked up.
Then does it get put in the fridge again or put out?
Then, either way, it is served.

How many times is it in the fridge, opened with the air from the family, transported getting back to a lower temp, put back into storage or set out?
Sounds like someone might get sick to me.
When I did catering, our clients expected fresh food, not leftovers.  Some caterers freeze some of their products but anyone I have talked to does not store things unless it is being marinated, pickled, a desert made a day in advance, or sauces frozen and reheated.
Another thought, if you go to a restaurant, are you willing to pay money for food sitting in the fridge for 2 or 3 days?  My family and friends would not.
Regardless, getting sick is the biggest problem you have. Do you have a license from the Health Department to do this?  There are  cleanliness issues here also.


Linq to Entities System.data.Objects

Question: I'm getting started on Linq to Entities and the example references a namespace called System.Data.Objects. My environment doesn't include this namespace and I can't find the DLL that contains it.  Anywone know where I'd find it?
Answer: I had the same problem.
I downloaded code for a Microsoft tutorial and started working with it, but I hadn't added any EDM items to the solution myself, so the reference was missing.
I used "Add New Item > ADO.Net Entity Data Model" in VS2010 to add a new, empty EDM to the solution.
Then I re-built the solution.  No compiler errors.
Then I deleted the empty EDM.
Adding the EDM to the solution added the necessary references.
Answer: If you have created a Website targetting .Net 3.5 [assuming you have 3.5 SP1 installed] then you have the support for ADO.NET Entity Framework in your Project. You can add an Entity model from Add New Item dialog. This means that your Project has a reference to System.Data and exposes the System.Data.Objects namespace. There's no case why it should not happen.
This namespace is very essential in developing applications with ADO.NET Entity Framework, since it provides you the Objects that can handle Entities , Entity Queries and Query Results.
For ex:
If you want to store all the Customers result-set returned by a Qry or Expression, then you can use following syntax to do so..
<code>ObjectResult<Customer> _Resultset = ctx.Customers;
</code>
Likewise you can use ObjectQuery to store Entity Queries.
You can refer to following link in order to learn more about this.
link text
Answer: Never mind. It appears to be part of the core framework. The tutorial I'm working from is from Microsoft so it isn't very clear... :-)

Linq to Entities System.data.Objects


Is this asbestos pipe insulation

Question: I have been doing work around a house I purchased. Never even thought pipe insulation would have or could have asbestos. My home was built in 1976 and is on Long Island NY (not sure if this helps.) I fixed a pipe leak and carelessly ripped off this insulation not even thinking. Just started gutting another room and found this. Could it contain asbestos being that it's from a home built in 1976? I know to be sure it would need testing, but is there a high chance this is?Thank you in advance!
Answer: It does not look like typical fiberglass. The era is right. Testing and reasonable care is certainly advisable, as is a reminder that most "effects of asbestos exposure" are from chronic exposure (folks that worked with the stuff) not whatever tiny bit you may have been exposed to in ripping one or two pipes open before going: Hmm, this doesn't look like what I was expecting.
The only way to know for sure is to have it tested at a lab (if you get exorbitant prices and scare tactics, keep shopping for a reasonable lab.)
Comment: « Folks that worked with the stuff » - heard stories that pipe fitters used to bunch the asbestos insultion into « snowballs » and throw them at each other...
Comment: I'm glad to see people saying that illness from asbestos comes from chronic, long term exposure in manufacturing or installation settings, rather than it just being present, but contained.  There was so much hysteria about it (probably driven by lawyers) earlier that you'd it was plutonium.

Is this asbestos pipe insulation


Weapon switch on the right side of the screen on Team Fortress 2?

Question: When I play TF2, when I switch weapons, the weapon switch menu used to be on the left. Now, I turn on my TF2 (On PC btw) and when I switch weapons, the weapon switch menu is on the right side. I have no idea why, and now I have to get used to it, albeit I want the menu to go back to the left side of the screen. :(
Comment: It's always been on the right for me too
Comment: It was always on the right side for me, albeit I turned on quick weapon swap about a year ago and haven't seen it since then
Comment: Same here - it's always been on the right.
Answer: So this is probably not the simple solution you hoped for, instead this uses HUD modding to manually move the weapon selection screen to the left. This is a very unelegant way and I'm not really sure if I would actually recommend it. But it works for sure. Also, you can easily get rid of it if you don't want it anymore.
First, navigate to your Steam folder or the Steam library tf2 is part of and navigate to SteamApps\common\Team Fortress 2\bin. Shift+Right click and click on Open command window here. Then type vpk ../tf/tf2_misc_dir.vpk and hit enter. Wait until the process is done (Note: This will probably take several minutes).
Then navigate to SteamApps\common\Team Fortress 2\tf\tf2_misc_dir\scripts and copy the file hudlayout.res to your clipboard. Then head over to SteamApps\common\Team Fortress 2\tf\custom and create a folder named HudFix. Inside that folder, create another folder named scripts. Inside scripts, paste the file.
Open hudlayout.res with your text editor of choice (Notepad will do just fine), and search for HudWeaponSelection. Beneath that, you will find several lines containing properties enclosed in curly brackets.
Replace the line
"RightMargin" "0"

with
"RightMargin" "760"

Note: This was tested on a 1920 * 1080 monitor. If your monitor resolution differs, the number 760 may be wrong and place the dialog either outside of the screen or right in the middle. In that case you have to experiment which number places the HUD element currectly.
Next, replace the line
"LargeBoxWide" "110"

with
"LargeBoxWide" "72"

This will prevent the currently selected weapon, which has a bigger box, randomly going out of the screen.
Save the file and (re)start tf2. If you now join a match, the weapon selection dialog appers on the left side as shown in the picture. It's a little different, because the selcted weapon box isn't as wide as it usually is, but it looks pretty good that way too.
After you're done, you can delete the folder SteamApps\common\Team Fortress 2\tf\tf2_misc_dir.
If you want to undo the modification entirely, just delete the HudFix folder you created in SteamApps\common\Team Fortress 2\tf\custom.


Why do we prefer DFS to find connected components?

Question: In finding the connected components in a graph, why do we use DFS, instead of BFS?
Comment: http://stackoverflow.com/questions/2869647/why-dfs-and-not-bfs-for-finding-cycle-in-graphs
Comment: Can you edit the question to clarify exactly *how* you were thinking to use BFS to find connected components?  That would provide us a more concrete basis for comparison.  Finding connected components is not just "use DFS and read off the answer"; there's some subtlety involved.
Answer: Two components $G_1(V_1, E_1)$ and $G_2(V_2, E_2)$ are connected if and only if there is a path between any vertex $v_1\in V_1$ and any other vertex $v_2\in V_2$. Here I'm assuming that both components are known to be connected, i.e., there is a path between any pair of vertices $v_i, v_j$ in any component (so that finding a path between any pair of vertices between both components fully connects them).
Hence, in finding connected components the procedure consists of starting in any vertex of $G_1$ and collecting all nodes that are reachable from it. Clearly, this can be done either with depth-first search or breadth-first search:

Breadth-first search visits nodes in order of their depth from the start state, $s$. This way, it finds all paths of length 1, then length 2 and so on. Its main advantages are: it guarantees it will eventually find a solution (given sufficient memory, see below) and that the solution found is optimal; also, it can avoid transposition by maintaining a CLOSED list that stores all nodes previously expanded. Its main disadvantage is that it takes an exponential amount of memory (unless the graph being traversed is known to be polynomial but this is a rare case indeed).
Detph-first search generates paths of arbitrary length until a goal is found or a threshold is reached. Its main advantage is that it takes memory which is linear in the depth of the search tree developed. Its main disadvantage is that it cannot guarantee that it will ever find a solution and, if it does, that the solution is optimal.

Now, considering the problem stated above it becomes clear that the goal is not to reach a specific goal but to collect nodes reachable from the goal state. For this, there is no specific need to be optimal so that the question reduces to whether the available memory might be enough or not: whereas Breadth-First search does not provide any guarantee, Depth-First search does.
That is why we use Depth-First Search Mostly because: one, there is no need to find an optimal solution; second, memory matters!
Cheers,
Comment: BFS takes an exponential amount of memory?  Exponential in what?  The memory used by BFS is at most linear in the number of vertices in the graph.
Comment: In the depth of the search tree generated: While Breadth-First Search takes $O(bˆd)$, Depth-First Search takes $O(d)$ ---with $b$ being the branching factor and $d$ the depth. By the way, it is not customary to use BFS for Breadth-First Search (even if the context helps) as it usually means "Best-First Search" which is an entirely different category of search algorithms.


Why didn't the Russian- and every other nuclear capable country- shut down their nukes?

Question: In Arrow season 4 episode 21, all of the world's nukes were activated- including Russia's. So at this point, it was obvious they were being hacked. So why didn't they simply shut them down, and why wasn't there more of a world collaboration (in fact I don't think there was any for that matter) for what was probably the highest possible state of emergency? Am I wrong in assuming that at the very least, there aren't any specialist government programmers like Felicity who could've made an effort in stopping Havenrock from being nuked?
Comment: [The Russians can't.](https://www.youtube.com/watch?v=2yfXgu37iyI)
Comment: And I would not be surprised if the policy is that, if one country is going to launch its missiles, we're going to launch ours.
Comment: [Nuclear War 101, 102, 103](https://gist.github.com/benders/141611)
Answer: Why couldn't the missile operators just stop the launch?
The missiles weren't being hacked, they were hacked; past tense. The operators had already lost control of their command systems. One of the Russian soldiers says this in the very first scene:

Soldier: At zero-three-sixteen, silos one through four rebooted and are now powering up to launch sequence.
Commander: What do you mean, "rebooted"?
Soldier: I've been locked out of the system...
Arrow Season 4 Episode 21: "Monument Point"

This is kind of the point of Rubicon, actually; if any country could circumvent it from their missile bases, it would rather undermine the intended purpose:

Lyla: [Rubicon is a] ballistic launch override protocol designed to prevent any country, even our own, from firing a nuclear missile.
Arrow Season 4 Episode 20: "Genesis"

Why wasn't there more international response?
Well, we don't really know what was going on outside of Star City; maybe there was some concerted international effort to stop the nukes. It seems unlikely it would have succeeded, though; Lyla herself says:

Lyla: Rubicon is a failsafe against nuclear Armageddon. ARGUS designed it to be 100% unhackable.
Arrow Season 4 Episode 21: "Monument Point"

Even Felicity and Noah, who are basically wizards, only manage to (almost) stop them by using hyper-advanced, experimental computer hardware.
Comment: True, but you can still manually remove the main source of power- .e.g. if someone takes control of my computer and the shutdown button doesn't work, I can still pull out the plug
Comment: @Logan545 Again, if Rubicon were easy to subvert then it would be pointless. I don't know much about how missile command centers work, but I know that they're designed to be *highly* tamper resistant
Comment: @JasonBaker: I think I agree with everything you said, except for the first sentence of the above comment.  If Rubicon was designed to prevent countries from firing nuclear missiles, then [other] actions taken to disable missiles, or otherwise prevent their launch and/or detonation, do not constitute a subversion the goals of Rubicon.
Answer: Because that would not have made a good show... Sadly a lot of the time the reason why there are flaws in plots such as this is because the  shows/movies are written to be action filled cliffhangers not for logic and what would happen in real life...


Due to the office timings (1 to 10 PM) doing workout late in the morning 10:00 AM has any impact on fitness?

Question: Due to the office timings (1 to 10 PM) i do workout/yoga late in the morning around 10:00 AM and also go to bed late in the night around 1or 2 AM. So does this time slot has any impact on the physical fitness or make workout less effective? If there is an impact on the mentioned things then what can be the ideal/best time to performing these activitis to get best out of it?
Answer: Himanshu,
First, congratulations on taking your health and fitness seriously. Taking the time to exercise is a big deal and I hope you make it a long term goal.
Many experts recommend morning exercises as the best, especially before breakfast.
Personally though, I don't believe that the time of the day matters. What matters is the consistency and intensity. 1000 calories burned during the day is the same as 1000 calories burned at night. If night workout fits with your schedule and you're consistent with it, you'll see optimal results. If day workout's your thing, you'll see identical results too.
I haven't read any article about the body's ability to lose calories based on the time of the day. However, a lot of people exercise in the evenings because it's much comfortable for them to do so. They are more relaxed, have more time, or have difficulty exercising in the mornings.
So, as long as you're consistent in your workout routines and maintain a good nutrition, you should be healthy and strong.
Equally important, don't make your workout time the only time you exercise. Try to incorporate exercising into your daily routine. A quick 10-rep squats after sitting in a chair for a while can give your body a quick calorie burn (in less than 20 seconds). So is a quick 10-rep push up.


Definition of Gravitational Potential Energy

Question: Suppose,we throw a ball of mass,m upward with an acceleration, A such that A>gravitational acceleration (g).clearly,the ball must stop after reaching it's maximum height, h.so,HOW SHOULD THE POTENTIAL ENERGY HERE BE CALCULATED?,Here the definition of the gravitational potential energy proves wrong. Gravitational potential energy is defined as the amount of work done in raising it from the ground to that height,which is the weight of the object.I can prove this definition this way,The work done on the ball=F*displacement=mAh which is greater that the weight of the ball. Am I wrong somewhere,then please tell?
Also,why do we define gravitational potential energy like that?(what is the reason behind that definition?)
Comment: "*Gravitational potential energy is defined as the amount of work done in raising it from the ground to that height,[...]*" **Agree**. "*[...] which is the weight of the object*" What, **no**. How can energy equal weight (a force)? This is not in the definition of gravitational potential energy.
Comment: http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html as regards gravitational P.E.
Answer: The definition of gravitional potential energy (at small heights) is $$U=mgh$$
Plug in the height $h$ and you have the $U$ associated with that point.
This potential energy is correctly equal to the amount of work done to bring the ball up there. But remember that the work is only being done while the force is throwing the ball. When throwing a ball, you accelerate it and then you let it go and it flies upwards, stops, and returns down. Your throwing force only works until you let go.
So when trying to calculate work with the formula $W=Fh$, then the $h$ is wrong. The force $F$ did not do work on the ball over the entire distance $h$. You don't know the distance (this distance would be the swing of the arm), so you can't use this formula in this case.
Of the same reason, the acceleration is not as you expect. The force might accelerate the ball with $a=A$, but as soon as you let go, and the force doesn't work anymore, this acceleration disappears. During the flight only gravity pulls, and therefore the ball only accelerates (downwards) because of gravity during the whole flight (and this acceleration is $a=-g$). This is why is slows down, stops and then speeds up downwards.
When you throw the ball, in the moment you let it go, you have done an amount of work on it, which has given it a lot of kinetic energy. This kinetic energy will now during the flight be converted into potential energy, while it flies up. When it reaches it's highest point, all kinetic energy is used up and no more can be converted into potential energy, so it doesn't go any higher.


Get Url from which the Redirection has been made

Question: I am using the symfony framework, with FOS User Bundle.
I have a URL https://my-domain/article/view/articleId on my website to display the content of an article. For this url, I have a security check with the ArticleVoter, that throws an AccessDenied Exception if the user is not logged in.
(You may ne asking yourself "Why didn't I just put the url behind my main firewall ?". Because, for some articles, the url is accessible for non logged-in users, it depends on some attributes of my article entities, and this is when the ArticleVoter comes in, in order to check these attributes).
Then I have an AccessDeniedSubscriber that implements EventSubscriberInterface and that goes like this (simplified version for the sake of clarity) :
public function onKernelException(GetResponseForExceptionEvent $event): void
{
    $exception = $event->getException();

    if ($exception instanceof AccessDeniedException && !self::isThrownByFirewall($exception)) {
        $redirectRoute = 'my_login';

        // Create your own response like in a custom access denied handler
        $response = new RedirectResponse($this->router->generate($redirectRoute));
        $event->setResponse($response);
        $event->stopPropagation();
    }
}

public static function getSubscribedEvents()
{
    return [
        // Define the priority to execute our subscriber before the one from the security component
        KernelEvents::EXCEPTION => ['onKernelException', 1],
    ];
}

So what happens is that an anonymous (non connected) user tries to access the url https://my-domain/article/view/articleId, an AccessDeniedExpetion is thrown by the ArticleVoter whose vote method will return an access denied, then the user is redirected to my_login route.
On the my login route, I am trying to determine from which route the user was redirected intially. The associated controller method for the login route goes like this :
public function loginAction(Request $request, AuthenticationUtils $authenticationUtils)
{
     $referer = $request->headers->get('referer');

     ... handle login rendering ...
}

Unfotunately, the referer is always null. Which I do understand because it is a server-side redirection, and not a redirection made by some clicking on a link, or by javascript.
So my question is, how do I manage to get the initial url that the user tried to access before being redirected to the login page ?
In my case, that would be https://my-domain/article/view/articleId with the correct article id.
Precision: To reply to @RyanNerd's answer, please find below the result of var_dump($request->headers) :
object(Symfony\Component\HttpFoundation\HeaderBag)#73 (2) { ["headers":protected]=> array(13) { ["host"]=> array(1) { [0]=> string(9) "localhost" } ["connection"]=> array(1) { [0]=> string(10) "keep-alive" } ["upgrade-insecure-requests"]=> array(1) { [0]=> string(1) "1" } ["user-agent"]=> array(1) { [0]=> string(121) "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.129 Safari/537.36" } ["accept"]=> array(1) { [0]=> string(124) "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9" } ["sec-fetch-site"]=> array(1) { [0]=> string(4) "none" } ["sec-fetch-mode"]=> array(1) { [0]=> string(8) "navigate" } ["sec-fetch-user"]=> array(1) { [0]=> string(2) "?1" } ["sec-fetch-dest"]=> array(1) { [0]=> string(8) "document" } ["accept-encoding"]=> array(1) { [0]=> string(17) "gzip, deflate, br" } ["accept-language"]=> array(1) { [0]=> string(23) "fr,en-US;q=0.9,en;q=0.8" } ["cookie"]=> array(1) { [0]=> string(561) "MY_SESSION_NAME_SESS=qih0m4nlmi5enljsni5lphkf9m; sf_redirect=%7B%22token%22%3A%22821c13%22%2C%22route%22%3A%22article_view%22%2C%22method%22%3A%22GET%22%2C%22controller%22%3A%7B%22class%22%3A%22AllArticle%5C%5CWebBundle%5C%5CController%5C%5CArticle%5C%5CArticleController%22%2C%22method%22%3A%22viewAction%22%2C%22file%22%3A%22%5C%2FUsers%5C%2Fjohn_fly%5C%2FallArticle%5C%2Fsrc%5C%2FAllArticle%5C%2FWebBundle%5C%2FController%5C%2FArticle%5C%2FArticleController.php%22%2C%22line%22%3A244%7D%2C%22status_code%22%3A302%2C%22status_text%22%3A%22Found%22%7D" } ["x-php-ob-level"]=> array(1) { [0]=> string(1) "1" } } ["cacheControl":protected]=> array(0) { } }

As you can see, in the 'cookie' value, there is a mention of 'sf_redirect', maybe there is somehow a way to retrieve the previous URL as the previous route is written in this string ("%22article_view%") ?
Comment: Programming by exception is a dark and dangerous path. That being said what does  `var_dump($request->headers)` look like? Are there any other headers being returned? This may provide a clue as to why 'referer' is null.
Comment: thx @RyanNerd for your reply, I added the result of the var_dump to my question on top
Answer: When you redirect to the login page, add a GET param redirect or from with the page URL as value, so that after the login success you can redirect to that URL.


How can you see from the transfer function of a system that it has feed forward/feedback elements?

Question: Furthermore, how can you see from the pole/zero plot if the system has feed forward/feedback elements?
Comment: You've got three "close" votes because your question appears to lack detail or clarity.  I suggest that if you change "How can" to "can" in the two places where it occurs, it'll be a pretty good question.  To really make it good, it's good practice to keep the title short, and to put the whole question in the title.  So "Using a transfer function to determine if a system has feedback" might be a good title, and then just move what is now the title into the body of the question.
Answer: It depends, if you have a graphical representation of your system, you can see that the system has feedback or feedforward elements.
Or you can look at the difference equation or transfer function. For example a discrete integrator $y[n] = x[n] - y[n-1]$ has feedback since the previous output is fed back to the current output. On the other hand, a differentiator $y[n] = x[n] - x[n-1]$ has no feedback as the output does not depend on previous outputs.
Edit : The transfer function does not tell everything. It is possible that pole/zero cancellation will hide away feedback elements.
Answer: Because the definition of "feedback" is contextual, you can't.
If you're talking about physical systems, any transfer function that's not of the form $\frac{A}{s^n}$ has feedback, because the only way to make a non-zero pole is with feedback.
So in theory, non-zero poles means there's feedback.  By this same theory, any zeros that aren't at infinity (i.e., a polynomial of any degree in the numerator) at least means that there are parallel paths -- but there's no way of knowing if these paths are due to feedforward, or simply parallel paths.
However, if you mean that you want to hypothesize a block diagram that has blocks with "full" transfer functions (polynomials of non-zero degree) in the numerator and the denominator, then no, you can't tell from the resulting transfer function whether there's feedback or not.
As an extreme example, if I were designing an industrial machine, I might want to buy a motor for it.  These days, it's more common than not to buy a motor that comes with its own built-in controller, to which you provide power, and commands for torque, speed, or certain motions.
If you were designing that motor controller, then your block diagram would have feedback, and possibly feed-forward.  But when I buy the motor, at best I'm just going to get the motor's transfer function*.  So for me that motor is just a transfer function, and I would just have to know that because I'm buying a motor-controller unit, that feedback is happening inside, somewhere.
* More likely, I'm going to get a guarantee that it's got certain maximum settling times, or minimum frequencies at which the amplitude drop and phase shift become significant.
Answer: All realizable causal IIR (Infinite Impulse Response) systems essentially have feedback. In discrete time systems this is given by poles that are not at the origin. (If the poles are inside the unit circle, we further know the system is stable).
Given a generic transfer function as follows with zeros in the numerator ($z_1, z_2 \ldots$) and poles in the denominator ($p_1, p_2 \ldots$):
$$H(z) = \frac{(z-z_1)(z-z_2) \ldots}{(z-p_1)(z-p_2) \ldots}$$
If the denominator has all poles equal to zeros (such that then denominator is simply $z^M$ for some power $M$, then the system does not have feedback, or at least is functionally identical to such a system that has no feedback (in case internally there is pole zero cancellation and a sub-component does have feedback which we would not be able to discern).


Do I have to prove all the theorems I mention in my paper?

Question: I am writing a paper on a big theorem in Algebra. But in order to supply as many proofs as I find interesting, I have to mention the theorems used to prove this Big Theorem. Do I have to include all the proofs of all the theorems I mentioned as well as relevant lemmas, even if they take me away from my topic for some time?
Comment: It is very difficult for undergraduates to write mathematics papers without the assistance of a faculty mentor to help them out (at least) with the conventions and mechanics of mathematical writing.  If you don't have a faculty mentor, it is extremely unlikely that you can get your paper into a form in which it will be seriously considered by a reputable journal.  (Maybe that's not your goal.  But if not, you have to tell us what it is, because the conventions depend upon the form.)  I hope you have a faculty mentor.  You should ask her about this.
Comment: Hey, @PeteL.Clark! Thank you for your thoughtful answer. To answer your implied questions: 1. My paper is simply a final year project to be read and graded by only my professors. I think the idea is to teach us how to write papers or something. 2. I do have a mentor, but I left this project to almost the last moment and after looking through my first draft she assured me all was well. I, however, have decided to try to shape it into a better form. And it's midnight where I am right now, and a cruel hour to contact one's professor.
Answer: Generally speaking, every statement in a published mathematics paper needs to have a proof somewhere that the reader can find it. (If this paper is for a class assignment rather than for publication, ask your instructor. The rules are also a little different for "expository" papers which are mainly intended to discuss previous work, rather than to present new results.)

If you use a result that is well known, you can mention it by name. It is not necessary to include a formal statement or proof,  nor a reference. This would apply to results that are found in most standard textbooks in your area, and that most people reading the paper would be familiar with.
If you use a result previously proved by someone else, give a reference to the paper containing their proof. It is often also helpful to give a formal statement of their theorem. You do not need to include the proof in your paper unless you think it will be specifically helpful to the reader (for instance, if you are planning to extend the same ideas yourself).
For an original theorem, not previously proved by anyone else,  give a complete and careful proof.
Comment: This is a great comment! Thank you for taking the time to type this all out. I have another question: because I am doing almost a collection of proofs, do you think including the original proof and published critique of it, is necessary? (eg. Gauss's 1799 proof for FTA, in a paper on FTA proofs)
Comment: I haven't ever written a historical survey, but I think you have to make your own judgment about what historical material is relevant.
Comment: Thanks for your prompt response! I've been considering it all day and have since decided against it.


How to intall whatsapp plus

Question: I downloaded whatsapp plus and I installed it on my phone...when am doing the initialization... where they ask you to put ur number there...I put my number there and when the thing was loading...they told me to install the new version on Google play store...uninstall original whatsapp and download whatsapp plus..I uninstalled the whatsapp and downloaded the whatsapp plus but still...they said that thing again...I tried doing they "call me thing" and they told me" it seems ur client has been modified...please download the new version from Google play store..my friend uses the same phone(Motorola droid RAZR) and the whatsapp plus is working perfectly for him...pls help me.
Answer: These happens sometimes. But after 1 or 2 try it will work.
Try these steps.
Make Sure you have perfectly running whatsapp installed with your important chat
1) Go to /sdcard/whatsapp/databases/
Delete all files using FILE MANAGER.
In Whatsapp Go to Settings>Chat settings>Backup Conversations to Backup your conversations ...
2) Uninstall OLDER WHATSAPP
3) Install Given WhatsApp+'s apk ...
4) Open it & Click AGREE & CONTINUE & enter your number
5) If you have done above steps well then there should be RESTORE BUTTON , Press it
(If you Dont get restore button or you get message like "Your app is not official Download from play store"
then
uninstall current whatsapp+ & install original whatsapp from play store , do verification process and then chat with 1-2 person again backup chats and then uninstall original whatsapp & install given WhatsApp+ You will get RESTORE button)
6) Wait ...it might take some long... Press Continue after it finishes

How to intall whatsapp plus


How to show the CUPS printer jobs history?

Question: I am printing some files from a remote computer to a network printer with the lpr command.
It apparently worked, but some minutes later when I typed lpstat or lpq, the job had already disappeared, it had probably already printed the file.
Is there a way to check the history or the log of my successfully completed jobs in the printer queue?
Comment: Of course I have no access to the printer right now, and therefore I cannot check if the file was printed or not.
Comment: Via the web interface it is easy.
Comment: What is that, could you explain a bit more?
Comment: http://localhost:631/.
Answer: Yes a program exists: lpstat - print cups status information
$ lpstat -W completed

-W which-jobs
     Specifies which jobs to  show,  completed  or  not-completed  (the
     default).  This option must appear before the -o option and/or any
     printer names, otherwise the default (not-completed) value will be
     used in the request to the scheduler.

Or if you prefer via the following web pages:
https://localhost:631/printers/[NameOfPrinter]?which_jobs=completed
http://localhost:631/jobs?which_jobs=completed

Kind regards
Comment: This will only show the completed for your userid. You can give a list of users like this: `lpstat -W completed -u user1,user2`.
Comment: Does lpstat have a rolling window for the completed print job history, or does it show all jobs that were ever completed?
Answer: The other answer when tried produced the following:
$ sudo lpstat -W completed
mfc-8480dn-1652         root              1024   Tue 28 Jan 2014 01:19:34 AM EST

Adding a user, saml gives you that user's history:
$ sudo lpstat -W completed -u saml | head -2
mfc-8480dn-1524         saml             23552   Thu 28 Nov 2013 10:45:44 AM EST
mfc-8480dn-1526         saml            699392   Sat 30 Nov 2013 10:34:34 AM EST

But the -u all mentioned in this U&L Q&A titled: View all user's printing jobs from the command line did nothing for me.
$ sudo lpstat -W completed -u all | head -2
$

Curiously I could do this:
$ sudo lpstat -W completed -u saml,root | head -3
mfc-8480dn-1524         saml             23552   Thu 28 Nov 2013 10:45:44 AM EST
mfc-8480dn-1526         saml            699392   Sat 30 Nov 2013 10:34:34 AM EST
mfc-8480dn-1652         root              1024   Tue 28 Jan 2014 01:19:34 AM EST

So one hackish way to do this would be to formalize a list of the users on your system and then add that as a subcommand to the -u argument like so:
$ sudo lpstat -W completed -u $(getent passwd | \
    awk -F: '{print $1}' | paste -sd ',')

Just to show that this sees all the users locally you can get a unique list of your users like so:
$ sudo lpstat -W completed -u $(getent passwd | \
    awk -F: '{print $1}' | paste -sd ',') | awk '{print $2}' | sort -u
ethan
root
sam
tammy

Issues?
One problem with this is if the user printing to CUPS does not have an account locally then they won't get displayed.
But if you have a directory that contains your LPD control files, typically it's /var/spool/cups, you'll notice a bunch of control files in there. These files are kept as a result of theMaxJobs` setting, which defaults to 500 when unset.
$ sudo ls -l /var/spool/cups/ | wc -l
502

Another source of usernames?
If you look through these files you'll notice that they contain usernames, and not just ones for accounts that are present on the system.
$ strings /var/spool/cups/* | grep -A 1 job-originating-user-name | head -5
job-originating-user-name
tammyB
--
job-originating-user-name
tammyB

So we could select all the entries that contain the username followed by the B.
$ sudo strings /var/spool/cups/* | grep -A 1 job-originating-user-name | \
    grep -oP '.*(?=B)' | sort -u
ethan
guest-AO22e7
root
sam
saml
slm
tammy

This list can then be adapted in the same way as we were originally using to take the list of users from getent passwd, like so:
$ sudo lpstat -W completed -u $(strings /var/spool/cups/* | \
    grep -A 1 job-originating-user-name | \
    grep -oP '.*(?=B)' |sort -u | paste -sd ',')
mfc-8480dn-1525         tammy           545792   Thu 28 Nov 2013 01:36:59 PM EST
mfc-8480dn-1526         saml            699392   Sat 30 Nov 2013 10:34:34 AM EST
mfc-8480dn-1652         root              1024   Tue 28 Jan 2014 01:19:34 AM EST
mfc-8480dn-1672         saml              1024   Sun 09 Feb 2014 01:56:26 PM EST

References

why is /var/spool/cups so huge?
Comment: Don pass usernames after `-u` and it returns all jobs of all users.
Answer: I think /var/log/cups/page_log etc. has the history of completed jobs.
An alternative is the web interface
http://localhost:631/

which also shows completed jobs. I'm not sure where the web interface gets its information from.
Answer: Every answer above seems to be only interested in completed jobs...
What if you are also interested in jobs which were canceled by the user or an admin? What if you want to know how many jobs were aborted by the print system itself, due to driver or other problems?
For these cases use:
lpstat -W all -o

Also, you may want to know more details about each job. In this case add -l for a long output format:
lpstat -l -W all -o

Answer: If you want to see jobs of other users, than for some reason specifying -u without value works (at least on my system):
lpstat -W completed -u

Answer: This command appears to clear history as well as any active jobs:
sudo cancel -a -x


Why some issues are not listed (skipped) in online list of journals?

Question: I want to know where I can find the articles published in "Journal für die reine und angewandte Mathematik" Volume 2013: Issues 675-682? These issues seem to be missing.
Comment: @MaartenBuis: Yes, if you look at the website, this journal numbers its issues sequentially across volumes.
Comment: Contact the journal directly.
Comment: are you sure that these are issues? This looks more like page numbers.
Answer: This looks like a bug in the publisher's website.  I would suggest contacting them directly to report it.  There seems to be a similar problem with the issues from 2012 and 2011, though not other years.
If you have a subscription to MathSciNet, you can see a list of all issues of the journal, including those, and click for a list of articles in those issues.  Unfortunately the DOI links for those papers seem to be broken as well (example), which also supports the hypothesis that this is a website bug, as DOIs are supposed to be permanently accessible.
If you're at a university, you might see if your library has those volumes on paper.  If not, and you need a specific paper, they may be able to get a scan from another university that does have it, via the interlibrary loan system.  Talk to your friendly university librarian; they'll know how to help.


Are SOQL query logs visible in managed package?

Question: I see that the queries made by the managed package are visible in the subscriber org. Is that the default behavior or can I limit the access to the SOQL query logs?
Answer: The actual query being made is shown and the number of rows returned. I cannot find specific documentation stating this is expected or if all queries are visible but it has been my experience that they are shown and cannot be hidden.
To make matters worse if you are worried about debug logs, customers can log a case with salesforce to get the debug logs unhidden for a period of time without your consent so keep that in mind if you have any IP you are debugging.
Comment: Never leave system debugs in production code... Maybe if you have a kill switch it's fine...
Comment: @Eric, Adrian, thanks. Thats a bit concerning if you want to have IP protection for your code and have a bunch of logs tracing the behavior. How do ISVs debug their code in subscriber orgs. Is it advisable to have a protected custom settings? If yes, that would also mean that you have to upgrade the package to toggle the setting for enabling/disabling logs since protected settings are not accessible outside managed package namespace.
Comment: @Brian - You can see your code when logged in via the LMA. As for the custom setting, you would log in via the LMA as well. Even if set and if you are not logged in the debug would be hidden. The only other way would be to write records to the DB. Keep in mind that any errors would be hidden text regardless unless you are logged in via the LMA
Comment: I see. Can you update protected custom settings when logged in via the LMA?
Comment: @Brian - Yes.....


Does every ninja become a jonin?

Question: In Naruto, a number of ninjas have become jonin, such as Neji and Kakashi. Does every ninja who becomes a chunin by definition become a jonin if they live long enough, or do some remain a chunin forever?
Comment: @Nil I believe you meant Iruka not Iraku. There is, to my knowledge, no such character.
Comment: Iraku is still a Chuunin and likely to stay a Chuunin.
Answer: No not everyone will become a Jonin. As you may have noticed, some ninja like Naruto don't even become Chuunin. Naruto is a rare example however.
In order to become a jonin, a ninja must be extremely skilled. Jōnin are generally able to use at least two types of elemental chakra, some genjutsu, and above average taijutsu skills.
They are usually also appointed to A and/or S ranked missions alone while those under the Jonin rank usually go with a team, or not at all.
Comment: Was about to say something about the second line but the third kinda held it up..
Comment: Awww but you didn't mention the Eternal Genin!!!

And Naruto wasn't a real genin since Shippuden, it's just that no one bothered to go "*Poof* You're a chuunin now, here's your vest"
Comment: If they held a Chuunin exam for him, the opponents will all surrender before fighting him, unless its people like Konohamaru.
Comment: There are people who are joked at forever being genin (might be filler only though). In the War, there was the fight between the previous mizukage, where the ninja fighting him were incredibly stupid and unable to comprehend that he was an illusion. Their only non taijutsu attacks were throwing weapons, which they had to throw and regather several times before Gaara stepped in.
Answer: If you think about this from a real world military viewpoint, it is like asking if every recruit becomes a commanding officer. The answer is no. Also as related, keep in mind that not all academy students become Chūnin, so that might apply here too.
And as per the wiki

It is as of yet unknown what one has to undergo to become one. It is
  mentioned that jōnin are appointed, however there was mention of a
  Jōnin Exam in the anime's Kurama Clan Arc. When a ninja becomes a
  jōnin, they may be assigned a three-man genin team to supervise.

And then there is also this guy.. Kosuke Maruboshi who even though by choice, has been a genin for over 50 years. This could also apply to your question.
Answer: Some become Chunin, some become Jonin. No one knows how they become Jonin, but in the Anime both appointments and exams are mentioned as a way to become Jonin. Only power- and skillful ninja will become Jonin.
Answer: I know it's like a ranking. You have to undergo an exam or a test to be one ranking higher...
I don't know if this sequence is correct: genin, chuunin, jonin...
In the 1st episode of naruto shippuuden, it is known that naruto is the only one that's still a genin, others were chuunin and jonin.
There are other ninjas that maybe are still on the same level or ranking.

Does every ninja become a jonin?


Rigid body physics: Why objects are attracted without any force applied?

Question: I am running a very simple simulation. However, something weird happens at the beginning of the simulations. It looks like the chair is pulling the monitor towards itself. Or the monitor is attracted to the chair. Why is that?
Comment: You baked the animation. If you go in the Properties panel > Scene > Rigid Body Cache, you can click on Free Bake and it will reset the animation. It doesn't explain why your initial animation worked this way though.
Comment: Did you try with the monitor itself, without the chair, does it happen anyway?
Comment: Where is the Origin point for both objects? I have seen the same result when the origin/centre point is not at the logical centre of the mesh.
Comment: @moonboots Sorry I forgot to free the cache and then upload the `.blend` file. But this happens almost every time regardless.
Comment: I don't know a lot about Physics and bake, but it looks like your file is still a baked version, if you click on Free Bake you can manipulate your objects and they fall normally
Answer: I tried your file, I found  that just following what @moonboots said above in comments should work, but sometimes... it doesn't, and it's weird!
Here is when I tried to "free" rigid body cache, and also to "free all bakes": it didn't work.

Then I tried a trick: apply scale, and it worked, the "screen" mesh fell in a more natural way, but after reaching the floor, it fell downwards...

So, I tried to apply also rotation, and in this way it worked how you probably expected:

Applying transforms (particularly scale and rotation) is always recommended when you simulate physics in blender, and most of the times it solves even weird issues like this one.
Comment: It's very weird why this keeps happening. This makes the physics simulation very unreliable :(
Answer: Such phenomena have to do with gravity (it is ca. 9.81 m/s^2) and mass. Check if the screen is heavier or, for one of the monitor's legs, gravity is more than 9.81 m/s^2. If either is true, give all parts of the monitor the same mass and the same gravity and see what happens.
Comment: I didn't speculate, I checked, and I found only the scene setting... could you please add details, it could be interesting.
Comment: sorry, what you mean by a mesh individual gravity setting? I think there's only one setting for the gravity force, in the scene settings... and that's share by all scene's objects...


Is there something like wine to run OSX apps on linux?

Question: This question answers why Linux can't run OSX apps, but is there some application similar to Wine that allows one to do so?
Comment: BSD systems can run GNU/linux apps with linux-compatibility enabled in kernel and userland.  However never heard of GNU/linux system being able to run BSD apps.
Answer: The Darling Project was started a short while ago and, while it is far from ready, is probably what you are looking for.  It is a Wine-like compatibility layer for running OS X applications on Linux, and looks pretty promising.
Answer: I don't think so but maybe you could possibly run a Hackintosh inside a VM.
Answer: Since wine is a re-implementation of the Windows API - you're looking for a re-implementation of the Macintosh API or various "kits" that Apple provides to let OSX apps link to the system frameworks. I don't know of any that fit the bill. The only thing even close is the Chamelion Project which brings the UIKit from iOS to Mac OS X.
Since I don't have a real library for you, Lion is allowed to be virtualized on Mac hardware. Perhaps that would work for your needs while you wait for a lighter implementation like wine?
There are about a hundred hits on Google about "how to run lion in vmware" and all basically point to the check for a server plist file presence check that the installer wants to see before it will proceed. Here is one that's fairly clear on the steps.
Comment: I didn't mention it in my question, but I was looking for a way to run it on PC hardware. But still thanks for your answer. But no worries, the question is more out of curiosity than out of urgent need
Comment: It's of course technically possible to do - just the license appears to discourage/prevent it. I'll edit my answer with some recent links on virtualizing OS X.
Answer: There's not a complete one.
Bear in mind that Mac OS X started life as NeXTSTEP, of which OpenStep is a compatible implementation. Or rather, was at one time, so the story goes. Since then at Apple the NextStep frameworks have evolved into Cocoa. Cocoa has over 10 years of additions, extensions and improvements that don't exist in OpenStep.
From what I understand (and this is only hearsay and probably not verifiable outside of Apple) some of the applications included in the Developer Preview of OS X (pre 10.0) would have been able to run on Linux (or Solaris or Windows for that matter) with OpenStep if you were able to get your hands on the source code and compile it. Some of the OpenStep applications (e.g., File Manager/Finder, Mail, Dock, Preview, et al.) look and function strikingly similar to the versions shipping in OS X 10.0, but with the Aqua skin.


Potential downtime during MSI upgrade

Question: I'm considering deploying websites to an Win2k3/IIS6 machine using a MSI built with WiX.
The website in question has a collection of .html, .jpg, .js, .swf & .asp files (no ASP.NET)
I'm doing a "Major Upgrade" when installing the MSI (i.e, it first uninstalls the old MSI, then installs the new MSI).  This is the only way I can figure to get the MSI to remove redundant files.
My question is; will there be any downtime for visitors to the site during the uninstall / reinstall process?
Or does IIS cache the files being used by currently open connections?
Answer: No, you'll definitely be in an outage.  It does not cache that info.
Answer: If you're that concerned about downtime, you should be thinking about load balancing and redundant webservers.

Potential downtime during MSI upgrade


In DNS Tunneling, What's the DNS Response packet size?

Question: In DNS Tunneling server use RR (CNAME, TXT, NULL) to send back data to the client. My question is what's the size of response packet and how much data can be sent using DNS response in DNS Tunnel ?
Answer: The maximum message size for DNS over UDP is 512 bytes. See RFC 5966 -

whose sizes exceed the DNS protocol's original 512-byte limit.

Or RFC 791

this size allows a data block of 512 octets plus 64 header octets to fit in a datagram

With EDNS (RFC 2671) a marker can be added allowing 4096 bytes - although in practice this often won't be accepted by older equipment / allowed through firewalls.
Alternatively DNS can run over TCP (RFC 5966) which allows unbounded messages.
The size actually used in real world techniques will depend on the payload and the perceived risk of detection from the attackers.

how much data can be sent using DNS response in DNS Tunnel ?

One attempt has (Van Leijenhorst, Chin, Lowe - 2008) was shown that 110 KB/s / 150ms latency could be achieved. I'd imagine that rate comes with a fairly high risk of detection from any organization attempting to monitor for it.
Comment: How much data can hold TXT, CNAME, and NULL records in the RDATA field?
Comment: Nowadays you can even run DNS over TLS and soon over HTTPS
Comment: @Claudia, per RFC1035 section 3.3, "standard" RR, such as TXT and CNAME (I do not know what NULL records are), are constrained to be character strings of up to 256 characters each. This is why long SPF/DKIM data in TXT is split over multiple records.


Should I include hobby SysAdmin (System Administration) activities in my CV?

Question: When I was younger I used to develop various modifications for some games and also run servers for them, which I maintained and kept up to date etc. I am now trying to find employment as a System Administrator. My question is: should this be included in my current resume? If so, how would I back it up if asked about it since I have no one to point to for feedback from it.
Answer: Even if you have no one to point to feedback, yes I'd included it - but under "hobbies" or "volunteer activities" rather than "experience." That gives employers a little bit more context about what you've done and in what capacity. They also have a way to know that it wasn't corporate/OJT experience, but it is a set of skills that you have, at least for that specific environment.
You also say "when you were younger" - you may want to include a date range with that volunteer-experience, since if it was in the last two years that will be potentially more relevant than if it was over a decade ago.


Titration with EDTA and Eriochrome black T

Question: I have a titration of Calcium Ions with EDTA. When I reach a volume of 9.6mL I the liquid turns slightly blue and after that the more I add the bluer it becomes. Now at what point is my titration finished? When I notice the first change or as soon as adding more EDTA doesnt change the color?
Comment: Suggestion: Prepare a stem solution (i.e. of known $c(\ce{Ca^{2+}}$).  Perform the titration several times.  Once "as if you were unaware about $c(\ce{Ca^{2+}}$" till the point all Calcium is chelated (which may be, taking known concentration of your stem solution and its volume, calculated in advance).  Keep a small amount of this solution, for example in a test tube.  Repeat the titration, *intentionally* adding more EDTA solution, than necessary; equally keep some of this solution as visual reference in a test tube. *Now* perform a titration with your unknown, with the visual aides beside.
Answer: Assuming you've used Erichrome Black-T as an indicator, the second your whole solution changes from wine red to blue, as in the moment you realise there is a change in colour, however light it is, that's your end point and you should stop.
Make sure you use a white tile under your conical flask and remember to pause the titration and swirl the flask little bit every now and then as you near the end point. That should even out the colour and make the change noticeable before it's too late.
Comment: I see, thanks. Could you also explain why this is so?
As far as I know ErichromeBlack-T is pink as long as there are metall ions that it can bind to, so while there are pink and blue particles (first change) there whould still be some Calcium not bound to EDTA but to ErioT
Comment: I'm a bit unsure about what your question means but I'll try to answer.Erichrome Black-T is actually blue in colour in its protonated form but turns wine red on complexing. That is why it is imperative to swirl the the contents of the flask, to distribute the reagent evenly so that any colour change is uniform. It's only when the whole solution uniformly turns the lightest blue possible that you must stop. There should be no trace of pink of course, but just like it's unadvisable to have a deep phenolpthalein pink while titrating, it's the same here. Does this answer your question?