What would be the challenges of my new telescope idea?

Question: I have an idea of using a black hole as a new design of a telescope by placing lenses in orbit around it. Since light bends around a black hole, it should be possible for us to place lenses in a coil around the black hole which would allow for much more room for magnification. This cloud allow us to see much farther than we have ever seen before. However this is just an idea and I would love to hear any criticisms or improvements anyone might have.
Comment: I am not sure how you think the bending of light around black holes leads to higher magnification of the telescope.
Comment: I'm guessing you have never designed an optical system.  Lenses at that distance from Earth are essentially useless for producing images at or near Earth.
Comment: I'm not saying we do it from earth, but that sometime in the future we will be able to to to a black hole and do it
Comment: That's not what I mean. I'm saying the black hole IS the telescope. The lenses are in orbit AROUND the black hole. The challenges for telescopes on earth is that if we want to see farther, telescopes would have to be cumbersome. This is a solution to that.
Comment: Well, if the black hole is the telescope, what are the lenses for? Your optical design is less than clear.
Answer: Generally, the reason you don't want to stack lenses is because each lens will lose/absorb/reflect some fraction of the light and we don't have a perfect lens. You want as few optical elements as possible. That's why telescopes generally have one large mirror/lens, and a few necessary smaller mirrors for redirecting the light.
Secondly, having a really really large optical instrument is helpful because a larger area collects more light, and a larger aperture is necessary to counter the diffraction effect: see https://en.wikipedia.org/wiki/Airy_disk. There's no way to get around a big cumbersome telescope, and big telescopes have many benefits!
In practice, if we are able to travel light-years to reach a black hole, we have already mastered the art of placing large masses into space. At that stage, we would be able to place huge telescopes in space, and as many of such telescopes as we would like. It would be much cheaper to launch millions of telescopes into the solar system, rather than send one to a black hole. Moreover, we wouldn't want to deal with a light-year-long data transfer time, so having the telescopes be near the center of civilization would be better.
We can use masses as telescopes using gravitational lensing. This does not require us to place lenses near those masses, but rather simply use those masses as the giant wonderful lenses that they are.
See:
https://en.wikipedia.org/wiki/Gravitational_lens
https://en.wikipedia.org/wiki/Einstein_Cross
https://en.wikipedia.org/wiki/Einstein_ring
Comment: Thank you so much! I know this question seemed rather dumb but I randomly thought about it while at work and I wanted to know if it could've been feasible. Thank you for your response, people like you are the reason the world continues to have generation after generation of people who love science with a passion!
Answer: Any gravitating body could in principle be used as a lens and as a part of a telescope. See e.g. this discussion about using the Sun as a lens for astronomical observations and possible interstellar communication:
Eshleman, V. R. (1979). Gravitational lens of the sun: its potential for observations and communications over interstellar distances. Science, 205(4411), 1133-1135, doi:10.1126/science.205.4411.1133.

Abstract:

The gravitational field of the sun acts as a spherical lens to magnify the intensity of radiation from a distant source along a semi-infinite focal line. A spacecraft anywhere on that line in principle could observe, eavesdrop, and communicate over interstellar distances, using equipment comparable in size and power with what is now used for interplanetary distances. If one neglects coronal effects, the maximum magnification factor for coherent radiation is inversely proportional to the wavelength, being 100 million at 1 millimeter. The principal difficulties are that the nearest point on the focal half-line is about 550 times the sun-earth distance, separate spacecraft would be needed to work with each stellar system of interest, and the solar corona would severely limit the intensity of coherent radiation while also restricting operations to relatively short wavelengths.

Note the figure of 550 AU appears because the Sun's large size doesn't allow us to use closer focal points. This distance is the main obstacle, since it means that if we wish to observe a single target using the Sun as a gravitational lens telescope we have to send a probe to 550 AU distance (Note that Voyager 1, for example, has traveled about 153 AU in 43+ years). New target for  observation would require sending another probe to 550 AU but in a different direction. See e.g. this relatively recent paper for discussion of benefits (such as magnification and amplification/gain for such a telescope) and challenges (besides 550 AU distances) of such solar gravitational telescope (and for references to other works).
On the other hand, a hypothetical civilization that has the ability to travel interstellar distances and has located  a black hole without an accretion disk surrounding it could indeed use it as a telescope and surround  it with multiple receivers and transmitters (if they also wish to use gravitational lensing for communication) in principle achieving much greater magnifications and gains than "the solar gravitational telescope" by increasing the focal distances.  But, on the other hand if such a civilization could easily send multiple probes to light years distances, many other gravitating bodies would suffice for the gravitational lens, not just a black hole.


How is the number of engineers required to build facilities calculated?

Question: I've noticed that the number of engineers required to build subsequent facilities (e.g. a second satellite uplink) is larger than the number required for the first facility. What factors determine the number of engineers required?
Comment: this may be dependent on the building.  I've often been able to build a new workshop yet cannot build a new sat uplink, and I think sat nexus requires more engineers than uplinks.
Comment: Are you sure the cost didn't grew due to the fact that you built your 2nd nexus on a lower level than the first?
Comment: @ShivanDragon yes I am building at lower levels, but is that the only factor involved?
Comment: @jwaddell well, as you build deeper the prices and maintenance costs go up. I've not noticed a price going up for a building ONLY because you already have some instances if it, but I may be mistaken
Answer: The number of engineers required for facilities depends on the number of similar facilities that have been previously built. (It appears engineers in the new XCOM are capable of working on an infinite number of tasks simultaneously, provided the tasks are sufficiently different from one another.)
Six engineers are required to build the first workshop, fifteen for the second, and ten additional for each additional workshop, the number of scientists required per laboratory is similar.
For satellite uplinks, the formula is (10 * SU + 15 * SN), where SU is the number of satellite uplinks already built (or under construction), and SN is the corresponding number of satellite nexuses. That is, you need ten engineers per satellite uplink, with the first uplink (already built at the start of the game) being free.
For satellite nexuses, the formula is 5 + (10 * SU + 15 * SN).
Satellite Uplinks vs. Satellite Nexuses
If we ignore adjacency bonuses, as nexuses provide double the uplinks for half again the number of engineers, this means the engineer cost of satellite nexuses is lower per satellite.
Factoring in adjacency bonuses, the upper limit is four satellites per uplink or six per nexus: each facility can particpate in four adjacency bonuses, but each adjacency bonus is shared between two facilities, so we get maximum two bonuses per facility. This 4:6 uplink ratio equals the 10:15 ratio of required engineers, so an infinite plane of satellite uplinks would provide the same number of uplinks per engineer as an infinite plane of satellite nexuses.
Since we'll never come close to approximating an infinite spread of satellite uplinks, in practice, nexuses will always have a lower engineer cost.
However, since they require alien materials to construct, and since having more engineers lowers the quantity of both money and alien materials needed to build everything, if you're short on materials, it's better to aggressively push the number of engineers, and build regular uplinks, saving the alien materials for armor and weapons.
If, on the other hand, you have a surplus of alien materials, the lower engineer, power, and space requirements of satellite nexuses provide more uplinks faster, and cost fewer credits.
Answer: It's not related to location - it's fixed based upon how many of the same type of building you already have i.e. the first Workshop requires 6 engineers, the second 15 engineers, the third is 25 engineers and so on... (I can't remember the rest - but it is fixed).
edit: having played another game - I noticed it increments workshops by +10 engineers per workshop, so the 4th is 35 engineers, the 5th is 45 engineers and so on...
Comment: Does difficulty play into it?
Comment: No - I've played Normal, Classic and a bit of Impossible, and its always 6 scientist/engineers to unlock the first lab/workshop, and 15 to unlock the second. After that I'm not 100% sure - but it doesnt seem to change.
Comment: fyi it climbs fast, I've seen a requirement of 60 engineers to build a sat nexus.


After minting 10,000 NFTs, do you list them for sale one by one? If so, then how long does it take to finish listing all of them?

Question: After minting 10,000 NFTs, do you list them for sale one by one? If so, then how long does it take to finish listing all of them?
Comment: Hello! This is an opinion-based question (and stack exchange isn't for opinion-based questions), and it might be better on something like Reddit. There isn't a single canonical answer to this, and it'll likely change rapidly as the ecosystem evolves.
Answer: If Your NFT follows ERC721/ERC1155. You can find your NFTs directly on Opensea or Rarrible or any other NFT marketplace explorer. So, you dont have to list them.
How to search your NFTs on marketplace?

Just search your Smart Contract Address on opensea, and you will se your collection.

If you are using wallet address which deployed the Smart Contrct, you can edit few things about your collection.
Note - Your NFT Smart Contract must have inherited ERC721/ERC1155.
i.e. This is Smart Contract and this is Collection
Comment: I mean how long does it take to price all of those NFTs?
Answer: You list the collection for sale. The address of the collection remains the same what changes is the token ID.


CubeMX HAL Libraries state or stateless

Question: I wonder if the STM32F4 CubeMX peripheral HAL libraries saves the internal state of the peripherals OR if they are stateless? I am concerned about this because I want to know the possibility and safety of mixing the use of HAL libraries with direct register accesses at places where time-saving is more critical.
Comment: Both.  You have to look at the source on a case by case basis for particular operations.  Mostly it is the more complex things where there is software state.
Comment: Mixing HAL functions and direct register acces should only be done if you are realy sure about the peripheral state. Usually it is not a good idea. If you need that time critical access, throw away the HAL driver and write your own driver for all cases.
Comment: @A.R.C. actually the HAL is incomplete enough that bypassing it is often needed for routine tasks after using it for initial setup.  But there is no general answer.
Answer: Look at the Low Level library (LL) this provides a much lighter weight wrapper around the registers.
You can generate the LL code from Cube by selecting LL from the advanced settings tab in the Project Manager section.
You should see a list of peripherals and be able to select HAL or LL.  I have not had much joy in mixing HAL and LL so generally pick one or the other for every peripheral. As for the state held by each peripheral the best place to start looking is the reference manual Cheers.
Comment: @MatsK: I still don't see how his (useful) answer violates the rules.  I'm a fairly long-timer user and **I** don't understand how his answer violates any rules.  If I don't get it, how do you expect a newbie to get it?  Please explain in detail what you mean.
Comment: @MatsK: Your comment would be **MUCH** better if you explained how this answer violates the rules.  In particular, the emphasis on looking at the reference manual is relevant.

CubeMX HAL Libraries state or stateless


How do i look at my minecraft screenshots?

Question: I take screenshots of cool stuff on minecraft but i don't know how to look at them or access them can someone help me out?
Comment: All of the screenshots are stored in `.minecraft\screenshots`. You can easily get there by opening the Resource pack folder and traversing up to the parent directory to find the screenshots folder
Answer: On windows, all screenshots are saved to .minecraft/screenshots. To get to .minecraft click Win and R and a program called 'run' will pop up. Then type in %AppData% then go to roaming and you should find it. Also, clicking 'saved screenshot as 'filename'' in game will bring you to the directory. On mac, This Site says to go to the directory /Library/Application Support/minecraft/screenshots


How can one interaction influence the way another interaction generates mass?

Question: This question arose as a follow-up of this one and applies generally to all interactions and all ways to generate mass.
To make it clear, I take here the example of the neutron, whose mass is in most part generated by the strong interaction (see this post), and the neutron star, inside which neutrons have a large gravitational binding energy (see this post).
When the neutron binds gravitationally to the neutron star, it loses a significant fraction of its mass. However most of the mass comes from the confining field of the strong force. I would naively expect that the strong interaction would behave the same way regardless if we are at the surface of a neutron star or at the surface of the Earth. Inside the neutron star, the high pressure has an effect of putting the quarks closer together and decreasing the neutron mass, but this is a separate effect (we can see that if we consider the case where the neutron is just outside the surface of the neutron star as compared with a long distance away -- the closer the neutron, the lower the neutron mass solely due to the gravitational potential). So how can the gravitational interaction influence the way the strong interaction generates the mass?
Answer:
When the neutron binds gravitationally to the neutron star, it loses a significant fraction of its mass.

Not immediately. When things falls they don't gain or lose energy. What really happens is more space is created above them and this makes them look (to people far away) a bit similar to falling. But the people far away don't think they are more massive. They think the star still has mass $M=C^3/(GT^22\pi)$ where $C$ is the circumference of the orbit of the people far away and $T$ is the period of the orbit of the people far away.
To understand this, first note what the word general in general relativity means. It doesn't mean that you can have acceleration (you can do that in special relativity). It means you deal with the general case when there aren't global inertial frames. All the physics you know is about inertial frames and there aren't global frames in general relativity.
So the frames themselves fall if you will. And if you have a shell of neutrons then as the star collapses the local frames fall, this creates more space outside the star which to far away people looks like the shell falling, but doesn't look like any local change at all to people far away, the spacetime out there does not change in the slightest. And the mass of the star does not change because of the collapsing shell.
If you imagine a funnel with a flat piece of disk shaped cardboard on the center then the people outside see a curved a spacetime, and the size and shape of the funnel is the mass they see.
When something falls what happens is that piece of cardboard is replaced with a smaller disk of cardboard. The spacetime farther out didn't change, the circumference didn't change. But now if you travelled in towards the cardboard and across it and back to the other side you would travel a larger diameter. So the circumference didn't change even though the diameter increased. That's because the funnel is curved and more curved space was created. (In reality, spacetime is curved so the star starts to also age more slowly relative to people outside.)
So the primary thing you think gravity does, the falling. Does. Not. Actually. Happen. Instead, space is created above you, your time slows down compared to other people above you and some more effects. But you don't fall.
What are these other effects? Remember how the cardboard disk was replaced with a smaller disk? So all the neutrons on the edge of that disk (the outer shell of the star) are now denser. The density of the shell increases. This is a tidal force, and is real and the neutrons in the shell move closer together and get more densely packed. In a realistic material this can increase the pressure and the temperature.
And that is key. The neutrons heat up from the increased density as the shell collapses and they radiate away that heat. When they radiate away that heat, the star becomes less massive. It becomes less massive because energy actually travels up in the form of light from the radiation from the heat.
If it slowly gets smaller it is because the temperature below has pressure keeping it large and as the temperature leaks out to the outside the pressure goes down and the shell collapses thus increasing the pressure again. And it is the energy that gets radiated away that allow it to get stuck as a smaller more compact object.  The more and more that gets radiated away allows it to get smaller and smaller.

I would naively expect that the strong interaction would behave the same way regardless if we are at the surface of a neutron star or at the surface of the Earth.

Yes and no. It is fundamentally the same. Except all interactions for fermions are modified by a exchange interaction due to the superselection rule associated with the Pauli Exclusion Principle. This does act like more pressure when things are closer together.
And of course, more dense does also affect the pressure and temperature so whole of thermodynamics and the phase of matter and the equation of state all matter.

Inside the neutron star, the high pressure has an effect of putting the quarks closer together and decreasing the neutron mass, but this is a separate effect (we can see that if we consider the case where the neutron is just outside the surface of the neutron star as compared with a long distance away -- the closer the neutron, the lower the neutron mass solely due to the gravitational potential).

That isn't right. Pressure does have a gravitational effect (just like stress and momentum and energy). But increased pressure is like more mass. And your statement that the gravitational potential (not a real thing) lowers a neutron mass (it doesn't), is completely and totally wrong.
Firstly, the mass of a star isn't now and never was the sun of the masses of the parts and anyone that told you otherwise lied. Secondly as the shell collapses the absolutely only reason the mass of the star goes down at all, is because some energy is radiated upwards by light as this happens.
If some energy goes up in the form of light then the energy behind is less and that's why the star eventually seems less massive. Its not until that light gets to you that you see and feel a less massive star.

So how can the gravitational interaction influence the way the strong interaction generates the mass?

It doesn't. The mass of each neutron is the same. The mass of the star was never equal to the sum of the masses of the neutrons in the star. When the neutron star lets energy escape that allows the layers to contract, that contraction creates new space between where the outer layer used to be and where it is now. That new space and its creation is what the far away people think is falling. The people on the star see it getting more dense and they see some energy getting radiated away. The increased density is from a tidal force and the fact that the surface area of each layer of star gets smaller as it contracts. But each layer moves through the newly created space the layer below it made.
You destroy the space below you as you move into it and you create space above you and since you create move above you than you destroy below you it looks to far away people as if you are falling. But its just destruction a and creation of space and because of the geometry your density increases. The increase in the density is real. And the energy from this process of getting denser in the sense of having less surface area for your layer, that's real. And that energy can be given away. And if you aren't collapsing at the free fall rate then there is additional pressure from the layer below you doing work as you fail to move inertially and that can also be radiated away.
Your decreased mass of the star is 100% entirely due to the energy radiated away.
Comment: relevant http://physics.stackexchange.com/questions/149744/does-the-mass-of-an-electron-change-with-its-energy-state


iPhone 4s turns itself off

Question: It started today and happened twice, my iPhone turns itself off even battery has 15%-17% charge, I can't open it, empty battery sign appears when I plug it in to charge, it begins to charge from 15%-17%, I couldn't find the reason, it has the latest iOS update, what could it be?
Answer: The clue is that it begins to charge at 15% or so.. you have some bad cells in the battery, so they are not considered for charging, but are part of the 100% the battery should have.
Take it in to Apple and have them look at it!
Comment: thanks @dave, so it is hardware problem, this isn't good because i got my iPhone from out of my country and there is no Apple store in my country, does this bad cell increase?
Answer: From what it sounds your battery is damaged, you should take your iPhone to a Apple Authorized Service center for further investigation.
Did this behaviour occur when you were outside, in cold? My 6S has some problems if I use it lightly in the cold, it shuts down when it's under 10% but I'm able to turn it back on if I force it two times.
You could try to charge it at 100% then listen to music, play some games, browse on mobile data, etc. and see if it does the same thing or it just goes to 1-2% (charge it, don't let it go 0% !).

iPhone 4s turns itself off


flip-flops vs logic gates

Question: When implementing something build with flip-flops and logic gates, why do we aim to minimize the number of flip-flops even if it produces more gates? Does it have to do with minimizing the delay, the energy or something else? Could someone explain this?
Comment: Are you talking about an FPGA?
Comment: Yes, but it is a bit of a theoretical question as I saw it stated in some article online but couldn't figure out the reasoning behind it.
Comment: Can you link to that article?
Comment: I'll try to find it again and I'll post it here if I do.
Answer: It boils down to efficient use of FPGA resources.
There aren't actually gates in a FPGA (in the usual sense of the term). There are, however, a lot of LUTs (Look-Up-Tables), multiplexers, adders, and flip-flops.
Here is a typical FPGA slice:

Now, you have to understand that, using very few LUTs, you can build rather complex combinatory logic. A single LUT can achieve the same result as a combination of multiple AND/OR/NOT gates.
On the other hand, a flip-flop is a flip-flop. There is no trick to lower the number of required elements, here.
So, usually, complexifying the combinatorial logic (the "gates") is not much of a problem (and, in some situations, it may not even lead to an increase of resources used). However, if you use more flip-flops, you will necessarily use more FPGA resources.
Also, the syntesis tools have much more ways to optimize combinatorial logic than optimizing the flip-flop usage. If you did your design right (e.g. there is no flip-flops that are duplicated or whose output isn't actually used), the tools can't do much to reduce flip-flop usage.
Answer: Modern FPGAs do not contain "gates" as such; the logic is implemented by LUTs (lookup tables) or MUXes (multiplexers). As such, the number of "gates" is not really the limitation in terms of either resources or delay.
With a LUT, the only key parameter is the number of inputs required to produce a given output function; the function itself can be completely arbitrary. It could be the equivalent of a single gate or dozens of gates, with no change in the "cost" of using it.
Therefore, the synthesis software for FPGAs only needs to pay attention to the number of FFs used and the number of LUTs used. Usually, there is one of each in the lowest-level "logic cell" in the FPGA's hierarchy.

flip-flops vs logic gates


Packages for API level

Question:
Hi, I'm new to Android development studio. My question is, if I am not using Android Emulator then do I need to install only these checked packages for every API level ?
Answer: Yes, that is the actual SDK, which is the source code, so definitely install it if you are targetting that API level. About Google API, depends on the use case. If you are going to talk to the internet or use Play Services you will need them for sure
Comment: Ok. It means Android SDK platform and sources for Android SDK of API levels which I want to use. What about Google API? Do I need to install that also ?

Packages for API level


IEEE 802.11 RTS/CTS Collision

Question: folks! I have the following question: in 802.11 protocol with rts/cts a collision occurs when two host try to send something to other hosts at same time (also an RTS)? in other words I did not understand when a collision occurs using this mechanism...
Answer: A small analogy for RTS/CTS: Consider a class with several students. If all students ask question at the same time, professor won't be able to hear anything but a noise (analogy to collision). But if a student wants to ask a question, he raises the hand (analogy to RTS) and professor says, proceed with the question (analogy to CTS).
A station(STA1) which wants send a data to STA2. It first would reserve the medium. STA 1 sends an RTS frame with a duration field in it. Duration indicates for how much time STA wants to reserve the medium
STA2 sends a CTS frame and other stations update their NAV (Network Allocation Vector), so that no other station will transmit any packet for the duration reserved by STA1.

IEEE 802.11 RTS/CTS Collision


Stored procedure with params in excel without Ado.net

Question: I have a stored procedure with params on sql server 2000 and i am trying to run it from excel 2003.
I did setup the DSN for the db and now when i am in MS Query form Excel, i tried to execute the SP using the syntax
{CALL mystoredProc (?,?,?)}
After i say ok it does ask me for the parameters after that the result set that i am getting has all the column names but no data.
The result set should have atleast 200 rows because i do get them if i run it using the QueryAnalyzer.
Please advice me i am a newbie and apologize if this is a silly question..
Thanks
Answer: Why not use ADO?
With ado you can use ADODB.Command, And pass parameters to object using append method.
Other way to your proc don´t work is if your parameter is date and a format is wrong.
if is a date try pass a date with format
<code>format(date(),"yyyy-mm-dd")
</code>
In Sql is a default format of dates.
[]´s


Dealing with missing data in SVD

Question: I am a newbie to machine learning and I am trying to apply the SVD on the movielens dataset for movie recommendation. I have a movie-user matrix where the row is the user id, the column is the movie id and the value is the rating.
Now, I would like to perform normalization on the movie-user matrix (subtract the data by users ratings mean). Then pass the normalized matrix to Scipy.sparse svds as follow:
<code>from scipy.sparse.linalg import svds
U, sigma, Vt = svds(R_demeaned, k = 50)
</code>
Now, I have the 2 method to do it:
Method 1.) Fill all the missing rating with 0 first, then calculate the user rating mean for normalization.
The predicted rating dataframe for method 1 by using svd is:

Method 2.) Calculate the user rating mean first and do the normalization, then replace the missing rating with 0.
The predicted rating dataframe for method 2 by using svd is:

I would like to know which method is better or there are other methods to do it. So far as I can observe from method 2, the predicted ratings for a user are quite similar. For example, user A may get 4.XX ratings for all movies. Meanwhile in method 1, there are more variation. I would like to know if there are something wrong.
Answer: I would recommend trying both methods to see which works better.
However, in my opinion, you should fill first and then normalise it. Imagine if you normalise it first, you could end up with some values very close to 0. So when you fill in the missing values with 0, you are inherently saying that these missing values had the same initial value as those that were normalised to 0, which would be incorrect.

Dealing with missing data in SVD


Why are all Rails helpers available to all views, all the time? Is there a way to disable this?

Question: Why can I access helper methods for one controller in the views for a different controller? Is there a way to disable this without hacking/patching Rails?
Comment: There is a new, more relevant answer. Check out Craig Walkers answer
Answer: @George Schreiber's method doesn't work as of Rails 3.1; the code has changed significantly.
However, there's now an even better way to disable this feature in Rails 3.1 (and hopefully later). In your config/application.rb, add this line:
config.action_controller.include_all_helpers = false

This will prevent ApplicationController from loading all of the helpers.
(For anyone who is interested, here's the pull request where the feature was created.)
Comment: I think this should be the default behavior.
Comment: I'm on 3.2. Does application.config here mean config/application.rb?
Comment: Will the ApplicationHelper get included as well if I disable include_all_helpers or I need to include it manually?
Comment: @AzizLight I'm not sure, since I haven't tested, that. However, I'd expect so, as ApplicationHelper is intended to be always be available in views as the "root" helper. It's the helpers from unrelated controllers that are of primary issue here.
Answer: The answer depends on the Rails version.
Rails >= 3.1
Change the include_all_helpers config to false in any environment where you want to apply the configuration. If you want the config to apply to all environments, change it in application.rb.
config.action_controller.include_all_helpers = false

When false, it will skip the inclusion.
Rails < 3.1
Delete the following line from ApplicationController
helper :all

In this way each controller will load its own helpers.
Comment: NOTE: The above comments are no longer valid, now that the answer has been updated to include Rails > 3.1
Comment: Note: Your ApplicationHelper will still be loaded. If you'd like to include any specific helpers all of the time, you can add `include HelperName` to the top of your ApplicationHelper.
Comment: This line doesn't exist for me in Rails 3.2, so this solution likely no longer applies to later versions of Rails.
Comment: I'd be interested in why this changed so significantly from Rails up version 3.1? Doesn't make much sense to me.
Comment: works for Rails 4.1, still needed. Why isn't this default ... it's better to include only particular Helpers where needed (as @vansan said)
Answer: In Rails 3, actioncontroller/base.rb (around line 224):
def self.inherited(klass)
  super
  klass.helper :all if klass.superclass == ActionController::Base
end

So yes, if you derive your class from ActionController::Base, all helpers will be included.
To come around this, call clear_helpers (AbstractClass::Helpers; included in ActionController::Base) at the beginning of your controller's code. Source code comment for clear_helpers:
# Clears up all existing helpers in this class, only keeping the helper
# with the same name as this class.

E.g.:
class ApplicationController < ActionController::Base
  clear_helpers
  ...
end

Comment: By the time you call `clear_helpers` the (performance) damage has already been done since all of the helpers have been loaded.  You really want to use `config.action_controller.include_all_helpers = false` (as @Craig Walker states, above), as it prevents the helpers from being loaded in the first place, thus gaining you some performance (perhaps most significantly in development mode).

Thanks for including the `actioncontroller/base.rb` snippet; it's always nice to see code and remove some of the mystery.
Comment: `clear_helpers` is still useful for rails 3.0 - `config.action_controller.include_all_helpers = false` is only rails 3.1 and above.
Answer: Actually in Rails 2, the default functionality of ActionController::Base was to include all helpers.
Changeset 6222 on 02/24/07 20:33:47 (3 years ago) by dhh:
Make it a default assumption that you want all helpers, all the time (yeah, yeah)
change:
class ApplicationController < ActionController::Base
  helper :all # include all helpers, all the time
end

As of Rails 3 beta 1, that is no longer the case as noted in the CHANGELOG:

Added that ActionController::Base now does helper :all instead of relying on the default ApplicationController in Rails to do it [DHH]
Comment: so does that mean in Rails 3, all helpers are by default all the time loaded?
Comment: @Nik surely yes, atleast for 3.2


What is the typical value for thickness of via plating?

Question: I use PCB tools and sometimes they ask for via plating thickness, what is a value that I can approximate most via plating thickness? What value would you use and why? If I wanted to bound the thickness (min,max) what values could I use?
Answer: Standard Spec for Beta-layout (https://us.beta-layout.com/pcb/technology/specifications/  Material construction tab) is 16-23 \$\mu \mathrm{m}\$ for copper in barrel.  I suppose you can ask for whatever you need, but this is their standard board.
Answer: Oshpark's standard 2- and 4-layer boards have 25.4um (1mil) via plating
Via plating thickness will affect electrical and thermal resistance of that via, which may be important depending on your application. Here is a handy calculator for determining via electrical and thermal resistance as a function of hole dia, plating thickness, and via length: http://circuitcalculator.com/wordpress/2006/03/12/pcb-via-calculator
I would check with your board house or CM to determine which standard plating thicknesses they offer, then use the calculator to pick from amongst those.
Comment: I'm well aware of what the values  are used for, what I'm wondering is how thick is the via plating and the tolerances. I have a calculator. Please make sure you read the question before answering. In other words, I would like numbers
Comment: I was hoping to address your "why" in "What value would you use and why?". You can put in different thickness values into the calculator to see which thicknesses meet your design requirements. I would think the particular value you want would be the intersection of what's offered by your board house and what meets your design constraints electrically and thermally. Am I missing something?
Comment: Yeah, I use a lot of different board houses I looked and I can't find the thickness for via plating, There are two questions about values of thickness, that is what I'm interested in
Comment: Oshpark's standard 2- and 4-layer boards have 25.4um (1mil) via plating: http://docs.oshpark.com/submitting-orders/drill-specs/


What is the difference in a Beastmaster's animals?

Question: A Beastmaster (homebrew class) has both an animal familiar and a companion. My question is, what is the difference? I've looked them over and can't find any.
Comment: Just looking through the list of features in the table of contents, I can see a number of differences between the two. What exactly is the part that confuses you? Also, keep in mind that dand wiki has a lot of homebrew material. The class that you are referencing looks to be unofficial homebrew content, not an official expansion of the ranger class.
Comment: Are you asking to compare this to what's in the book, or specifically the variations/variants listed at that site?  In asking this question, what problem are you trying to solve.  Does this influence your choice of PC, your choice of Ranger sub class, or something else related to the game you are playing?
Answer: The most notable difference between animal familiars and companions is that the familiar can not directly attack. See the evolution section of the link you provided and note:

Unnatural Selection
At level 5, the familiar may take evolution points from the companion's evolution to use on themselves. The familiar can only take evolutions that it meets the prerequisites for. Remember, the familiar CANNOT attack.

Also note the hit dice calculations. Your familiar has half your hit points rounded down, and your companion uses whatever hit dice are listed in their monster manual entry.


Convergence of empirical measures in Wasserstein distance

Question: Let $X_1, X_2, \ldots$ be iid random variables with common distribution $\gamma$, the standard Gaussian distribution on $\mathbb {R}$, and let
$\mu_n = \frac 1n \sum_{i=1}^n \delta_{X_i}$, $n \geq 1$, be the empirical measures based on the sample $X_1, X_2, \ldots$. Is there anything known on the rate of convergence of $E (W_p (\mu_n, \gamma))$ as $n \to \infty$, where $W_p$ is the Wassertein distance with exponent $p \geq 1$? Same question if the $X_i$'s are standard
Gaussian in $\mathbb{R}^k$.
Answer: A complete literature can be found in the article of Fournier and Guillin https://link.springer.com/article/10.1007/s00440-014-0583-7
Comment: I was interested in the exact rates in this Gaussian example. Does this reference address this issue?
Comment: The rate in the one-dimensional case is different from the one holding in higher dimensions. For $\mathbb{R}$, I suggest to have a look at the book by Bobkov and Ledoux: One-dimensional empirical measures, order statistics, and Kantorovich transport distances.
Comment: Thank you, but there is nothing in higher dimension in the Bobkov-Ledoux Memoir?
Comment: Unfortunately, no. The reason is simple, the situation in one dimension is relatively different from higher dimensional case. In one dimension a Bahadur representation type of theorem can be used whereas no such thing exists (yet) for $d \geq 2$. Bobkov and Ledoux decided to treat only the 1D case in their book.
Comment: There is recent progress on the exact rates for multidimensional Gaussians: https://arxiv.org/abs/1911.07579


Detect if a tab was POSTed to from Google Chrome extension

Question: Is there a way to detect if a particular tab was POSTed to via a Google Chrome extension? I am working on an extension that includes an "undo closed tab" type of function and would like to handle tabs which were POSTed to differently than GET pages.
Answer: I believe a solution exists in chrome.webRequest.onBeforeRequest via the 'method' return parameter. I will report back with a working code example once I have time to try this out.


Sales Cloud EE license

Question: We have a few users with Sales Cloud EE users. We are looking to potentially switch a few of the users to chatter users. Can we swap that user type and go from Sales Cloud EE --> Chatter plus or chatter free or would we need to deactivate the sales cloud user and then create a new profile?
Answer: You can't "downgrade" users, so you will need to create new users for these users.

Sales Cloud EE license


Update dBase III DBF file which has 1100 columns in c#

Question: How to Update dBase III DBF file which has 1100 columns in c# ? I tried "Microsoft.Jet.Oledb.4.0" and "vfpoledb" providers but they are not able to update. I read that there a  255 columns limit with oledb. But even when I try to update a single column, an exception "Query too complex" is thrown. I am able to read data from this DBF though, if I limit the columns in select query. Is there any way, I can update this DBF in c# ?
Comment: I would expect `System.Data.Odbc` to be more native to formats of that era. But still, it's an old format.
Answer: Take a look at my answer here regarding SyBase Advantage Local Server.  It allows reading VFP (dBASE) style .DBF files but also allows for files larger than the 2 gig limit and 255 columns.  It has its own "Provider" you can download and run with.  It's VERY similar with the OleDb providers (as they are all based on interface objects such as ICommand, IConnection, IDataAdapter, IParameter, etc)


Making the webform submission results as a link in Drupal 6?

Question: I am a newbie in using Drupal 6 and I am still walking in the dark in terms of module familiarization and module configurations. Lately, I installed the Webform module for my site to be able to let users submit a request. In my Webform, there is a Select View field where it lists all contents of a specific Content Type on my Drupal site. I was able to use a View as a datasource for my normal webform select component via the Webform View Reference Component that's whay I have a custom <select> field which are views on my Webform. Now, after a successful submission of the form, I go to Results and found the data which are saved upon submission. What I want is that, on my Results page, the selected option from the list, which basically are views, will be a link to their respective views. This is how it looks like,
I am really lost right here. Can someone lend me a hand on how to make this possible? Cheers.
Answer: Install this module http://drupal.org/project/webform_report it will be displayed in table format and also you can customize the report & also It will create separate content type.
Below is the screen shot for webform report, you can also view, edit & delete from report itself.


Has a human ever perished in space?

Question: Apollo 13 returned safely. The Challenger was leaving when it exploded. The Columbia was coming back when it burned up, as was that Russian guy who was profiled on National Public Radio (NPR) and that recent book. Has any human ever died in space?
Comment: You need to better-define your question.  Technically speaking, "Earth's pull" lasts to the non-existant edge of the universe.  Do you mean, say, the political definition of "space" which is 60+ miles?  Or in Earth orbit?  Or something else?
Comment: @SB01: what is "that recent book"?
Comment: @Peter Mortensen: I believe the article/book he's referring to is [this](http://www.npr.org/blogs/krulwich/2011/05/02/134597833/cosmonaut-crashed-into-earth-crying-in-rage).
Answer: There have only been 3 recorded deaths that occurred in space (that is, greater than 60 miles above the Earth). The crew of the Russian capsule Soyuz 11, died when their capsule depressurised during preparations for re-entry. It wasn't known they had died until the re-entry capsule was opened on Earth as communications had been lost with the capsule during re-entry. All three crew members died as a result of a loss of pressure in the capsule. These are the only recorded fatalities to have occurred outside of the Earths atmosphere and in space. You can read more about the series of events that lead to the accident here. There have been no deaths outside of the Earths gravitational pull (if this is what you are inferring) though since no humans have ever left it and as Stuart says, it technically extends to the edge of the universe.
Comment: The only humans that could be said to have "left Earth's pull" in *any sense at all* are those that have traveled close enough to the Moon that that body's gravity dominated over the Earth's gravity, but seeing as how the Earth's gravity holds the Moon itself in orbit, that's not much of an "escape".
Comment: I read the question as referring to a kind of SciFi scenario, where a human is sucked into vacuum due to a hull breach, etc.

Has a human ever perished in space?


maximum file size capable of fitting onto a 700MB ISO CD-RW

Question: I'm making a TrueCrypt volume and want it to fit the entire available space on a 700MB, ISO formatted, CD-RW disc without overburning.
Comment: That's fine for that particular batch of discs. Different ones may have different limits.
Comment: Owing to the nature of the medium, there is no exact maximum size. The same goes for any physical storage medium (including hard drives). So going for *at least* 700MB in the manufacturing process is probably how they do it. That's where I'd put my flag in the ground anyway.
Comment: I finally done it. The answer is **719320 kB** that is rough 702.4 MB. Adding 1 kB more causes the recorder to go in overburn mode which is dangerous with CD-RW. (I've deleted my irrelevant comments of my rough estimation, this is exact value).
Answer: You need to check how many free blocks are there on particular media. Then you make ISO of respective size.
<code>cdrecord dev=/dev/sr0 -atip
...
ATIP info from disk:
...
  ATIP start of lead in:  -11077 (97:34/23)
  ATIP start of lead out: 359849 (79:59/74)
...
Manufacturer: Mitsubishi Chemical Corporation
</code>
My CD-RW has 359849 2k-sized blocks totalling 736,970,752 of data
32k+32k+3x2k will be used for ISO9660 service information. If you add joliet rockridge and hfs naming extensions it will be even more space wasted.
the "standard" limitation is for 640MB disks with many older drives not reading bigger disks, for 700MB it is more flexible.
In theory there is a lead-out are of size of lead-in after the end of programmable area which can be written with data fully and safely, but CD readers will be more likely to reject drive which does not have proper "session end" and lead-out marks.
Answer: The ISO specification imposes a limit of 2GB for a single file.
So for a CDROM the maximum size will be the size of the CD - 700MB (less a little for lead-in, lead-out and file tables)
For bigger file sizes the UDF system is preferred.
Comment: @user43300 i remember when i would back up DVDs to CD i used to be able to fit around 706 MB on a disc (no overburning). i never checked the actual size of the files on the disc in KB though.
Comment: was it cd-r or cd-rw ?


Do US banks lend your money to foreign banks during the weekend while you cannot make withdrawals?

Question: I was told by an economics teacher once that banks take advantage of the weekend being closed to deposit on hand cash in foreign banks, specifically Mexican and South American banks. The lending bank would then receive high interest rate for that short period of time but their funds were unsecured. The primary reason they do this was that banks could make a higher yield than what they pay out themselves. The teacher made it sound like this is a common practice but I'm having trouble finding information about it.
Comment: I've never heard of such a thing, but it doesn't make a lot of sense to me. Most banks make money on the cash deposits by lending it out to other customers in the form of car and house loans where they can get a pretty decent rate of return with not all that much risk.
Comment: Seriously doubt that they lend *cash* to anyone, particularly banks outside the country.  Just the logistics of picking it up after close on Friday (payday for many people), crossing the border with it in both directions, then returning it before open on Monday morning would be infeasible.
There could be ways for banks to make interest while they're closed, possibly sending money electronically to follow the sun, but they definitely need it back by morning.
Comment: If this is an actual practice, I don't see how it would work.  If most banks only operate during business days, what's the use?  (If a tree falls in the woods, but no one is there to hear it, does it make a sound?)  That is what purpose would it serve to the borrowing institution?  How would this be different from the [overnight market](http://en.wikipedia.org/wiki/Overnight_market)?
Comment: I'm not sure about commercial banks but the FED lends money to foreign banks all the time....to the tune of trillions of dollars: http://edfriendly.blogspot.com/2011/04/federal-reserve-discount-window.html
Comment: @muro I believe the crux of the question is the potentially higher risk lending of the money while it is "locked up."  I actually came across the [discount window](http://en.wikipedia.org/wiki/Discount_window) article while looking for something on short-term, inter-bank lending (overnight market).
Answer: Most of your money doesn't exist as physical cash, but simply as numbers in a ledger. At any given time, banks expect their clients to withdraw a certain percentage of their balances... For instance, checking accounts are frequently drawn down to zero, savings accounts might be emptied once our twice a year, CDs are almost never withdrawn, etc.
To cover those withdrawals, banks keep a certain amount of physical cash on hand, and an additional amount remains on the ledgers. The rest gets loaned out to their customers for use in buying homes, cars, credit cards,etc. Anything they can't loan out directly gets deposited with the federal reserve or loaned directly to other institutions who need it. However, those last two options tend to be short term (ie overnight) loans.
With debit cards functioning 24/7, you could get cash at an atm or make a purchase anytime of the day our night. The weekend has nothing to do with it.
Which is a long way of saying "No, they do it all the time, not just on weekends" ;)
Comment: [Fractional Reserve Banking](http://en.wikipedia.org/wiki/Fractional-reserve_banking) for more details.
Answer: You might be thinking of the Overnight market/Overnight Rate. This lending happens constantly.
Comment: Yes.  Foreign banks are just closed on the weekends as US banks, but (especially in Asia) it's daytime during the US night.


What is the purpose of beating aravot on Hoshanna Rabba?

Question: I'm not asking about the origin of this minhag. I'm inquiring of the purpose of the actual beating of the aravot on Hoshanna Rabba - i.e., the action, itself. What does the hitting of the aravot represent or do and what is its connection to Hoshanna Rabba specifically?
Comment: If you look in the statement of intent in any siddur for beating the hoshanos it will invariably state that the purpose is to be *mematek et ha'gevurot* - to "sweeten" the harsh attribute of judgement. Specifically we beat the hoshanos on the ground 5 times to sweeten the 5 attributes of judgement. Exactly what these are and how it does that, I have no idea.
Comment: @TheGRAPKE the gemara only indicates you do it twice. There must be a non kabbalistic reason for this practice
Comment: Note some Rishonim hold you are supposed to wave the willows not beat them
Comment: @DoubleAA Agree with what you are saying. So I guess it is a question of symbolism.
Answer: According to Rabbi Mansour, the purpose of beating the aravot against the ground is to beat the lips of the Satan such that he will be unable to testify against us in the coming year. The connection to Hoshanah Rabah is that on that day our final judgement is sealed. As he explains   "The reason for this custom relates to the comment of the Midrash that the leaves of the Araba symbolize the lips. Specifically, the Arabot represent the lips of the Satan, and we bang them on the ground in order to silence the Satan so he does not prosecute against us. Hoshana Rabba is the day when our final judgment is sealed, and we therefore seek to silence the Satan in an effort to guarantee a favorable judgment. Furthermore, the Arizal (Rav Yishak Luria of Safed, 1534-1572) instructed that one should have in mind while banging the Arabot that the five beatings should correspond to the five letters in the Hebrew alphabet that appear only at the end of words (the final Kaf, the final Mem, the final Nun, the final Feh, and the final Sadi). These letters signify the Geburot – the harsh judgments in the world – and we bang the Arabot on the ground to express our desire to eliminate these judgments. The custom of the Arabot also commemorates the practice that was observed in the Bet Hamikdash to walk around the altar with an Araba on each day of Succot, and seven times on Hoshana Rabba. We commemorate this ritual by using Arabot on the day of Hoshana Rabba."
Answer: Rav Shimshon Raphael Hirsch writes in Chorev here (second paragraph) that after we have encircled the Bimah seven times we beat the Aravos as a means to remove any sense of Ga'avah (haughtiness / arrogance) from our hearts and the belief that our successes come from us alone. He says that the Aravah is symbolic of the 'chomer' lit. the material i.e. it is something that we receive from nature - it grows from the tree, wild and untreated, much like our natural abilities and talents. And with our human deficiencies, we wrongly assume that these talents are solely due to us. Therefore we specifically beat the Aravos to remind ourselves that everything is G-d given and beat out the natural arrogance that might express itself.


Is the modded Skyrim play time inclusive of unmodded Skyrim play time?

Question: I've been playing Skyrim SE recently and I wanted to see my total modded and unmodded playtime. So I'm trying to see the time played when loading a character. It says I've played 58 hours on the unmodded save and 106 hours on the modded save.
So does the 106 hours on the modded save include the 58 on the unmodded save? I personally think it doesn't because it also shows 6 mins as a prisoner at the start of the game separately.
Comment: Sorry, but how can we possibly know that? First of all: where did you get those playtimes? Are they of the same or of different characters? And I think you already have your answer, because the full intro as 'Prisoner' [takes less than 6 minutes](https://www.youtube.com/watch?v=8RnyEnUZIqg).
Comment: @Joachim: You're still "Prisoner" while you have the character creation menu open, since you don't pick a new name until you finalize your appearance. I'm not sure if it counts that time. It arguably *shouldn't* because the game is paused, but I have never paid much attention to the statistics menu so I have no idea whether Bethesda thought of that.
Comment: @Kevin That's a very good point! I think it does count as playing time. Playing time usually still counts when the game is paused, because it usually counts the time the time a program is running (here with the added condition that you're in-game as a character, not just in the main menu, but that can be verified through Steam, if playing through that).
Comment: @Joachim When you open the game, click on load and you'll see your saves. Click on a save and you'll see some number in this format : 109.11.15. 109 is the number of hours, 11 the number of minutes and 15 the number of seconds of played as that character.
Comment: @Zohair Thanks, I know, it just wasn't clear from your post :) If you had used a different tool, that would have been important.
Answer: If they're the same character: yes. Otherwise, no.
The same rules still applies for even if you multiverse time travel, save scum, etc.
Unless you have a mod that modifies your playtime stat, just having mods alone will not affect the functions that govern the playtime counter...
As for Steam's counter: That's a seperate thing altogether.
Comment: Yes they are the same character.  and thanks!


Question related to Cauchy Schwarz Inequality

Question: If $x$ and $y$ are two vectors such that $\|y\|_2=1$ and $x$ is in the non-negative orthant then how to show that the condition $$x^Ty\leq1$$ leads to the condition that $$\|x\|_2\leq1.$$ I know that Cauchy Schwarz Inequality means that $$\|x^Ty\|_2\leq \|x\|_2\|y\|_2$$ but I still do not understand how in this case $$x^Ty=\|x^Ty\|_2.$$ Any help in this regard will be much appreciated. Thanks in advance.
Comment: Any vector $x$ orthogonal to $y$ trivially satisfies $x^\top y = 0 \leq 1$ but $\| x \|$ can be as large as you want. However, if you want $x^\top y \leq 1$ for **all** $y$ such that $\| y \| = 1$, it is straightforward.
Comment: Take $y = \frac{x}{\| x \|}$ and see what $x^\top y \leq 1$ gives you in that case.
Comment: @VHarisop yes I want it for all $y$. How to show that? What straight forward thing I am missing here?
Comment: @VHarisop why consider only $y=\frac{x}{\|x\|_2}$ why not any other $y=\frac{p}{\|p\|_2}$?
Comment: Because this is the "most pathological" example you can find in the unit sphere. After you've solved this example, all the others follow from Cauchy-Schwarz.
Answer: It is not true. For example, in $\mathbb R^2$ take $x=(2,0)$, $y=(0,1)$. Then
$$
x^Ty=0,\ \ \|x\|_2=2.
$$
Answer: If you need $x^\top y \leq 1$ to hold for all $y$ such that $\| y \| = 1$, try the following: take $y = \frac{x}{\| x \|}$. In that case
$$
x^\top y = \frac{x^\top x}{\| x \|} = \frac{\|x\|^2}{\|x\|} = \| x \|
$$
So we know that $x^\top y \leq 1, \; \forall y : \|y\| =1 \Rightarrow \| x \| \leq 1$
Now, check that any $x$ that satisfies $\|x \| \leq 1$ satisfies $x^\top y \leq 1$: using the Cauchy-Schwarz inequality, you will get
$
x^\top y \leq \| x \| \| y \| \leq 1 \Rightarrow x^\top y \leq 1
$.
Comment: @FrankMoses: Yes, this is essentially what you are doing by considering $y$ collinear to $x$.
Comment: Is it right to say that (I think this was what you meant from pathalogical case) "we should take $y=\frac{x}{\|x\|}$ because for any other $y$ of the form $y=\frac{p}{\|p\|}$ will always produce a smaller value than for $y=\frac{x}{\|x\|}$ case due to Cauchy Schwarz Inequality, which becomes equality only when $y$ is a linear combination of $x$"?


What are the differences between an AC and DC coil relay?

Question: If a DC coil was just a coil, it would saturate. If an AC coil was just a coil, the magnetic field would drop to zero at 120 Hz and the relay would chatter. Presumably the designs in some way prevent these problems. How? And does it prevent me from putting DC on an AC-rated coil?
Comment: why would the field collapse at 120Hz
Comment: @JonRB Because the AC voltage passes through zero twice per 60 Hz cycle (assuming 60 Hz line frequency). No voltage, no current, no magnetic field. Or so I understand. If that's not accurate, I'd love to know about that!
Comment: sorry, I misread part of the question (ie it works at 60hz but not at 120). Yes part of the field collapses, there is usually another coil that produces a field out of phase of the main excitation field. This contributes to the retaining force and thus allows the relay to ride through zero crossings of the AC http://www.britannica.com/technology/shading-coil
Answer: The DC coil relay has a resistance from the copper wire typically used. The current is limited by that resistance.
AC coil relays have inductance as well (the DC relays have inductance as well, of course, but it does not affect the 'on' current). They also typically have a shading ring that acts as a shorted turn in order to cause a magnetic field 90° out of phase with that from the coil, so that the total magnitude of flux does not drop to zero at the zero crossings.

Edit: As Andy says, an AC relay will work on (greatly reduced) DC. Of course you can also make a DC relay work on AC by adding stuff to it (if you use a capacitor filter you might need only 18VAC (RMS) to operate a 24V relay, and 24VAC would cause it to overheat and fail early).
Comment: +1 from me for the link. Never thought about driving an AC relay from DC or vice-versa.
Comment: After putting shader ring still it is chattaring. there is special type of material in core in solonoid Ac coil
Answer: A Dc relay coil has resistance that limits the dc current. An AC coil relies on its impedance for governing the current. An AC relay will remain contact closed due to mechanical inertia and a little mechanical hysteresis and, the fact that an alternating north and south pole both attract the relay armature.
Putting dc on an ac coil should work fine but be prepared for the resistance to be low. In other words, If relay is rated at 24 volts ac don't use 24 volts dc.
Answer: In theory AC coils can be driven with DC as long as you limit the DC coil current to the level of the AC holding current (to keep the coil from overheating).  For larger relays i.e. contactors, a DC current limited to the equivalent AC holding current is often insufficient to actuate the contactor effectively.  Since the coil inductance of the contactor increases once the contactor is closed the AC holding current is less than the AC pull in current.  DC contactors usually have some mechanism to shift between a coil pull in current and the holding current.  This is the difference between an AC coil and a DC coil in large relays.  The flux shunt (shader ring) is external to the coil and only shorts a portion of the core.
Answer: Recently, I had a problem with a batch of four 2-pole plug-in relays that had coils marked as 240V AC. When connected to the supply, three of the four merely 'chattered' and failed to 'pull-in' the contacts; the fourth operated correctly.
Believing the problem to be one of poor manufacturing quality control, I received some replacements from the same supplier which subsequently worked OK.
I assumed that the three faulty relays had been fitted in error with DC coils.  Consequently, rather than binning them, I overcame the problem by internally fitting 1000 PIV DIL bridge rectifiers allowing the relay coils to operate normally on raw DC (i.e. no filtering).
Perhaps the use of 1000PIV devices was overkill, but I wanted the rectifiers to have a sufficiently high withstand voltage to the back emf generated when the relay released.
Additionally, as the relay coils were now operating on DC (albeit raw DC), I could have fitted a single rectifier wired in inverse parallel to the polarity of the raw DC to snub the back emf, thus permitting use of a bridge rectifier with a lower PIV, but a lack of sufficient space within the relay precluded this solution.
Comment: a bridge rectifier would act as a DC snubber (free wheeling), so 350V diodes would have been fine, but 1N4007 are so cheap that there's nothing gained by using weaker diodes.
Answer: If you use a full wave bridge, the diodes in the bridge should act as free-wheeling diodes, and should snub the back EMF.


Proxy issue for generating PDF from XHtml source text using Flying Saucer

Question: I am facing some problem to generate a PDF using FLying Saucer from a XHTML source.
I use the working following code :
<code>ITextRenderer renderer = new ITextRenderer();

renderer.setDocumentFromString(pageSourceCode);
</code>
I would like to include some images in my source for the pdf to be generated. It implies having a href pointing to the images.
It seems that the href to the image has to be the complete URL else it does not find the image. On my local environment, I got no problem to generate the pdf with the image, but on the distant farm of server wich is behind a proxy, I have no image.

It does not recognize the URL used from the web (https://mydomain.com/mysite/...)
Using the server name + port works but is dirty since I have a farm of server (2 cluster with different port). So I can never determine which server is providing the data. If I point to a single server and if that server goes down, I won t have the image any more.
I have tried to configure a http/https proxy on the server jvm.properties (used for tomcat configuration) but it did not work.

My configuration :
tomcat5.5.28
jre1.6_13
Usage of https for all navigation
Usage of a proxy
java.args=-Duser.language=fr -Duser.region=FRx -Xms1024m -Xmx1024m -XX:MaPermSize=128m -Dhttp.proxyHost=myProxy -Dhttp.proxyPort=80 -Dnetworkaddress.cache.ttl=300  -DnonProxyHosts=myServerHost -Dhttps.nonProxyHosts=myServerHost-Dhttp.nonProxyHosts=myServerHost
Thanks in advance for your help.
Answer: have you tried "http://localhost:"+request.getServerPort()+"/mysite/etc"


How to go from iptables to pf?

Question: Hi how to is possible to convert iptables rules into pf rules file?
Comment: Are you talking about Pfsense?
Answer: One method would be to use the application FWBuilder. It is primarily a graphical application that can be used to manage firewall rule sets. It also, however, has an import/export feature that can be used to translate between different firewall types. And yes, both iptables and pf are supported.

So it looks like FWBuilder is more tricksome than I remembered. While you can import rulesets from Cisco and IPTables, you cannot directly export. I went through the process of translating a desktop IPTables ruleset from iptables to pf, and it was a fair amount of effort. Whether it is easier than learning pf syntax and doing a manual conversion, I don't know. That being said, this is the process I took to go from iptables -> pf using fwbuilder.

File -> Import Policy -> Select iptables rule file
Set Platform = iptables, click Next
Change New Firewall type to pf with appropriate Version and Host OS settings

Create new Interface object

Change Direction in Policy frame to their correct direction (inbound/output/both)
Add newly created interface to rules where appropriate
Save  to FWBuilder format -- File -> Save As -> Select filename to save as

From this point I used an iterative process to filter out any parse problems that fwbuilder had, primiarily the direction issues above and things like my rule to drop XMAS packets. I did this by issuing the compile commands directly, instead of using the interface.

fwb_pf -v -f ~/sandbox/firewallwork/desktop_firewall -d ~/sandbox -o ~/sandbox/firewallwork/New\ Firewall.fw 'New Firewall'
If no errors, stop, else
Fix problem in interface
File -> Save
goto 1

This will, eventually, leave you with two files that contain your new pf firewall config, New Firewall.fw, and New Firewall.conf. To reiterate above, after refreshing my memory on the process I am a lot less excited about it as a truly useful solution. However, by comparing the output beside your original ruleset it may help you get a better understanding of how the pf rulesets are developed so you can start managing them yourself.
Comment: How to import I see, but there is't clear how export?
Comment: Admittedly, I cheaped out with the answer. I'll try to update with better instructions (and maybe screenshots) later when I have more time.
Comment: Import current iptables rules (fwbuilder can import only iptables and cisco ACL rules), change firewall type to PF, press "Compile" button.
Answer: Assuming your talking about Pfsense. No, this isn't possible, you'll have to enter them by hand.
Comment: pf is the OpenBSD software firewall. pfSense, being FreeBSD, uses pf for the firewall portion of the distro.
Comment: I think you're spot on either way -- the OP will need to conceptualize the rules and translate them from iptables syntax to pf syntax.  As far as I know there's no tool that does this.
Comment: well my reply is most likely wrong then... Please move along, noting to see here...

How to go from iptables to pf?


Using application.xml instead of ejb-jar.xml

Question: I have a project that contain many ejb-module, each has it is own ejb-jar.xml.
All of these module  are packaged in a single ear.
i need to centralize my deployment descriptor in one file,
can i use the application.xml for that issue? can i use ejb-jar.xml in the ear ? or is there other solution?
Answer: No, not in an EAR. Each jar with EJB:s is his own separate module. If you cannot describe through annotations all the things you want to describe, then resort to the ejb-jar.xml file and put him in the sacred META-INF directory of the jar file that contains the beans. The application.xml file is mostly for describing things that is true for the entire application, environment entries being one thing that immediately comes to mind. Separation of modules and separation of ejb-jar.xml also separates the namespace and makes it possible for you to have multiple beans and environment entries defined with the same name without any clashes.
If you don't need this modularization, then it is possible for you to package everything in a WAR file instead and use only one ejb-jar.xml file for the application, put in the WEB-INF directory. In fact, would you first package your beans in a jar file together with an ejb-jar.xml file that you throw into a subdirectory called lib in your WAR file,  then these descriptors (the XML files) will be ignored.


Proving that two crossbars of a bisector intersect the midpoint of one of the crossbars

Question: This is sort of an involved question. I've proved parts of it already but now I'm stuck. Here is the question:
Let ∆ABC be such that AB is not congruent to AC. Let D be the point of intersection of the bisector of $\angle A$ and the perpendicular bisector of side BC. Let E, F, and G be the feet of the perpendiculars dropped from D to lines AB, AC, BC ⃡, respectively. Prove that
(a) D lies outside of the triangle on the circle through ABC [which I did by proving that both the bisector of $\angle A$ and the perpendicular bisector of segment BC bisect arc BC on the circumcircle of $\triangle ABC$.]
And given that one of E or F lies inside the triangle and the other outside, prove
(b) E, F, and G are collinear.
So my ideas so far are:
-my strategy is to prove that EF intersects the midpoint of BC (point G).
-By the crossbar theorem, we know that segments EF and BC intersecting AD exist. (although I don't know if it's completely necessary to state it for BC since it's obvious that the bisector of $\angle A$ intersects opposite side BC.)
-Since it is given that "one of E or F lies inside the triangle and the other outside", we know that EF must intersect BC at some point.
-I think that the angle bisector theorem may be helpful (if a point is on the angle bisector then this point is equidistant from the sides of the angle) because from there we can construct congruent triangles and maybe that will be enlightening.
Before I began the problem, I constructed everything with a straight-edge and compass so I have a visual of everything.
I think that $\triangle GFC$ and $\triangle EGB$ may be congruent because $\angle G$ would be the same measurement in both and we know that BG is congruent to CG. I don't know if that would help show that E, G, and F are collinear though.
Also, if anyone knows why these proofs would be useful, I would love to know. I think I read somewhere that it may be useful in navigation at sea or something. That's super interesting if it's true.
Answer: Hint: You already showed that $D$ lies on the circumference of $ABC$.
Hence, apply the theorem which says that the foot of the perpendiculars from $D$ are collinear.

If you want to prove that, consider the cyclic quads $DEGC$ and $DFGB$.


iTunes does not recognize folder structure when importing music

Question: I have imported a lot of Music into iTunes in a folder structure:
<Artist>/<Album>/<Song>. Unfortunately iTunes has not recognized these information, so that now I just have a lot of songs without artist or album information. (Actually I think that these has always worked until now).
Do you have any idea what went wrong here and what I could do, to get this information into iTunes (and the iCloud)?
Comment: iTunes recognises the tags put into the mp3 or similar files and not the file structure, you need to fins a tool that will tag the music - this will depend on which OS you are on
Comment: I'm on OSX Lion.
Answer: Doug's AppleScripts for iTunes are a good place to start looking for any iTunes scripting solutions.
In your case Tags Mirror Artist-Album-Filename looks very promising:

This script copies the names of the Artist folder, Album folder and filename (minus its extension) of the selected tracks' file paths to each corresponding track's artist, album and song name tags, respectively.
For best results your files and folders should be arranged in this format:
Artist folder / Album folder / filename.xxx
Answer: Select some songs (an album or songs from one of your previous folders) r-Click, select 'New playlist from selection'.  And a new playlist in your ipod is created..


What nuances distinguish statuó, cónstituó, and ínstituó?

Question: I'm asking mostly in the context of living Latin and trying to figure out how to say things like "I decided," "I started a blog," "I set up an organization," "I instituted a policy," and so on, for all of which some version of statuó seems appropriate.
Answer: Excerpted from Döderlein's Handbook of Latin Synonymes:

Instituere; Instaurare; Restituere; Restaurare. Instituere means to establish a profane, instaurare, a sacred, or honorable, or generally important institution, such as sacrifices, sacred games, wars and battles. Hence is instituere itself a usual, instaurare, a solemn, select expression. In the same manner restituere is distinguished from restaurare.
Destinare; Obstinare; Decernere; Statuere; Constituere. 1. Destinare and obstinare denote forming a resolution as a psychological, whereas decernere and statuere as a political, act. 2. Destinare means to form a decided resolution, by which a thing is set at rest; obstinare, to form an unalterable resolution, whereby a man perseveres with obstinacy and doggedness. 3. Decernere denotes the final result of a formal consultation, or, at least, of a deliberation approaching the nature and seriousness of a collegial discussion; statuere, to settle the termination of an uncertain state, and constituere is the word employed, if the subject or object of the transaction is a multitude. Cic. Fr. Tull. Hoc judicium sic expectatur, ut non unæ rei statui, sed omnibus constitui putetur.

I've also heard condere in reference to organizations and websites, e.g. Nuperrime situm/sedem ad linguam Latinam pertinentem aliquot homines in rete universali condiderunt.
Comment: Hmm. I like Döderlein but I'm always hesitant to take him as the last word. For example, in *Letters to Atticus* 47 (III.1), Cicero writes *Intellexi ad iter id quod constitui nihil mihi optius cadere posse quam ut tu me quam primum consequare,* and neither he nor Atticus is a multitude.
Comment: Good point. Neither does his note on *decernere* seem to apply to uses of that word referring to a simple decision by one person, e.g. Liv. 2.45 "reliquam aetatem a republica procul habendam decrevi."
Perhaps, then, we should take these distinctions as applying *only* where these words differ in meaning from the rest. (And unfortunately no clues are given as to how often they differ.)
Comment: After poking around Smith's Copious and Critical (*man* does Google books suck on a phone) I'm beginning to suspect that statuo has a sense of putting something down and constituo has a sense of putting something down and fixing it in place. This would certainly fit with the way prefixes work in Latin verbs, and with the meanings of "con-".  However this is at the moment pure conjecture.   Will add citations when am home.


Definition of "agent"?

Question: I apologize if this thread doesn't belong here but it seemed like a good place to start. I'm working on the final project for my math's degree on the topic of belief revision, mostly based on the approach described on Sven Ove Hanson's article called "Revision of Belief Sets and Belief Bases". One of the concepts that is constantly mentioned throughout the text but is never explicitly defined is "agent". I more or less understand what it stands for, which appears to be "anything that has the ability to execute an algorithm". However, I would like to give a more precise definition and I just don't know where to look. It doesn't have to (and actually shouldn't) be too technical. I only need a concise definition that gives some context to the word. Ideally I would also like to provide a reference for it, so I would be grateful if you could point me to some article or book where I can find this information.
Comment: See [SEP, Agency](https://plato.stanford.edu/entries/agency/):"*an agent is a being with the capacity to act*".
Answer: Philosophically speaking, 'agency' is a euphemism for the concept of free will, a euphemism meant to sidestep all of the knotty problems that arise whenever one talks about free will. With that in mind, an 'agent' is anything that senses its environment and makes (implicitly non-deterministic) alterations in its state. That parenthetical is necessary — we need it to distinguish simple mechanical sensors from beings that are capable of conscious decision-making — but it is left undiscussed in many cases because authors would like to avoid metaphysical entanglements.
Put in more simple terms, we have discussions bounded by these constraints:

No one wants to say that a mercury thermostat or photoelectric sensor is the same 'kind' or 'class' of object as a human (or presumptive AI) consciousness, even though the former technically make 'decisions'
Many people want to avoid invoking the concept of a soul, or of any transcendent, metaphysical 'essence' that constitutes the capacity for free will, thought, or reason; sometimes because they distrust those concepts and sometimes because those concepts are superfluous and distract from their main point

'Agency' is a quick way of pointing at the capacity to make non-deterministic decisions without getting bogged in the tar-baby of the metaphysics of consciousness.
Answer: I think the book "Artificial Intelligence, A Modern Approach" by Stuart Russel and Peter Norvig gives a good definition. In my 1995 edition, right at the beginning of chapter 2 "An agent is everything that can be viewed as perceiving it's environment through sensors and acting upon that environment through effectors".
Comment: @YuriAlexandrovich Really, it is only things like digital braking and engine control systems which have agency, most of the vehicle is dumb as schmuck.
Comment: According to this definition, a car would have agency, and I don't mean a self-driving kind either.
Comment: @YuriAlexandrovich, yes according to this definition very simple things could also be said to be an agent.
Comment: Yes, and I don't think we should regard cars as having agency
Comment: @YuriAlexandrovich And why is your opinion the measure stick of what agency should be defined as?
Comment: @JD > "*And why is your opinion the measure stick of what agency should be defined as?*" -- Thank you for asking. We call it consciousness. We have the agency when we a) have an intention to accomplish something and b) actually *know* how the desired effect can be achieved. W/o (b) you can't have agency/consciousness whether you admit it to yourself, or not.
Answer: A useful distinction can be drawn between an agent and a free agent.
A plain agent is any entity which receives information, process it and acts on the outcome. Software agents are, or were, common in hierarchical software applications, where an agent would typically run on a client host and interact with the main service on a remote system.
A free agent embodies the philosophical notion of free will, being able to make decisions and think up its own actions, independently from any inputs received.
The fact that the term is not defined in the text you ask about makes it difficult to know how (or even whether) the author was handling this distinction.

Definition of "agent"?


How can I add WFS layer URL in WPS request builder in GeoServer 2.17?

Question: I tried adding my layer URL under reference input type but no luck. I'm trying to create buffer.
Answer: This is because the Buffer operation is expecting a Geometry and you are providing a FeatureCollection. So you probably want to use the vec:BufferFeatureCollection process instead.
Comment: You can't a Geometry is a single object while a WFS returns a collection of features - maybe you need to ask a new question that describes what you are actually trying to do with the WPS
Comment: Cool let me check
Comment: Hi, but them how can I use the geometry from geoserver wfs url in the reference? (e.g. for JTS:area)


Does a corroded starter drain battery?

Question: I know a "bad" starter can drain battery, but am unsure what constitutes "bad enough" to allow a starter to drain the battery. Suppose a hypothetical coolant leak has corroded one's starter. Would that cause a drain on the battery?
Comment: Can you describe what you are experiencing a little better?
Answer: A 'bad' starter wont drain the battery while it is not being used to start the car, if that is what you mean.  If the starter has a damaged commutator, it will not run at full power and so could mean that you have to turn the engine over longer to get the engine to start which will use more energy from the battery.  Even if coolant is leaking on the starter, unless the starter is running there will be no electricity near the starter, so the water will not cause a short. The outward appearance of the starter should not be used to judge its internal condition.  I would be surprised if you starter is draining your battery.  If the starter starts the engine well, then it is more than likely not causing your problem.
Comment: Too low to upvote answer :(. Advance ran their thing and did not think it was an alternator or battery problem. I was a little suspicious as I have never had to turn it over very long to start, but I also doubt an alternator issue because the car ran for the whole work week after the first jump before needing another one, and I seem only the require jumps after letting the car sit for about a day without use. In addition, no light dimming or engine rpm going to 0.
I thought starter because I do have a coolant leak, which can corrode that. Thanks for clearing up how starters work :)
Answer: Yes starter will drain battery n ruin battery n some time ruin alternator ..I have ran into this few times . Once cops mechanic kept replacing battery's n alternators when was starter frying them . I told parts store . They thanked me for clearing it up . With new starter 14 years later no more problem n few same many times. It will take out core in batterys first

Does a corroded starter drain battery?


procodile syntax errors trying to start staytus

Question: Running Ubuntu on AWS to do a proof of concept for staytus. I am super green and have waded through most of my issues to get to this point.
staytus tutorial is what I've been following, and perhaps it's my lack of skills, but I've found it to be left wanting.
stuff I've had to do beyond the tutorials recommendations to continue through the installation :
CREATE DATABASE `staytus` CHARSET utf8mb4 COLLATE utf8mb4_unicode_ci;
CREATE USER `staytus`@`127.0.0.1` IDENTIFIED BY '*********';
GRANT ALL ON `staytus`.* TO `staytus`@`127.0.0.1`;

staytus:

Ruby 1.99 was installed did this and got ruby 2.4

sudo apt-add-repository ppa:brightbox/ruby-ng
sudo apt-get update
sudo apt-get install ruby2.4
sudo apt-get install ruby2.4-dev

need to update gemfile to show ruby '2.4.4' and then gem install bundler

last error I got:
ubuntu@ip-172-31-26-129:/opt/staytus/staytus$ sudo -u staytus procodile start --dev
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:160: warning: else without rescue is useless
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': /var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:57: syntax error, unexpected keyword_do_block, expecting keyword_end (SyntaxError)
...ort_by { |k,v| k.to_s }.each do |method, options|
...                               ^
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:57: syntax error, unexpected '|', expecting '='
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:144: syntax error, unexpected keyword_do_block, expecting keyword_end
          instances.each do |instance|
                           ^
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:145: syntax error, unexpected tSTRING_BEG, expecting keyword_do or '{' or '('
            puts "Started".color(32) + " #{inst...
                  ^
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:155: syntax error, unexpected keyword_do_block, expecting keyword_end
...upervisor(@config, @options) do |supervisor|
...                               ^
/var/lib/gems/1.9.1/gems/procodile-1.0.19/lib/procodile/cli.rb:162: syntax error, unexpected keyword_end, expecting $end
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /var/lib/gems/1.9.1/gems/procodile-1.0.19/bin/procodile:16:in `<top (required)>'
        from /usr/local/bin/procodile:23:in `load'
        from /usr/local/bin/procodile:23:in `'

I've seen this block of error prior when trying to do installations, but they aren't exactly google able. Thank you for the help in advance
Answer: Use this command which should fix your issue
gem install procodile

I had the same issue. The above fixed the issue
Comment: Can you elaborate, why this has fixed your issue?


Does `BorderlessMaximizedWindows=false` still work with Kubuntu 13.10?

Question: Since the upgrade to Kubuntu 13.10 today I lost the borders around maximized windows. How can I re-enable them? I tried this and of course changed true to false, but it doesn't work.
Comment: As I understand it, there are two separate options here.

`BorderlessMaximizedWindows=true` causes the title bar and borders to not be shown for maximized windows.  Setting this to false is not what causes borders to be shown for full screen windows.

`MoveResizeMaximizedWindows=true` causes the borders to be shown on maximized windows so that they can be resized by dragging the borders.  This option was removed from kwin in KDE 4.11 and has been reimplemented as a part of the oxygen window decoration for KDE 4.12.
Answer: The ability to show borders on maximized windows was removed from kwin in KDE 4.11.  You can find more information about this decision in bug 299245.  Basically, the implementation was a hack that caused multiple bugs, and the developer said that the proper place for this feature was in the decoration rather than in kwin.
So, this feature was added to the oxygen window decoration.  Unfortunately, this change has not made it into a stable KDE release yet.  Once released, the option will be available by going to System Settings->Workspace Appearance.  Then, if you're using the oxygen decoration, you'll be able to click on the 'Configure Decoration' button, go to the 'Fine Tuning' tab, and check 'Display window borders for maximized windows'.
Comment: I prefer 'Plastik' above 'Oxygen'. I did find a [reference](https://git.reviewboard.kde.org/r/109931/diff/) to a script to replace the functionality, but it doesn't seem to be finalized yet. Nor do I have any clue how to install and test it anyway.
Comment: Is it me, but currently (using KDE Plasma 5 and Breeze) the title bar is shown by default when maximized and there is no option to hide it... Is this the other way around?
Comment: jippie was talking about just the resize borders, rather than the title bar and the borders.  The BorderlessMaximizedWindows setting removes the title bar and borders, and it defaults to false.  The other setting to show resize borders for maximized windows has been removed from kwin and added to the oxygen decoration.


Fastest Speed of Sound

Question: Obviously, sound (like every other causal phenomena) may not travel faster than the speed of light. I know that materials with a high bulk modulus and low density will typically have faster speeds of sound, but is there a theoretical limit due to either a condition relating the density and bulk modulus, or some relativistic condition beneath the propagation of sound?
Comment: For an ideal ultrarelativistic gas it is $\frac{c}{\sqrt{3}}$
Comment: the restriction is relativistic, you know in advance that the speed (or information) cannot be larger than the speed in  vacuum.
Comment: See also http://physics.stackexchange.com/q/54684/
Comment: Related: [What is the speed of sound in space?](http://physics.stackexchange.com/q/162184/), which discusses another example where the speed of sound is comparable to the speed of light (baryonic acoustic oscillations).
Answer: There is no restriction other than $c_s<c$. Relativistic plasmas and fluids explore this regime. A weakly coupled quark gluon plasma has $c_s=c/\sqrt{3}$. Even higher speeds are reached in neutron stars, see Is the speed of sound almost as high as the speed of light in neutron stars? .
The speed of sound is related to the adiabatic compressibility
$$
c_s^2 = \left(\frac{\partial P}{\partial \rho}\right)_s \, .
$$
This quantity also enters the neutron structure via the TOV equation. It constrains, in particular, the maximum mass and the mass-radius relation. The recent observation of a 2-solar mass neutron star implies that $c_s$ becomes quite large, certainly bigger than $0.5c$.
Adendum: See here for a more quantitative analysis based on the existence of a 2 M(solar) neutron star, and equ.(10) of this paper for a theoretical limit in which we can shows that $c_s\to c$.
Comment: No equation from the link you provided indicates that $c_s > c/\sqrt3$. Is there a material where we'd expect $c_s > 0.6c$, or could one reasonably say that $c_s < c/\sqrt3$?
Comment: @user121330  Added a postscript
Answer: Science Progresses!
Per the above link, the theoretical limit for the speed of sound in condensed matter is
$$
v_{max} = \alpha c \sqrt{\frac{m_e}{2m_p}} \approx \frac{c}{8304}
$$
where $\alpha$ is the fine structure constant, $c$ is the speed of light, $m_e$ is the mass of the electron and $m_p$ is the mass of the proton.
Comment: It is so satisfying that 4 years later the answer to this question changed because of new science.
Comment: It's an amusing paper, but it's not rigorous (just dimensional analysis), and it only applies to solids for which the speed of sound is determined by the bulk modulus, and the scale for the bulk modulus is set by a typical chemical bonding energy. Certainly not relevant to relativistic fluids, such as neutron matter or the quark gluon plasma.
Comment: Note that the equation refers to hydrogen; for nuclear matter the relevant mass-ratio would be something involving quark masses, and the fine structure constant replaced by the strong nuclear force counterpart. It is not a general answer to the question, despite the nice paper.

Fastest Speed of Sound


Has there ever been a successful application of Liquid Democracy in recent (non-ancient) history?

Question: While looking for the best application of a Democratic regime, people tend to look either at Athenian Democracy (as applied in Ancient Athens that is) or at Grassroots/Liquid Democracy (as supposedly promoted by (some?) Pirate Parties worldwide). Direct democracy applications are not viable for various reasons, mainly due to the vast number of voters and disinterest by the citizens (that's why most prefer to just vote for their representatives). Grassroots/Liquid is an interesting hybrid between direct and representative democracy. Have there ever been any successful applications of the model, and if not then why?
Comment: It seems that there has been not yet been any serious experimentation, neither successful nor otherwise, for very many political ideas. Liquid Democracy in particular is a quite new idea that needs the internet to be successfully applied.
Comment: Note that liquid democrcy and anonymous vote don't really mix. I'd also argue that council democracy is pretty close (imperative mandate, possibility to immediatly take someones position if they loose support).
Comment: There are many parties and administrations that use it "informally", i.e. with a not-binding power. I'm not sure if being non-binding satisfies your question.
Answer: After the Pirate Party of Germany introduced Liquid Feedback (a variant of Liquid Democracy tools) internally and got a lot of press coverage for it, the Landkreis Friesland is now introducing LQFB as a tool to gather more diverse opinions by its citizens.
As this is still an experiment and the first public one it will be interesting to watch how the project evolves.
Press coverage in German about the initiative can be found at ZEIT Online
Answer: The traditional, relatively modern (i.e. post-Middle Ages), example given for grassroots democracy is the historic New England town meeting system (particularly in the colonial period before the formation of the United States of America and when the realities of sea travel limited the British role in governing the American colonies). A closely related system of self-governance, under the Mayflower Compact, is also often mentioned. These systems have largely been replaced with Representative Town Meeting systems.
Protestant church governance in the United States has also historically been radically decentralized, with New England's Congregational Churches, governed democratically at the individual congregation level (i.e. on a church building by church building basis), as an early example, in contrast to the highly centralized organizations of the European established Protestant denominations like the Anglican Church and the Lutheran Church at the time the U.S. denominations or movements were initiated. Many modern U.S. Protestant churches continue to have this form of democratic governance by their members.
Liquid democracy, as I understand it, seems to involve voters providing limited proxies to representatives, as opposed to providing proxies to family members or other trusted people participating in traditional elections in traditional representative democracies on behalf of both themselves and the family members for whom they have proxies (which a number of countries have tried and a few continue to use).
The closest real world example I can think of to the "liquid democracy" concept is proxy voting by shareholders in private corporations, although in practice, it is usually highly undemocratic even as to shareholders. The U.S. State of Oregon considered something along these lines in the year 1912, but ultimately rejected the proposal (which was still crude compared to the issue delineated liquid democracy proxy concept).
It isn't at all obvious to me that "grassroots democracy" as defined at the link in the question, and "liquid democracy" defined at the link in the question, significantly overlap in any way, or can be merged in some trivial and obvious manner. If there is a hybrid system that could be based upon these concepts, the question fails to describe it and it is not common knowledge even among political scientists who work in the area of electoral democracy.