What constitutes a gauge theory? Help me understand electromagnetism as the prototype of all gauge theories

Question: In The Geometry of Physics and Knots, Sir Michael Atiyah says that electromagnetism is the prototype of all gauge theories:

The prototype of all gauge theories is electromagnetism. From the geometrical point of view the electromagnetic potential $a_{\mu}$ $(\mu=1,\cdots,4)$ defines a connection for a $U(1)$ bundle over Minkowski space $M$. The field is the corresponding curvature
  $$f_{\mu,\nu}=\partial_{\mu}a_{\nu}-\partial_\nu a_{\mu} \hspace{10px}(\partial_{\mu}=\partial/\partial x_{\mu}).$$
  Maxwell's equations in vacuo take the form
  $$df=0, \hspace{10px}d^*f=0$$
  where $f$ is now viewed as a $2$-form, $d$ is the exterior derivative and $d^{*}$ is its formal adjoint (relative to the Minkowski metric).

He then goes on to say how non-Abelian gauge theories are obtained by modifications to the example above.
So, my goal is to understand the part that I have quoted it from the book, page $2$. I know what electric and gravitational potentials are. I understand how having some potential function defined on a space leads to forces acting on each particle in the field. My first question arises naturally now:

How does a potential field (electric potential) give a curvature on space? In what sense $f_{\mu,\nu}$ is a curvature? And what is the intuition behind associating a connection to a potential field?
How does Maxwell's equations take that form? I know what exterior derivative is. How is $f$ viewed as a $2$-form? And how is $d^{*}$ defined?
What is a $U(1)$ bundle? Does this mean that each fiber is diffeomorphic to $U(1)$? What's special about $U(1)$ here?

I know that reading this book is a huge leap for me considering my mathematical background, but my goal is to read and analyze the first chapter step by step with your help in a month. Maybe if I understood the first chapter well, I would attempt the second chapter too. So, I earnestly request you to bear with me and try to write your answers with minimal mathematical background please. Thanks in advance.
Edit: my mathematical background:

I have passed two courses in fundamentals of physics covering classical mechanics, thermodynamics and the classical theories of electromagnetism and gravitation. I have self-studied special relativity and I know that general relativity attempts to generalize SR to curved space-time by Einstein's field equations.
I haven't passed a differential geometry course yet, but I have read Barret O'Neill's Elementary Differential Geometry up to the chapter where the shape operator is defined and Do Carmo's Differential Forms and Applications, excluding Gauss-Bonnet's proof. I have also looked at some chapters from Lie's Introduction to Smooth Manifolds and I know what covariant differentiation, geodesics, Lie derivative, Lie algebra and Lie groups are.
I know what a vector bundle is (it's the abstraction of tangle bundle. Right?). I don't know much about principal bundles, except the explanations and definitions on Wikipedia.
Comment: @stressedout A very nice book that you might want to check out is "Gauge fields, knots and gravity" by John Baez and Javier P. Munian. It develops (or reviews) a lot of the necessary concepts in differential geometry in the first chapter (with application to electromagnetism) before turning attention to gauge fields. Highly recommended.
Comment: You ask for answers assuming a "minimal mathematical background". Can you say *something* about your actual background in math and physics? I see you said you know what the exterior derivative is (in what settings?), but what about other topics?
Comment: @KCd I updated my answer. Please let me know if it's clear now. Thanks for your time.
Comment: I'm pretty sure this has been asked and answered before, at several different levels, on the physics site or here.
Comment: @LeeDavidChungLin I prefer to have someone with a math background answer this question to be honest. Due to my background, I understand better when a mathematician explains a concept like this rather than a physicist. And I looked for this on MSE, but didn't find any. If you found any similar question, feel free to post it as related.
Answer: Sorry to answer a question about one book by mentioning others, but... have you looked at Naber's "Topology, Geometry, and Gauge Fields: Foundations" (he wrote a follow-up book "Topology, Geometry, and Gauge Fields: Interactions") or Frenkel's "The Geometry of Physics"?
Comment: No, I had never heard about them before. I googled them. They seem to be very good books. Thanks. Although, the last one is available at my university's library and it's 655 pages which seems suitable for a two-semester course! Plus, I'm eager to understand Atiyah's book. :(


SSL handshake exception: unable to find valid certification path to requested target

Question: We are using a third party payment service for our web users to be able to pay for courses. Once payment is completed however, payu attempts to connect to our site and run an IPN. However, they get the following error:
<code>javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX     path building failed:
sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid     certification path to requested target
</code>
Since our certificates are completely correct and up to scratch I can only imagine their services need to have the root CA certificate installed in their java trustore. Is this correct? Or is there something else we are missing?
Comment: If you have just a normal public CA it should be already in their certificate store. But there might be SNI needed to access your site and older versions of Java do not support it. Please check your site against [SSLLabs](https://www.ssllabs.com/ssltest/analyze.html).
Comment: Thanks Steffen, even without using SNI we're coming up short.
Comment: It would probably be a good idea if you would tell us the URL which causes the problem, so that we don't need to guess about possible causes.
Answer: The trust chain for this site is like this:
<code>[0] /CN=www.vitalityinstitute.co.za
[1] /CN=Entrust Certification Authority - L1K
[2] /CN=Entrust Root Certification Authority - G2
[R] /OU=(c) 2006 Entrust, Inc./CN=Entrust Root Certification Authority
</code>
The first 3 certificates are send by your server and the last one should be in the local certificate store as a trusted certificate. But, according to this PDF from 2010 this certificate is not included in the trust store by default. Instead older certificates from Entrust are included.
Thus there are probably two options:

add the missing certificate as trusted to Java
or add the missing certificate to the chain of the web server. Since this certificate itself is signed by an older certificate from Entrust and this older certificate is probably included in Javas trust store this might just work. Unfortunately, this might cause problems with OpenSSL based clients as described here.
Comment: ... since 6u24 circa 2011-02 **also** alias `entrustrootcag2` name `Owner: CN=Entrust Root Certification Authority - G2, OU="(c) 2009 Entrust, Inc.
- for authorized use only", OU=See www.entrust.net/legal-terms, O="Entrust, Inc.
", C=US`, SHA1 `8C:F4:27:FD`, which has the same key as cert [2] and (as then expected) verifies cert [1]. Of course the application at payu could be using a truststore that has been changed from the default, or even an entirely custom trustmanager.
Comment: I don't know what Java that PDF is, but I've tested most (Sun/Oracle) JDKs since 6u17 circa 2009-10 and all included alias `entrustevca`, name `CN=Entrust Root Certification Authority, OU="(c) 2006 Entrust, Inc.", OU=
www.entrust.net/CPS is incorporated by reference, O="Entrust, Inc.", C=US`, SHA1 fingerprint beginning `B3:1E:B1:B7` which verifies cert [2], and ...
Comment: @dave_thompson_085 That sounds very likely indeed, as payu is able to run the ipn when we test using their staging services, it is only when using their production services that the SSL exception is thrown, which makes me think their trustore is different between the production and development environments.
Answer: The typical reason why this type of errors come up is because the server does not provide a full certificate chain during handshake. That usually happen when the intermediate CA isn't installed.
Comment: You can check the chain with openSSL: connect to your server with `openssl s_client -connect :443 -showcerts`
Comment: According to SSLLabs we have provided a full certificate chain - or is there perhaps more to this? And do we need to install an intermediate CA?
Comment: Usually, yes: you need to install the intermediary CA because most client do not have them, they only carry the roots.


Alpha-Beta pruning in chess?

Question: How can alpha-beta pruning be made safe for a chess program? Pruning away a certain set of nodes is safe when the tree is complete and it is known that no other new nodes can be found deeper in the tree, but obviously this is not the case in chess. If my program prunes away a move that looks bad because it sacrifices valuable material that may just be the sacrificial move that would have checkmated the opponent in another few additional ply (or just gain a material and/or tactical advantage)?
Comment: Alpha-beta cutoffs have nothing to do with what you're describing.  The [horizon effect](http://en.wikipedia.org/wiki/Horizon_effect) afflicts all fixed depth minimax searches whether you use alpha-beta pruning or not.
Comment: Not research-level theoretical computer science. Migrating to [CS.SE].
Answer: What you are describing is unrelated to alpha-beta pruning.  The tendency of fixed-depth minimax searches to badly underestimate or overestimate positional scores in dynamic situations is known as the horizon effect.  The problem is due to static evaluation being used on positions unsuited to such evaluation.  Static evaluation scores a position based on things like material, pawn structure, and king mobility, properties that can be determined without extending the search tree.  But some positions are transitional, needing more search to identify their nature.
The standard way to deal with this problem is to recursively apply a quiescence function to all terminal nodes that extends the search an extra ply if there are checks, forced captures or promotions available at that node.  If no such moves are available, then the position is considered quiet and the usual static evaluation function is called.

Alpha-Beta pruning in chess?


Strongest Magnet Material - Theoretical Limits?

Question: In terms of field strength the current best (AFAIK) is NdFeB with small magnets capable of producing strengths on the order of one Tesla. Is there a theoretical upper limit on the field strength of magnetic materials under normal conditions? What might it be? In other words, how much stronger can common magnets get?
Answer: Check out the answer to this related question: How to wire a STRONG electromagnet?
Note that the saturation of an electromagnetic core is happening because the external field can't do any more to line up the tiny magnetic domains of a given material.. so there's the limit. Idk about specific materials, but for any given material it's limit will be related to the bend on that graph. (once you turn the field off and crystalize domains etc the strength goes back down to whatever)


What was Vashishta's role in Ramayana?

Question: What role did sage Vashishta play in the Ramayana? He was Rama's guru, but then, is there any other significant role? What weapons did he give Rama?
Comment: No, Vasishta didn't give Rama weapons.  Vishwamitra did, though.
Comment: Being the Guru is the most important roles of all.
Answer: It was in Vasishta's gurukula that Rama did his education. Also, Vashishtha was the one who impressed upon Dasaratha to send Rama with Vishwamitra to the forest for slaying the rakshasas who were not allowing the sages to perform  yagna. This later also led to the marriage of Rama with Sita.
Comment: I have noticed many ancient names are not capitalized in English as is common for nouns, especially names. Is this some known practice or just simple ignorance. I'm not complaining about English etiquette, but want to find out if there is a writing custom. For example, in the above comment only Rama is capitalized by not Dasaratha, Vishwamitra, and Vasishta. Is that following some custom?
Comment: Sorry, this is ignorance!!
Answer: Dashratha does not have a son who can take charge of his vast kingdom. Rishi Vasishtha advises him to worship Goddess Kamakshi in Kanchi in present-day Tamil Nadu state. She blesses him to have 4 sons. He then performs a putresti (putra-kameshti) yagnya under the guidance of Rishi Rishyasringa [Ref: Brahmanda Purana 4.40.88-142; Skanda Purana 126.96.36.199].
Rishi Vishwamitra asks King Dasharatha that God Rama be sent with him to the forest to kill the Rakshasa obstructing his yagnya. Dasharatha hesitatingly says that his son is not even 16 years old. Dasharatha adds that the task is difficult for him too, but offers to accompany the learned sage. Vishwamitra starts to get angry. Rishi Vasishtha steps in and says that the Suryavanshi tradition demands that the king keep his word and that he should comply with Vishwamitra's request [Ref: Valmiki Ramayana 1.19-21].
God Rama is asked to appear in the court. Vasishtha explains to God Rama the summary of the Vedic scriptures in a brilliant spiritual text called 'Yoga Vasishtha'. Then, God Rama and Lakshmana accompany Vishwamitra.
Later in Mithila, there is a mutual agreement that Janaka's younger daughter Urmila would wed Lakshmana. Janaka's 2 nieces Mandavi and Shrutakirti would marry Bharata and Shatrughna respectively. God Rama and Sita, and the other 3 couples get married at present-day Janakpur in Nepal, under the auspices of Rishis Vasishtha, Vishwamitra and Shatananda [Ref: Valmiki Ramayana 1.71-73].
It has been 12 years since the wedding of God Rama and Sita. King Dasharatha has a terrible dream with inauspicious signs. Royal astrologers tell him that such dreams foretell a calamity, perhaps his death itself. Dasharatha immediately decides to make God Rama the next king of Ayodhya (as guided by Rishis Vasishtha and Vaamadeva) [Ref: Valmiki Ramayana 2.1-3].
Royal messengers had been sent (by Vasishtha) to recall Bharata and Shatrughna from Kaikeya to Ayodhya [Valmiki Ramayana 2.68].
A huge convoy led by Bharata leaves Ayodhya to get God Rama back. He is accompanied by Rishi Vasishtha [Valmiki Ramayana 2.83-90].
Ravana had insulted and kept in captivity Rishi Vasishtha as he had refused to teach the Rakshasa the Veda [Puranic Encylopaedia pg 646].
When Yamaraja (the demigod of death) came to meet God Rama, Lakshmana was instructed not to allow anyone inside. God Rama says whoever enters and interrupts them would be executed. At that very time, Rishi Durvasa arrives saying he wants to meet God Rama but was stopped by Lakshmana. Durvasa says if he is not allowed inside, he will curse all of Ayodhya. Lakshmana decides that it is better to be executed by his elder brother than let the residents of Ayodhya suffer. He goes inside and interrupts God Rama. Durvasa is allowed inside. Later, God Rama approaches Rishi Vasishtha for advice on how to keep His word. How could such a beloved brother be executed, but there is a dilemma of Dharma? Vasishtha says Lakshmana can leave God Rama and that would mean death for him. Thus, God Rama banishes Lakshmana for upholding Dharma. Lakshmana merges with his original form of Adi-shesha on the banks of River Sarayu, which still exists today as Sahasra-dhara-tirtha. God Rama grieves when He sees a fiery flame reach towards the sky [Ramayana 7.116-119; Skanda Purana 184.108.40.206, 6.99-100].
Lastly, Vasishtha did not provide weapons to God Rama, but Vishwamitra did. And so did Rishi Agastya who granted God Rama a bow made by Prajapati Vishwakarma with a never-ending quiver of arrows [Ref: Valmiki Ramayana 3.12].
[Source: 'Essence of the Fifth Veda' by Gaurang Damani pgs 2, 4, 6, 8, 9, 16, 63, 64].

What was Vashishta's role in Ramayana?


How do I freeze a frame of a Mantaflow Smoke sim?

Question: I have a 75 frame smoke sim (Mantaflow- made in 2.82 release version) , with noise baked on it- all good so far.
Now- how do I freeze, say frame 75 and have the same smoke, static, for as long as I want?
Basically I want to use the resulting smoke trail, as simulated on frame 75, but without any animation and I want it to stay/last as long as I want/need.
I've tried deleting all the cache files from whatever.0001 to whatever.0074 and renaming all the left over frame 75 files to whatever.0001 (like you could do to fool the 'old' smoke sim) and I don't get terribly much of anything happening smoke wise… any ideas?
Answer: I've managed to do it 'work around' style, but my god it's an UGLY way to do it!
Basically I copied all the different cache files for the last frame (75 in my case), created another set of cache folders, then pasted 400 copies of the same file and 'bulk renamed' them (fantastic free Windows app!!!) so Iended up with 400 copies of each 'frame 75' file numbered sequentially from 0001-0400.
So that's 400 copies of config_0075 renamed to; config_0001 - config_0400 files in the 'config' folder.
4800 density_0001 to density_0400, densityIn_0001-densityIn_0400 etc.etc. in the 'data' folder.
and 1200 files similarly in the 'noise' folder.
And it works!!! But it's a huge ball ache to do (and 30Gb of disk for a relatively simple sim), so if anyone knows a more elegant way to do it, I really am all-ears!
Cheers!
Mark
Answer: I had the same problem and tried to address the issue with keyframing the time scale value. I had a problem with setting the time scale to zero (perhaps due to something else) so I keyframed it to 0.001 from 1.000. For reasons I don't understand, even though I 'downshifted', the size of the cache files and adaptive domain size continued to grow at a not-so-apparently slowed rate. Its size in the viewport and rendered images while decidedly slower was not frozen satisfactorily.
In my case, I had a 180 frame total loop of which I wanted the domain to evolve in first 15 and thereafter be frozen for frames 16-180. On my first attempt I baked the first 15 frames and then duplicated frame 15 for 165 copies, renaming them sequentially. (Ditto your kudos on the Bulk Rename Utility, BTW.) I ran into difficulty with the duplicated frames in the cache not being picked up for the 'playing' the 180 frame loop. I'm punching  above my weight class in dealing with the nuances of Mantaflow and haven't found an explanation for this. I did come up with a workaround by baking all 180 frames and then discarding 16-180, and substituting 165 copies of frame 15. This worked up to a point. I am now able to see a viewport view of all 180 frames but can only render frames 1-45 without it crashing. (I can render all 180 frames if the domain cache contains only the first 15 (?) steps. The 180 renderings show the domain data for those steps and complete the remainder without it showing.)
Obviously, the crashes may be unrelated but given they happen in both cycles (CPU and GPU) and Eevee I suspect at least some association with Mantaflow.  I've run the Blender file in the Windows command terminal and monitored the memory required and the upper demands are well within my system's capabilities. If I render the full loop with the domain present but switched off via the outliner pane, it works properly.
Given this is a workaround in the first place I'm not sure if this is worth pursuing in its current state but thought I'd relate my experience in case others run into a similar issue.
I'm running on an AMD 8-core processor, 32G of RAM and a Nvidia GTX960 with 4G of video memory. I'm working in Blender 2.90.1 but have similar issues in 2.80


Why do Auntie Em and Uncle Henry not have counterparts in Oz?

Question: In the film version of The Wizard of Oz several characters play two roles - one from the scenes in Kansas before the tornado, and a second as a counterpart character in Oz. Miss Gulch is the Wicked Witch of the West, the three farmhands are the Scarecrow, Tinman, and Lion, and Professor Marvel is the Wizard of Oz. The only characters from Kansas who do not have counterpart roles are Auntie Em and Uncle Henry. Is there a particular symbolic reason for this?
Comment: You are attempting to ask why injecting the dream into the script where it was not in the book without rewriting the entire plot results in a less than expected result.
Answer: Neither Dorothy nor Toto had alternates in Oz and it wasn't established in the film that they HAD to have alternate versions of themselves.
Ultimately, it stands to reason that the writers realized that to impart the moral of the story only required Scarecrow, Tin Man, Lion, and Professor Marvel.
Comment: Hello and Welcome to Movies & TV Stack Exchange. While your answer might be helpful, it would benefit from a source to confirm where in the film it is established that ever single person had to have a counterpart? Just because some people do, doesn't mean everyone has to unless this is explained in exposition or by the executive producers. In addition the film is based on source material.
Comment: And the witch, obviously...


How to prevent a yoga mat from skidding?

Question: I have a yoga/exercise mat. But due to tiled flooring, it slips a lot and I find it difficult to do, say, jumping jacks on it.
Any ideas on how to make it stick to the floor?
I tried putting a rug under it. That helps but it still moves.
Comment: Put a [non-slip mat](https://www.amazon.co.uk/Auckpure-Multipurpose-Non-Slip-80X130CM-Waterproof/dp/B08F41ZVYM/) under it.
Comment: Buy a [Amazon.co.uk : yoga mats non slip](https://www.amazon.co.uk/yoga-mats-non-slip/s?k=yoga+mats+non+slip)
Comment: If you only move it occasionally, spray only the mat with a low-grip fabric contact adhesive, e.g., https://swankyden.com/best-spray-adhesive-for-fabric/, *and let it dry before putting mat on floor*, to allow mat to be removed later.
Answer: I have a tiled floor too. The best thing I've found for mats and rugs so far is Ikea's Stopp Filt product (not the same thing as the one just called Stopp). This stuff is surprisingly grippy, much more than those mesh/net designs.
Since you have to buy way too much for a single yoga mat, you can just fold it in layers till it works. Or put it under other rugs.
Answer: Place a wet towel underneath the mat. I use a wet towel on the shower floor to keep from slipping. the grip is rock solid.


Multi Ring Buffer "Outside polygons only" option grayed out - ArcGIS

Question: I'm trying to create a multi-ring buffer around a shapefile of ~400 points in ArcMap 10.6.1 using the Multiple Ring Buffer tool. I need the buffers to be "doughnut style" not "swiss cheese" style, where the larger rings may overlap each other or with smaller rings but remain separate annulus-shaped features. The "Outside polygons only" option for this tool becomes grayed out as soon as I enter the points shapefile I want to use for buffering. Is there a way around this where the option is not grayed out? I would like to be able to use this simple checkbox. Because the larger rings overlap with the smaller rings, I can't use Erase to create the rings either.
Answer: If you check the help in the tool you will see that the Outside Polygons Only option in the Multiple Ring Buffer tool works only for polygons and it is not activated for points or lines:
Input polygon:

The workaround for this is to create individual buffers at various distances and then use Union tool to merge them all in one feature class. The Union tool will work as the difference tool and merge in the same time. This means it will keep the outside portion of the big polygon with no overlap with the smaller polygons.
Comment: Of course, I need to read the instructions better. The union thing would work with a low number of polygons, but with ~500 points x 9 distances, I gave up on running Union after it ran overnight and didn't finish. However, I found a workaround by using the 1km buffer polygon as the input to the Multiple Ring Buffer Tool and subtracted 1km from all the original distance intervals. Then the Outside Polygons Only option worked.


Does anyone else make a product like the Leviton Smart Wi-Fi 4 Button Controller DW4BC-1BW?

Question: I have an x10 system in part of my home that controls outlets that are on separate circuits from the wall x10 wall switches. I would like to put wifi outlet plugs in the existing outlets (removing the x10 receivers) and replace the wall x10 transmitter switches with wifi switches. This would allow me to control the outlets with voice, phone, or the wall switch. I have only found 1 wall mounted wifi switch/controller that will control wifi outlets- the Leviton DW4BC-1BW. Unfortunately they run nearly $100 each. I would need 8 of them, so ouch. Anyone aware of a similarly functioning product, maybe something from China, that would be cheaper? Thanks!
Comment: Do you really need Wi-Fi? You'll probably have a lot more choice in the Zigbee world.
Comment: would like to stick with wifi.
Answer: SonOff has "Smart Wall Switches" that can be used as you describe and is also powered by mains.
They can also be re-programmable with other firmware like tasmota or with your own code as it is a ESP8266 MCU inside the device.
I have two "TX Series" that I have loaded Tasmota firmware on and that I have configure it to send the state of the buttons to a MQTT broker.
Then I have Node-Red and it is doing the logic control like: if button is pressed then do send toggle state to wallswitch.
The MQTT broker (mosquitto) is then sending the commands to the SonOff devices that is controlling light bulbs and blinds.
So the "TX Series" is the one I am referring to.
Comment: Thanks for your reply. Do you have a model number? The only SonOff wall switches I can find are hardwired to the outlets they control- they don't allow control of smart plugs as far as I can tell.
Comment: I have updated my answer, but see the "TX Series" as two parts. One is the button that can send its state to "something" (in my case a MQTT broker). Two is the output part that can control a electric load like a light bulb. And then it has a local rule that say "push button = toggle state on the output". NOTE I use an alternative software/firmware on the SonOff device called Tasmota since the chip inside the SonOff devices is either a ESP8266 or ESP32 that you can program yourself if you would like to.
Comment: Ok, thanks. A little more involved than what I am looking for...
Comment: Perhaps it can be done with the standard firmware, but that I don't know anything about.


Feedburner - Multiple feeds in multiple languages

Question: I run a website that available in Dutch and English. Each 'language version' has its own blog and related RSS feed. I want to publish these RSS feeds through Feedburner. Obviously I would like to have the Feedburner UI for the Dutch readers in Dutch and the English speaker readers in English.
Does anybody know how to manage multiple feeds with multiple languages in Feedburner.
Answer: You will likely need to setup separate Feedburner feeds for each language since the source RSS will be coming from two different places.  Within the feed settings on Feedburner, if you go to the 'Optimize' tab, the 'BrowserFriendly' section allows you to change the language of the theme that is used by Feedburner so that your subscribers have a language specific interface (the ClearFeed theme has both an English and Dutch version).
From the 'Publicize' tab, the 'Email Subscriptions' setting has a similar setting, although the number of languages there is fairly limited (it does not look like Dutch is an option).


AXI stream slave

Question: I have question about the AXI stream transfer. In the AXI stream slave, I want to capture the data coming through the AXI_Slave_TDATA and then process them (for example multiply each byte of data by 2) and then further transfer it as stream to another IP.
What is the VHDL code which processes the stream of data entering the AXI_Slave Stream and then transfer them as stream to another IP?
Any example would be helpful
Comment: I think you will need to add more information. Particularly since the processing example you give is so trivial (shift the data bits, no logic involved). What IP do you have already for this protocol? Do you have any experience working with this protocol already?
Answer: You can take a look at the Xilinx AXI-stream IP example design.
In Vivado, open a new empty project. Tools -> Create and package a new AXI ip. And go through the wizard.
When asked, choose "Create a new AXI peripheral". This will generate example code for you for what an AXI peripheral looks like and create a barebones project to start from.
Answer: Until you provide more details, the only answers that can be provided are about AXI stream in general.
Transfers in AXI stream take place when tvalid and tready are both high, with tvalid being driven from the source and tready being driven from the sink. When your core is ready to receive data, set tready high and read in fresh data when tvalid and tready are high. To send data, drive tvalid high along with the new data. Hold the data constant and tvalid high until you get a high level on tready. Then either drop tvalid or place new data on tdata. If you want to use a framed protocol, mark the last transfer of the frame with high tlast. Make sure to only transfer tlast when you transfer tdata (only accept or update when both tvalid and tready are high).
Comment: I have a data stored in the DDR, I want to read the data from it using the DMA through M_AXI_MM2S port and then send the stream from the DMA through M_AXIS_MM2S port to my AXI stream slave of my IP. In my IP, I want to capture the streaming data coming through the S_AXIS_TDATA and then do some calculations on them and then send them back as stream to the DDR through the S_AXIS_S2MM port of the DMA.
Answer: You have posted several FPGA and PL implementation questions. The problem is that these questions are complex and are not easy to answer. I would suggest you to watch several dozens of Xilinx video tutorials just to have a general idea of how things work and what should be expected. Then, there are hundreds of datasheets like UG761 that you can read and get your answers from. Use Xilinx DOC Navigator. Dive into it.
You are asking for an example - I bet there is one somewhere in the documentation - and it is not as simple as two lines of code. Follow these series of videos - the guy focuses on PS-DDR-DMA transfers and gives an introduction to AXI4 interface.
Comment: I watched all the videos you mentioned and understood completely how the DMA works and the AXI protocols. I did not post several FPGA and implementation questions. My question was just how to process a stream data to custom IP and then forward them to another IP. Thanks Alex for the explanation which is totally clear. My question if there is simple example to translate the explanations into VHDL. you know there are some issues we have to take into account like the delay for 32 cycles after resetting the IP....etc.

AXI stream slave


What has priority, Kavod or Oneg Shabbat

Question: Does Halacha have a preference for either Oneg Shabbat or Kavod Shabbat? For example, if a fancy suit would make a Jew uncomfortable (like during summer) that would be Kavod Shabbat but hurt Oneg Shabbat, and not wearing it would be the opposite. Which one should be picked?
Answer: Halachapedia addresses a similar scenario here:

If wearing a nice tie or jacket on Shabbat in the summer makes a person very hot and uncomfortable some say that it is preferable to wear fancy respectable clothing but not be uncomfortable than to be even fancier and uncomfortable.

The footnote there reads as follows:

Chashukei Chemed Shabbat 25b quotes the Chazon Ish as saying that if a person feels that wearing a tie on Shabbat will make him hot and uncomfortable then it isn't a fulfillment of Kavod Shabbat, honoring Shabbat, if it won't also fulfill Oneg Shabbat, enjoying Shabbat. Or Avraham Brachot siman 79 quotes this as well from the Maaseh Ish p. 134. Chut Shani Shabbat ch. 1 fnt. 2 is cited to agree with the Chazon Ish and explain that it isn't a condition in Kavod Shabbat to be Oneg Shabbat but that it isn't a violation to Kavod Shabbat to remove the uncomfortable clothing when it isn't Oneg Shabbat. Rivevot Efraim 3:554 agrees. See Shalmei Yohanatan 242:1 p. 32 who poses the question whether it preferable to use challah that is store bought and nicer since it is more Kavod Shabbat or the tastier homemade bread since it is more Oneg Shabbat.


Trackbacks in PHP

Question: I'm writing a custom blog engine and would like to have trackbacks similar to Wordpress. I could look at the Wordpress source, but I'd really prefer a tutorial of some sort and so far I haven't been able to find one. Are there any good tutorials for implementing trackbacks or pingbacks in PHP5?
Answer: Implementing trackbacks isn't that hard at all. Here you can find the official specification and an example at the bottom.
Answer: Trackbacks are fine, but they're very prone to spam, since there's no verification of their origin.  You use a simple discovery method to find the trackpack entrypoint; look for RDF in the target site's source.  Then it's simply a RESTful POST request to the destination site's trackback entrypoint passing the requisite trackback information.  The specification linked by Sebastian Hörl should give you plenty of information for that.
Pingbacks are a bit more complex, because they require both XMLRPC and a linkback.  So while you can just use REST calls to send anonymous trackbacks to any site, sites that you pingback will look at your site to determine whether you're actually linking to the pinged page.
Also, implementing Pingback on the receiving end is a little more complicated, because you have to verify the ping.  I suppose you don't have to, but it would be foolish not to, otherwise it would be susceptible to the same spamming techniques as Trackbacks.
The Pingback specification has all the information you need to get that implementation off the ground.
Another recommendation is rather than wade through the inevitably dense and spaghetti-ish WordPress Pingback code, have a look at Habari's self-contained Pingback implementation, which isolates some of the key features of the task into more easily digestible units.
Answer: Dunno what trackbacks are, all I've noticed of them is that they clutter up websites with messy blocks of text, often interpersed with article comments.
But if they allow you to see who is linking to you in real time without log analysis:
You could use the HTTP-Referer (sic) header to see if people have linked to your article.
Each article would have a map of these referrers (and a count, so you can organise by popularity). You then implement a basic crawler that visits new referrers to scrape content.
I'm sure there's a third party implementation available as well. Or a specification. As the other poster has linked to, so, err, ignore this.
Comment: I believe this is a "Refback". But it is heavily prone to spamming, in that you get referrers from sites that aren't actually linking to you via an HTML anchor. Together with "Pingbacks" and "Trackbacks" (mentioned above) they are grouped under the term "Linkbacks".

Trackbacks in PHP


How do I secure my new web server (Server 2008)?

Question: I've just put my new server up on an IP address with a domain pointing to it. I need to be able to remote admin it. I've opened the firewall for Remote Desktop and HTTP traffic. Is this going to be secure enough? I guess I should probably rename the administrator user...
Answer: Should be sufficient, as long as you use a crazy-complex password for the admin account, and make sure your http server is security-patched and up-to-date.
Also, I hope firewall != Windows Firewall.
Edit: +1 for EHaskin's suggestion of changing RD port, if only to reduce the bruteforce spam that your FW will have to endure, but never think that security == obscurity.
Answer: The absolute minimum you should do is change the Remote Desktop port, change the Admin username, and have a very strong admin password.
Answer: Any chance you can set up your server as a VPN endpoint?  Then you would only have the VPN ports and the HTTP ports open.  When you want to RDP to the server, you would connect to the VPN first and then you're good to go.
Only reason is, if my memory serves me right, RDP traffic is not encrypted.
This is how I run my IIS server at home, works very well.
Answer: Windows Server 2008 supports VPN capabilities.  You can configure your remote access policies by using the Network Policy and Access Services.  I believe this needs to be installed as a role before you can use it.  Also, simply changing the RDP port on your firewall will not prevent an experienced hacker from still getting to your server.  A simple port scan would reveal open ports.


How does the Mueller investigation compare with former investigations in terms of length of time, money spent, and indictments?

Question: How does the Mueller investigation compare with former investigations, of a similar size and scope, in terms of length of time, money spent, and indictments brought? What are some of the extreme examples of these categories that have occurred?
Answer: FiveThirtyEight has a chart and an article that compares the current special counsel investigation to previous ones, accurate as of November 29, 2018.

It's worth noting that the current special counsel has indicted many people abroad. Here's an older chart that labels the number of indictments of people abroad. As this article explains:

There's a problem, though, with simply comparing the number of indictments in different investigations. That approach assumes that all the indictments are in the same general category. For the Mueller investigation, they aren't.
The vast majority of the people charged by Mueller live abroad — specifically, in Russia. This means that unless the 25 Russians individuals accused of crimes related to election interference travel abroad, are arrested and turned over to the U.S. or are extradited by Russia, Mueller can't actually prosecute them.

FiveThirtyEight also has an older article, with comparisons to previous special investigations in detail.

For the cost of the current and previous special investigations, PolitiFact has some of the numbers:

So, if you add up the direct and indirect costs through Sept. 30, 2018, [the cost of the Mueller investigation] works out to $25.2 million.
[ ... ]
As we've reported, the investigation of President Bill Clinton over Whitewater and Monica Lewinsky — headed by Kenneth Starr and then by his successor, Robert Ray — cost upward of $52 million over half a decade. Another $40 million-plus went toward a handful of other Clinton-era independent counsel investigations.
Meanwhile, the investigation of Reagan administration officials over the Iran-Contra Affair, headed by Lawrence Walsh, topped out at more than $47 million over a roughly similar period.
Those figures are direct costs only, and are not adjusted for inflation. Their price tags would be far higher if converted to 2018 dollars.
Comment: @Brythan Then again, did any of the other investigations have anywhere near the same level of foreign involvement?  I don't think the other investigations failed to give such indictments simply out of practicality, but because they didn't really have the opportunity. They also might not be entirely without teeth here: property holdings by these individuals within the US (or certain cooperating jurisdictions) could be confiscated, assets frozen, etc. A small price compared to the consequences of a successful prosecution, but not without effect.
Comment: No mention of the fact that the Mueller investigation has actually made more money than spent?
Comment: Note that the indictments are artificially inflated by indicting foreign citizens who are unlikely to be prosecuted.  Previous investigations did very little (if any) of that.  If we drop those indictments, there have been far fewer.  Most of the two big bubbles disappear.
Comment: @Brythan That is true, I've updated the post to include an older chart that labels the indictments of people abroad for clarity.
Comment: Now that the investigation has concluded, [here's an updated chart](https://fivethirtyeight.com/wp-content/uploads/2019/03/atd.MUELLER-CONVICTIONS-31.png?w=575), albeit in a slightly different format.


The Command "File.BatchNewTeamProject" is not available in VSTS 2010

Question: I am trying to automate the creation of team projects using a command built into VS 2010 called File.BatchNewTeamProject using a project settings file as an argument, and executing this as a process silently in the background from my c# win app. I am aware of a requirement that Team Explorer has to be open and connected to TFS before this command can run, however, when I start a process with devenv.exe, a fresh instance of visual studio pops up with Team Explorer not connected by default. How can I solve this problem?
Answer: I added an extra command statement and the whole thing started working.
devenv /command view.TeamExplorer /command "File.BatchNewTeamProject "C:\vss2TeamFoundation\ProjectCreation\Ajax.xml
Answer: Just posted this project on Codeplex: http://tfsprojprovisioner.codeplex.com/
Includes an assembly that hides BatchNewTeamProject behind a simple programmatic interface, and a sample desktop app that you can use to create and provision new Team Projects.  Must be run on a computer that has VS 2010 installed, and by a user that has permissions to create new projects.  But it doesn't require you to interact with VS directly, at all.


Safari has some menu items that are dimmed and won't work. Is there a way to make them active?

Question: I'm running High Sierra on my older iMac. I use Safari as my browser. Under several of the Safari, menu items are certain options that are dimmed out and can't be run. Specifically, under the History menu is an option to open all windows from the last session. This could come in handy but that menu item appears to not be active and clicking on it has no effect. Is there any way to use these menu items?
Comment: I don't think it has to do with the Mac's age. I'm running High Sierra on my iMac late 2015, and those Safari menu items are dimmed on my machine as well.
Comment: Does it actually re-open all your windows from the last session [without using the menu] each time you relaunch it ? That would be dependent on system prefs ? General > Close windows when quitting...
Answer:
Safari has some menu items that are dimmed and won't work. Is there a
  way to make them active?

Two possibilities that I can think of.
1- If you clear the history before quitting Safari, it wont be there when you restart.

2- If you have a setting in Safari to clear the history after set time.


what version bundle does pythonnet_py35_dotnet work in?

Question:
Windows 8.1
Visual Studio 2015
NuGet pythonnet_py35_dotnet 2.3.0

I've tried about 15 different versions.
Conda with python 3.5.0, 3.5.2, 3.5.4, 3.7.5 and many more different versions...
Python 3.5.0, 3.5.4, 3.7.5, 3.6.2 and many more different versions...
PythonNet 2.4.0, 2.3.0, 2.2.0
Does this library even work?
In the worst case scenario, I get an error.

Fatal Python error: Py_Initialize: Unable to get the locale encoding

In the most ordinary case.

Unable to load DLL 'python35': The specified module could not be
  found. (Exception from HRESULT: 0x8007007E

I've been trying to solve the problem for 30 hours straight...
<code>    Dim tmpPath As String = "C:\Program Files (x86)\Anaconda3\;" + Environment.GetEnvironmentVariable("PATH", EnvironmentVariableTarget.Machine)
    Environment.SetEnvironmentVariable("PATH", tmpPath, EnvironmentVariableTarget.Process)
    Environment.SetEnvironmentVariable("PYTHONHOME", "C:\Program Files (x86)\Anaconda3\;", EnvironmentVariableTarget.Process)
    Environment.SetEnvironmentVariable("PYTHONPATH ", "C:\Program Files (x86)\Anaconda3\Lib\;", EnvironmentVariableTarget.Process)

    PythonEngine.Initialize()

    Using Py.GIL()

    End Using
</code>
Answer: I am successfully working with pythonnet for a while, with different pythonnet and Python versions (3.5, 3.6, also Anaconda).
You may have a path (whitespaces) or permissions issue, try installing Anaconda into "C:\Anaconda3".
Also make sure the bitness is matched between Python, pythonnet and .NET project platform - all three should be 64-bit (or all three 32-bit).


How much is moderate violation to normality for one sample t-test?

Question: I want to test if my data have a mean equal to zero (Ho: mu=0). The sample size in not large (n=21). My variable is numerical with average -0.10 and sample_sd 0.05. I don't know the population variance. The only test I know to do this, is t-test. I checked for normality using Shapiro Wilks and it gets rejected (therefore no evidence of normality). I understand that  t is relatively robust to moderate violations of the normality assumption; what is not clear to me is, how much is moderate? I do need to run other test like this for other samples. May someone be so kind to provide me with examples of how much can I deviate in skewness and kurtosis for sample sizes between 12 and 25? What else can I do to test against zero if I can't use t-test? Thank you in advance
Comment: Your plot does not match your description of the data: although it reflects 21 values, they clearly do not have the mean, max, or min you report. Moreover, it is impossible for a dataset of 21 numbers between -0.8 and 2 to have a SD of just 0.05.
Comment: Very true. Sorry to be misleading. I added the one described in the question to the left. As I mentioned, I want to know how much is a moderate deviation from normality in the different samples I have. Hopefully with two images I can get even better feedback.
Answer: What makes "mild deviations" mild depends on you and what you're doing. Deviations from normality (or indeed any other assumptions) will impact the significance level and the power of a one-sample t-test.
For example, a heavy tailed distribution will tend to have a true  type I error rate that's lower than the one that you'd get if the data were normal with the same critical region, and somewhat lower power (higher type II error rate) partly because of that shift in type I error.
In some cases this kind of thing mightn't matter very much (I usually don't really care all that much if my type I error rate is 3% instead of 5%, as long as I have some sense of the size and direction of the impact). In other cases it might be more important.
It's possible to use simulation to make some assessment of how badly those things might be affected for various kinds and sizes of non-normality, so you can decide then whether you're prepared to deal with that.
In large samples, the normality assumption tends to become less critical (though the other assumptions - like independence - tend to remain relatively important).
If you know a good approximation to the distributional shape, sometimes another parametric assumption can be made (for example, if the distribution for this kind of data tends to be somewhat right skew, one might be able to approximate it reasonably well with a gamma distribution and use a GLM to do the test).
Some people use transformations, but there are two problems: (i) with small samples, you can't reasonably assess whether the transformation does sufficiently improve the  distribution of the test statistic; and (ii) you're no longer testing for a shift in mean of the original distribution (if you're not focused on means, that may not matter, and with some additional assumptions, you may still be able to make a suitable conclusion about means). Similar comments apply to rank-based tests; if you don't care explicitly about mean-shift alternatives or you're prepared to make some additional assumptions, the fact that they're not directly testing for mean-shifts may not matter to you.
One solution if you really don't have a pretty solid idea of what the shape might be but really do want to be testing for a shift in mean is to look at a permutation test.
Comment: I've done permutation tests to compare two samples assigning random labels. How does it work with only one sample? If you have a link to read about it will be of great help. Another question, is there a number in skewness (for instance less than 1) where I could say is symmetric enough? is there a number for kurtosis to say is close enough to 0 (normal kurtosis)?
Comment: As my answer already suggests, what is 'close enough' for your purposes depends on your purposes and your tolerance for effects on significance and power. I can't tell you where to draw those lines and I expect my own ideas of 'close enough' for my applications would seem strange choices for yourself and your applications. For a one sample permutation test of location, if you are prepared to assume symmetry about $\mu_0$ under the null, then you can subtract $\mu_0$ from each observation and write each deviation observation as the magnitude (absolute value) x its sign, &then permute the signs.
Comment: Aside from my previous suggestions, there's also the possibility of a one sample test based off a bootstrap confidence interval (which doesn't assume symmetry). However, the bootstrap tends to work better with larger samples than this.
Answer: You could provide a plot of the data so we are able to comment on the violation of normality.
On the other hand, you could perform a permutation t-test to test the mean. This would not assume any distribution.
Comment: I just uploaded a couple of histograms. The one described in the question and another one more skewed.


OEM engine oil alterrnative

Question: My motorcycles recommended engine oil grade is 10w30 synthetic.I am not happy with the OEM oil and looking for an alternative ,to my surprise there is not even a single manufacturer providing same grade in synthetic.I am able to find mineral and semi synth oil in same same grade i.e 10w30.
For synthetics I have 10w40,10w50,20w50 options available.
Can i switch to semi synth or upgrade to a heavier 10w40 fully synth
Comment: This has been asked before...
Comment: What are you not happy with?
Comment: engine gets heated up quite fast.also i feel change in performance after i have ridden around 100kms
Comment: Is the bike overheating?
Comment: I would not say overheated but just riding around 2 kms it gets very hot that i can feel the heat at my legs. Outside temp is below 30 degrees these days.
Answer: Motul makes 10w30 fully synthetic for 4 stroke bikes. It is coded 7100. You csn use semi synthetic unless the manufacturer specifically recommends fully synthetic although it will have some drawback. You can learn more about oil types or whether you can use a heavy oil if you search this site.
Comment: I do not find Motul 10w30 in my country
Comment: If you have any honda dealers around you may ask them if they have any. Large manifacturers sell their oil and at least some hondas use 10w30
Answer: First, it would help to know the make/model/year of your bike, and to some degree the environment you ride in. Usually the manufacturer will provide a few different grade options depending on conditions.
That said, in most cases (not all) you can use whatever grade of oil is most applicable to your riding conditions. If you're putting around town in a cooler environment you need different oil than if you're desert racing. This tends to be true regardless of the OE spec because they base it off of the most average riding conditions they can expect.
It is important to know what the numbers and ratings of oils are to better determine, for yourself, if it's a good choice to change. This is a pretty good place to start.
Now, it has also been common practice to change grades over time, as the engine becomes more "broken-in". Vavoline mentions how bearing clearences can change over time.
The safest bet would be to use the oil they spec. Then the next safest is using the same gear from about maker. Then, next down would be using 5w30,and after that using 10w40.
In your specific case, I'm hesitant to say anything definitive without knowing the make/model.
What I will say definitely though, is the engine coming to running temp fast is not an issue (in and of itself), unless it's also overheating or having other issues. You mention a change in performance as well. If the bike is new, you should generally expect some change, and if it has a lot of miles on it, you will also generally get some change. If this change is noticeable in a single ride then there may be something else wrong.
Answer: Use 5W-30 synthetic. The 5W means the motorcycle engine starts better in the extreme cold. Well, I'm sure you won't be riding your motorcycle in -20 degrees Celsius, so you won't see the benefits of 5W-30 when compared to 10W-30.
There should be no drawback when using 5W-30 at the usual working temperatures of an engine, if the oil is fresh. At the operating temperature, both are SAE 30 assuming the viscosity improvers haven't degraded.
Note a 5W-30 is actually SAE 5 mixed with viscosity improvers, and 10W-30 is actually SAE 10 mixed with viscosity improvers. So just in case the viscosity improvers degrade with time (thus giving you SAE 5 or SAE 10 oil), you'll want to change the oil at some point of time. But that should be obvious anyway; nobody rides with the same oil in for the lifetime of the engine.
I don't see any drawback of using 5W-30 vs 10W-30 assuming you will change the oil before the viscosity improvers degrade.

OEM engine oil alterrnative


Idioms and Formal Usage

Question: Certain websites devoted to idioms claim that they--idioms-- are not used in formal conversations or writing; that is, they claim that these creatures are always used in only informal situations. As the site is a British website, is this a common belief among the British, or is this a mere hiccup in the website? For "according to" and "at first glance"
are far from "informal".
Comment: Are you referring to phrases.org.uk? best to link to the site in your question, and to the claims about what is informal.
Answer: One problem is that the term idiom has various definitions. I would be surprised if the websites you refer to make the claim that a.) according to and at first glance are idioms and b.) such idioms are not used in formal conversations or writing. This is simply not true.
A narrower definition of idiom is that an idiom is a phrase whose meaning cannot be deduced from its constituent words. Examples are:

She's having kittens. = She feeling nervous.
You get up my nose. = You irritate me.

It is true that informal language abounds with idioms in this sense of the word. It is also true that you would not expect to see some of these idioms in formal conversations or writing. It is not true, however, to claim that no idioms are found in such situations.
Here are just three examples from the Telegraph, a serious British newspaper:

The latest can of worms to burst open is the notion that an
  independent Scotland should have a properly funded pension scheme.
Iran decides to give its Western adversaries a taste of their own
  medicine.
Disagreements within the Conservative Party about Europe dominate the
  headlines this morning, but it's a storm in a teacup compared to
  Labour's deep ideological divisions.

Idioms and Formal Usage


Book about a panda who ate liquorice and used a sudoku square to do magic

Question: I read a book when I was younger that my grandma brought me and loved it. It was about a panda who ate liquorice and used a sudoku square to do magic. I really would like to find this book for my son
Comment: Welcome to the site! Can you remember any more details about this book? E.g. when did you read it (we don't know when you were young), how old was it at that time, was it written in English, which country did you read it in, what was the target age group, was it a picture book or with lots of words, how long was it, ... ? Any little detail could help to narrow down the searches.
Comment: I must have been about 8 or 9 and I remember the book had a white cover with the panda on. I seem to think his name may have been izzy but not sure. It wasnt a picture book and I think the author was male as I seem to recall my grandma had it signed by him. Hope this helps
Comment: Ohhh it woukd have been late 80s early 90s
Answer: Issi Noho.

Issi Noho is a panda with magic powers. His magic results from completing the vacant square in a series of magic number squares that he inherited from his Chinese ancestors. In a magic square the numbers must come to the same total in whichever direction they are added up, horizontally, vertically or diagonally. Issi's poor arithmetic causes frequent mathematical errors, with unexpected and humorous consequences.
Comment: Thank you so much for your help
Comment: @user10665 Are you the same person who posted this question? It seems like you've lost access to your original account, but you can [merge your accounts](https://literature.stackexchange.com/help/merging-accounts). Then you'll be able to comment freely on this thread (instead of posting as "answers"), and also mark this answer as accepted if it's correct.


not able to see recovery mode on my Redmi1s after I installed cyanogemod 11

Question: After I installed cyanogenmod 11 on my new redmi 1s using clockworkmod recovery. I found that I am not able to see the recovery screen as the mi boot time logo is hiding the recovery screen.Can any one help me???
I have followed this tutorial.Please help me.
Answer: Boot time logo cannot hide the recovery. Try to reinstall recovery from fastboot:
<code>fasboot flash recovery clockworkmod_recovery.img
</code>


How can I roll back Ubuntu?

Question: I face a very frequent problem: I am new to Ubuntu and I like experimenting with different softwares and environments, but either after installing them or after uninstalling them I can not log in into my machine after reboot. Something goes wrong at boot and I left with access to terminal only. Can someone suggest me what should I do? Can I restore my Ubuntu a previous point (I have not created a restore point before installing any package because I don't know how to do it)?
Answer: I suggest you to either:

read how to install Ubuntu on BTRFS. Btrfs is a filesystem that can roll back in time.
In your Ubuntu, install Virtualbox, then install Ubuntu into VirtualBox and do your experiments there. It is safe and also provides the rollback capability. Trnsfer only changes you are sure of into your working Ubuntu.

The second one is easier.

How can I roll back Ubuntu?


Any popular experiment in chemistry that digital signal processing played a crucial role in?

Question: Is there any famous experiment in chemistry where digital signal processing played an important part? I don't mean using a machine that relies on such techniques (they all do) but an experiment where signals were first recorded (e.g. temperature/pressure/voltage/etc. as a function of time) and then analyzed with some sophisticated methods (e.g. time-frequency analysis with wavelets).
Comment: You mean something like [Spectrograph](http://en.wikipedia.org/wiki/Spectrograph)
Comment: No, I don't mean that.
Comment: I do not understand. How would you imagine doing an experiment with signal recording without a machine?
Do you mean some pioneering experiment, where a guy measured some curve, which had no sense. Than he applied signal processing by hand and voila, nice results?
And it was so nice, he build it into automated machine with DSP.
Answer: Many spectroscopic experiments record analog data, which is subsequently Fourier transformed to give a frequency-domain spectrum. For example, in a nuclear magnetic resonance experiment, the signal emitted by an excited nucleus is picked up as a voltage in an RF coil for 1-2 sec. This time-domain signal is processed then converted to the frequency domain by FT.
As for 'popular', Richard Ernst won the 1991 Nobel in Chemistry for his contributions to this field of research.
Comment: Many mass spectrometers (ion cyclotron and also "orbitrap" instruments) now also work by Fourier transform.
Answer: How about the patch-clamp technique in neurochemistry?  This technique allowed the study of ionic currents through single molecules of a voltage-gated ion channel.  It won the Nobel prize in 1991.
The signal processing required to simply observe the single-molecule opening and closing events is fairly minor.  However, processing single-molecule ion-current data for multiple experiments when inhibitors are titrated, or to compare ensemble-average kinetics to single- molecule kinetics, can be fairly complex.
Here is a paper from 1998 on single-molecule enzymology; it's an extension of the patch-clamp experiments developed for the specific case of single-molecule ion channels: http://www.sciencemag.org/content/282/5395/1877.full  Read the paper for a better description of some of the signal processing involved.


Old kombucha scoby

Question: I have some kombucha scoby that I haven't been taking care of and has been doing it's own thing for almost a year. It seems to still be alive. Is it still safe to use and will it still make kombucha?
Answer: I've been in the same situation (and am again... I'm a terrible scoby parent).
It's dormant and likely quite weak and/or imbalanced, but as long as it's not moldy or severely discolored, you should be able to bring it back. That said, you probably want to give it a boost. I usually buy a bottle of plain kombucha (GT's for me) and use that as starter tea instead of the ancient tea vinegar you have.
In my experience the well-controlled and cultivated commercial kombucha is pretty vigorous stuff, and a new, healthier scoby usually grows with the first batch. If any of your senses say yuck though, just let it go. These days you can get a new scoby fairly easily.
As always with kombucha, taste and smell are your best guides. A pH test kit is also your friend, with pH between 2.5-3.5 being ideal.

Old kombucha scoby


How to patch code in someone else's project hosted on github

Question: I have cloned someone's open source code hosted on github and made some changes to fix things on various platforms. I'd like to feed this back into the system. How do I contribute this into github so that others get the benefits?
Comment: For hopefully obvious reasons, you cannot do this yourself, at least not in someone else's repo. What you can do depends on the project. Most of the larger projects have something like a "How to contribute" section in their documentation. Does your project have anything like that?
Comment: possible duplicate of [How do I contribute to other's code in GitHub?](http://stackoverflow.com/questions/4384776/how-do-i-contribute-to-others-code-in-github)
Answer: I think you want to create a Pull Request.

Push your fixes to a branch of your fork on GitHub
Visit the GitHub page of the original project
Click on Pull Requests, it's somewhere in the top-right
Click on New pull request
Click on compare across forks
Select your own fork + branch
Review the differences. Make sure it's clean.
Create the pull request, and maybe get in touch with the maintainers

UPDATE
If you don't have your fork on GitHub yet, then you have to create that first:

Go to the project's page on GitHub
Click on Fork, it's somewhere in the top-right
Clone your fork to your PC
Make your changes and commit (you can split to multiple smaller commits, it's actually better)
Push your local branch back to GitHub
Now your fork on GitHub has your changes, ready create a Pull Request from it, as explained above.
Answer: If you are contributor:

Create your own branch using git branch [branch_name]
Make your changes in the code of that branch
If you have added new files you will need the git add .(adds all the new files) command to notify GIT of your new files. After this GIT will start tracking the new file.
Commit the changes git commit -am "Commit message"
Push your changes using git push origin [branch_name]

PS: You are not allowed to make changes to the main branch(called master) on a public repository unless you are a contributor.
After this the admin will check the changes you have made and if he approves they will be merged with the master branch.
If you are not a contributor:
Create a pull request as suggested by @janos
I'm not sure if you are looking for a GIT tutorial but here is one to start with anyway.
http://www.vogella.com/articles/Git/article.html
Comment: Usually you can't push to the remote repo at all unless you've given permissions. That applies to all branches. Also, one can send pull requests for any branch, not just master.
Answer: In github, you can send a pull request to patch code in someone else's project. I've started a project to help people making their first GitHub pull request. You can do the hands-on tutorial to make your first PR here
The workflow is simple as

Fork the repo in github..
Clone the repo to your local machine.
Make a branch for changes and make necessary changes
Push your changes to your fork on GitHub git push origin branch-name
Go to your fork on GitHub to see a Compare and pull request button
Click on it and give necessary details


What percentage of verbs in English take both a direct and an indirect object?

Question: I am assuming that the number of transitive verbs that take both a direct and indirect object in English is a subset of those that just take just a direct object.  Does anyone know how much smaller?  How does this compare with other languages (e.g., German, Spanish, Russian)?  Are there databases that can be used to explore this further?
Comment: Some would argue that verbs can be indirectly monotransitive – that is, have an indirect object without a direct object (e.g., "I told you already!"). Others would argue that the indirect object gets bumped up to direct object status in such cases. I don't personally buy that, but some do. Even so, I don't think there are any verbs that can _only_ take indirect objects: every IO verb can also, sometimes optionally, take a DO in the same construction.
Comment: There are also verbs that can't take only a direct object on its own: *I put the book, *I gave the letter.
Comment: @JanusBahsJacquet  You read my mind because this was also something I had been wondering about, but I didn't want to complicate the initial post.  Thank you for your contribution.
Comment: Despite doing a full linguistic degree, I never learnt the difference between "direct" and "indirect" objects. They're all just complements to me!
Comment: @JanusBahsJacquet otoh, such verbs never explicitly mark their complement as an indirect object, e.g. it's always "I told the man already" not "I told to the man already". This isn't necessarily a problem though as there are doubtless explanations of the "I gave him it" vs "I gave it to him" (and regional "I give it him") reordering that would cover this too
Answer: As John Lawler notes in his comment, the big problem is defining the class of verbs you mean. Typically, we use "give" as an example of a verb that can have both an indirect and direct object: you can say "I gave Bob a cake". You can also say "I gave a cake to Bob": does that version count as an example of IO+DO? Let's say "no" (if you say "yes" then the answer probably has to be "pretty much all of them"). An Indirect object is an "object-like NP" that doesn't have a preposition. So we also have "I sent Bob a letter" which has two bare O-like nominals.
We also have "I baked Bob a cake", which we could derive from "I baked a cake for Bob". Now we can start looking for verbs that we can do "for" people, and see to what extend we can shift "V NP1 for NP2" to "V NP2 NP1", i.e. "dug Bob a hole", "skinned Bob a vole", "bought Bob a pole", and also try more examples of the analogous shift with "to", like "threw Bob a rope", "showed Bob the dope".
So a paraphrase of the question could be: which verbs engage in the V NP prep-NP ~ V NP NP alternation? For example, *"introduced Bob the pope", *"lobbied Bob his vote". As far as I can tell, "underneath" can never enter into that relation (*"I found the table the book" meaning "I found the book underneath the table"). It's not always or obviously always prepositional phrases, though, for example "They elected Bob mayor" matches "They elected Bob to be(come) mayor" or "They elected Bob as mayor", without movement or a second PP (unless you think of "as mayor" as a PP). Of course nobody would say that "I hammered the metal flat" has an IO and a DO, because "flat" is an adjective, not an NP.
In Bantu languages, the general answer is "everything", because there are valence-increasing suffixes that allow all forts of thematic roles to be integrated as bare NPs.
Comment: Benefactive constructions like _buy a book for Bob_ can do Dative to _buy Bob a book_ if Bob winds up with the book; otherwise not. Cf _I fixed John a hamburger_ and *_I fixed John the roof_.
Comment: @jlawler  Good points all.  Another thing I got to pondering is the concept of transitivity, independent of any language in which it is conveyed.  For example, if the concept of "give" is something that can involve both a direct object and an indirect object, then wouldn't this really be true in any language?  What would cause statistical variance, then, are the number of words any given language has for verbs like "give" and its synonyms that share this same quality.
Comment: According to some sources, German has eight times as many words as English.  And, if verbs that can take both a direct object and an indirect object are a small subset of the total number of verbs, it would be logical to conclude that disparities between languages will be multiplied accordingly.
Comment: So, for example, if one out of four verbs in English can take both a direct and indirect object and only one in six in some other language that has more verbs in total, it is possible that the number of concepts we have that involve both types of object is virtually the same and that the difference, statistically, is merely a reflection of a larger total number of verbs.
Comment: It's not a question of number of verbs - it's a question of what an object is, and that's a big rabbit hole. The semantics of subjects is determined by the verb -- agent, experiencer, instrument, etc, depending. Indirect objects are even more determined by the verb -- mostly they're receivers, with dative verbs, but occasionally they're resultatives, like _elect_. But the semantic relations between the verb and its direct object are part of the meaning of the verb, and vary wildly from verb to verb. And, of course, from context to context, and from speaker to speaker.
Answer: I am not a linguist by trade, so I don't know if I should even attempt to answer this, but here it goes because curiosity is killing this cat.  In an attempt to answer my own question, I simply took the first most frequent 25 verbs in English and then compared their transitivity to a randomized selection of the most frequent 1,000.  I did this simply because I was interested in how they compared.
There were some flaws in this methodology because I did not use the same source for the most frequent 25 as I did for the most frequent 1,000, but if you want to take a look at a rough idea of the distribution, look below:

So, after doing that, I attempted to do the same thing with German verbs.  As with the English, the first most frequent 25 German verbs were from a list of infinitives.  That may be the most consistent aspect of the methodology.
But the methodology for the collection of German words is contaminated with flaws as well because I also did not use the same lists for the first most frequent 25 and the most frequent 1,000.  In addition to that, the English list did not appear to show any past tense forms, but it did have conjugated present tense forms.  This is not true for the German which appeared to contain not only all conjugations, but all tenses as well.  Nevertheless, perhaps some of you can draw some conclusions out of these charts just the same.  (Or, perhaps this poor answer will inspire someone to contribute a better one!)  The German distributions are below:

Due to the lack of rigor in the methodology and my lack of experience with linguistics, I won't draw too many conclusions from these charts, but what I didn't know before I began this attempt to answer my question is just how many verbs in English aren't just either exclusively transitive or exclusively intransitive.  Many verbs can play different roles along the transitivity spectrum depending on context.  Even so, it does appear that this is less of a tendency with German verbs.  Of particular note is the larger percentage of exclusively transitive and intransitive German verbs compared to those for English.  (44% vs. 32% for transitive and 20% vs. 12% for intransitive.)  Perhaps this is common among languages with case systems.  Perhaps in languages without case systems, verbs become more flexible in usage which affects their transitivity as well.
For the record, I used WordReference.com to assign transitivity for both the English verbs and the German verbs.

*Note: I realize that this answer morphed into content regarding transitivity rather than what percentage of verbs take both a direct and an indirect object, so let me close by saying that when I looked at the randomly selected transitive verbs that were included in the tabulations for this answer and determined which ones took both a direct and indirect object, I came to this estimate:

Granted, the numbers are really, really small, and the methodology was a bit shoddy, so these estimates are a bit of a leap.  In fact, if this were an academic paper, I would never settle for such limitations, but as it is, especially due to a lack of experience and linguistic toolsets, I'm content to leave it at this for now.
Comment: It's pretty good for back-of-the-envelope figuring. As you noticed, the real problem is defining "transitive", and much ink and many pixels have been spilled doing it. The syntactic theory I follow says that 3-argument predicates, like _give_ or _elect,_ distinguish the two NPs in their VP semantically, and usually also syntactically. "Indirect object" is a term syntacticians don't use much, because it almost always implies the existence of a "direct object" as well, which may not be present in a given sentence. For that matter, neither object may be present: _I gave at the office._
Comment: @jlawler  You're so kind and supportive.  I love it!  In fact, posting on a forum on a topic of which you are not much of an expert is a bit daunting, but I have found the members of this stack exchange to be polite and encouraging ... much more so than some others I've been a part of!  BTW, I read your bio.  Very interesting.  I look forward to exploring your links further.
Comment: You might find [Relational Grammar](https://www.google.com/search?q=relational+grammar) useful. It starts off by assuming that the grammatical relations of subject, object, and indirect object (called simply "1,2,3" in the theory) are the basis of syntactic analysis. Passive in Relational Grammar is "promote 2 to 1", i.e, make the object into the subject. Dative is "promote 3 to 2", i.e, convert _give a book to Bob_ into _give Bob a book_. If you do dative before passive, you get _Bob was bought a book_.
Comment: @jlawler  Sounds very interesting.  Thank you for the recommendation!


Balloon out of gas but won't accept fuel

Question: So several times now I go to refuel a balloon but I don't have the option of pouring in the fuel when I'm at the funnel unit (in particular, Wailing Wind and Rot 'N' Rusties territories). Even when I come back, I have to bring my own fuel and the container still won't accept my refueling (yes, I'm positive the balloon is still out of fuel and can't rise).
Comment: Ok, I'm buying this game...
Answer: I had this same issue....What you actually need to do here is go to the balloon itself with a fuel tank and add gas to it there. There is a small gas tank on the side of the balloon which it gets the gas from...not very obvious, but it is there.
Comment: it took me 3 tries at this before I got mad and went to the balloon...the thing we were trying to add gas to before is the wench for the balloon I believe.


Farthing / Riding

Question: A 'farthing' is an an administrative quadrant (4 parts)
Is a 'riding' an administrative triant (3 parts)?
How do we call each of the parts of a circle divided in 3 parts? Not 'one third' but another other term(s)... Does the word 'triant' exist? Thanks in advance!
Comment: Where are you planning to use this term?  In the US "farthing" means simply "small amount", while "triant" is a misspelling if "tyrant".
Answer: You're actually correct about the etymologies: they are cognate with four and three, respectively.

Farthing is from Old English fēorthing, from fēortha "fourth" + -ing 1.
Riding is from Middle English triding, Old English *thriding, which is from Old Norse thridjungr "third part"; the Middle English t- was a variant of Old English th-; this -t was later lost by assimilation to -t in east, west, which commonly preceded our word (e.g. east triding → east riding).

As to the third part of a circle, I don't think any word exists that is in common use. You could invent a new word, like *triant, if you like. Or you could use a semi-obsolete word and repurpose it:

Tierce

an old measure of capacity equivalent to one third of a pipe, or 42 wine gallons.
a cask or vessel holding this quantity.
Also terce. Ecclesiastical. the third of the seven canonical hours, or the service for it, originally fixed for the third hour of the day (or 9 a.m.).
Fencing. the third of eight defensive positions.
Piquet. a sequence of three cards of the same suit, as an ace, king, and queen (tierce major), or a king, queen, and jack (tierce minor).
Obsolete. a third or third part.

(Etymologies and quotation based on Dictionary.com, s.v.)
Comment: Since this answer seems like it's going to sit at the top of the replies, it'd be better if @Alephzero's discovery of 'trine' were added in place of the mistaken idea that no such word already exists.
Comment: Latin has a word *trient–*, so I'd use that rather than change the vowel.
Answer: A farthing is an obsolete British coin, a quarter of an old (pre-decimal) penny. As far as I know it is only used for an administrative region in the works of Tolkien. A riding is one of the three former administrative regions of the county of Yorkshire. The word is not used in this sense in any other context.
I agree with Cerberus that there isn't a word for the third of a circle.
Comment: I believe electoral constituencies are called *ridings* in Canada, at least by some, and local party organisations are *riding associations*.
Comment: @JamesMcLeod You learn something new every day! Following links from your Wikipedia article I see that Iceland used to be divided into four farthings, which is presumably where Tolkien got the idea for the divisions of the Shire.
Comment: https://en.wikipedia.org/wiki/Electoral_district_(Canada)
Answer: An the English word meaning "1/3 of a circle" is trine, but it is now only used in astrology.
It was also used in astronomy to describe the position of two objects when there was an angle of approximately 120 degrees between, until modern accurate measurements made such an imprecise term redundant.
Answer:
How do we call each of the parts of a circle divided in 3 parts?

Like with alephzero's trine, this mostly just shows up in astrology, but another term that shows up is
trigon

Astrology  (a) A set of three signs of the zodiac, distant 120° from each other, as if at the angles of an equilateral triangle... (Also figurative or allusively.)  (b) The aspect of two planets distant 120° from each other...

It's usually just an archaic synonym for 'triangle', though. Its adjective 'trigonal' is more often used more generally for 120° angles in, e.g., geology, chemistry, & crystallography. Such angles are 'triequal'. 'Triradiate' and 'triradial' are used for things shooting out from a source in three sections/directions but not necessarily at equal angles from one another. Another synonym is
triplicity

a. spec. in Astrology. A combination of three of the twelve signs of the zodiac, each sign being distant 120° or the third part of a circle from the other two...

but its figurative and allusive uses are marked as completely obsolete right now.

Farthing / Riding