Is there a limit to how much spacetime can be curved?

Question: My question is about the fact that general relativity predicts bending of space and time. However, I have not seen anything regarding the limits to how much space or time can be bent.
So my question is that is there anything (like a hypothetical short ranged repulsive force) that actually puts some limit or is any 'warping' possible.
Comment: How would you quantify the "amount" of warping? The magnitude of the Ricci scalar at a point seems like a natural candidate.
Comment: @NiharKarve Ricci scalar is not the right indicator because it can be zero even when the curvature tensor diverges.
Comment: @AndrewSteane you're right. I was just trying to think of a way in which OP's idea of magnitude of curvature could be converted to GR terms. Of course you'll actually need to analyse *all* the scalars that can be formed from the Riemann curvature tensor to see whether there's a honest spacetime singularity
Comment: We can probably use the principal invariants of the curvature tensor, such as the Kretschmann scalar.
Answer: The short answer is no. The components of the Riemann tensor, which is an object measuring the curvature of the manifold at a point, can take any real number value. In classical GR, there's nothing which bounds these numbers, but when they diverge to infinity at a point it indicates a problem with our classical description (see discussions of singularities in GR for more details about this).
Comment: I don't think the components are in any sense physically meaningful. Seems to me that unless the Riemann tensor is identically 0 you can make any one component arbitrarily large by applying diffeomorphisms of spacetime.
Comment: Yes exactly we  can make any component arbitaryily large so no limit
Answer: The only thing I would consider to be a "limit" to spacetime curving in general relativity is a gravitational singularity:

A gravitational singularity, spacetime singularity or simply singularity is a location in spacetime where the mass and gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system.

Gravitational Singularity - Wikipedia
The most common singularities are in black holes where the density and thus the "warping" of spacetime is considered to be infinite. However, this singularity cannot be described by general relativity - a theory of quantum gravity would be needed.

This answer does not make any statement on how much curvature can be described by general relativity; I am probably missing something here (I also only have a basic non-mathematical understanding of GR). I hope someone else can go into more detail.


What are the origins of these two uniforms?

Question: We believe these two are father and son from the early 20th century.
Possibly German/Austro-Hungarian/Polish region but unsure.
Any assistance would be greatly appreciated, I understand that is little to go off.
Comment: @njuffa - I'd say there's enough (really helpful) content in those first two comments to make a proper answer, at least by identification question standards. Feel free to do so when you feel comfortable with it.
Answer: I was encouraged to write an answer by a moderator. I am neither an expert on military uniforms nor German student corporations, but recognized some identifying elements shown in the pictures.
I think these could be two pictures of the same person, at a younger stage as a university student and later as a member of the military. The uniform in the first picture appears to be a Luftwaffe uniform, about 1935-1945, by its cut and emblems. So most likely this person was German. The second picture shows a university student who is a member of a student corporation who could be German, Austrian, or Swiss. Because all self-organized student associations were forced to disband early under Nazi rule in Germany and Austria (many were re-constituted following WW2), if the person was German this would date the second picture to before 1933 and the first to some time after 1935.
The second picture could yield more clues about the person, if we could decode the information contained in it. We cannot date the picture with accuracy, as the ceremonial dress depicted has remained largely unchanged over the past one hundred years or more. Most student corporations using such dress originated in the mid to late 1800s. They may be called Corps, Landsmannschaft, Burschenschaft, etc. with different rules and regulations applicable, partially set by the umbrella organization they belong to, e.g. Coburger Convent, Deutsche Burschenschaft.
This ceremonial dress (Vollwichs) is worn by the executive leadership (elected annually) of a student corporation during particular official functions. Typically the leadership of a student corporation comprises three people referred to as Chargierte. The presence of a fox tail on the hat of the person identifies them as holding the office of Fuchsmajor: an older student who acts as a mentor and instructor to new members. This suggests the person depicted may have been around age 23 or 24.
The presence of the épée indicates that the members of the student corporation engage in academic fencing, either mandatory (schlagend) or optionally (fakultativ schlagend). As far as I know, the type of guard on the weapon differs by university and could be either the bell type shown here or a basket type. Student corporations are associated with a particular university or university town. Identifying marks are the colors as shown on the sash for example, and the Zirkel, an elaborate monogram unique to each student corporation, partially visible on the sash here. Most student corporations use a tri-color scheme, with the middle color frequently white (heraldic: argent) and sometimes yellow (heraldic: or). Since this is a black & white photograph, it is difficult to determine the colors.
Most Zirkels contain a capital letter 'F', and this seems to be the case here. Of particular interest is the connection (or lack thereof) between the major horizontal stroke and the vertical stroke, here a simple small loop. I looked through Wikipedia's list of Zirkels but could not find a good match to the partially visible one in the photograph. One possibility is that the student corporation this person belonged to no longer exists, possibly because it was associated with a university in regions that now belong to Poland (e.g. Breslau) or Russia (e.g. Königsberg).
As for the military uniform in the first picture, the "wings" on tunic and cap are clearly the emblem of the Luftwaffe. No part of the uniform indicates an NCO or officer rank to me. The collar patches on the collar tips appear to be of uniform color, but I cannot determine what color that may have been. So I think this is most likely an enlisted man. The background is not aesthetically pleasing, suggesting this picture may have been taken in the field rather than at home. An airman depicted with no visible infrastructure in the background leads me to hypothesize that this person may have been part of antiaircraft personnel, in particular part of a crew manning an AA gun emplacement in the country side.
Comment: The second photo is not only taken in the field but by an amateur - the subject is out of focus! Makes the enlisted assumption more likely, IMO.


Configuration of Apache and Tomcat for load balancing

Question: I'm trying to setup Apache as a load balancer for 2 Tomcat instances with session affinity.
The goal is to have the session stick to one server but to have next session (when it's changed by the backend server) to go to the next available server (let's say using round-robin algorithm for easier implementation). When using the "jvmRoute" in Tomcat and an equivalent "route" in Apache the actual value that does the routing is the route name which does not change and all requests are routed always to the same backend server for a single client.
I found out so far that there's an chicken/egg problem when using just the JSESSIONID cookie. Let's consider the following setup:
2 Tomcat servers listening on ports 8009 and 8010 (AJP13)
1 Apache server with the following configuration
<code><Proxy balancer://hello-cluster>
    BalancerMember ajp://127.0.0.1:8009/hello
    BalancerMember ajp://127.0.0.1:8010/hello
</Proxy>

ProxyPass /hello balancer://hello-cluster stickysession=JSESSIONID
</code>
And here's the scenario:

The first request has no cookie so Apache selects the next available server in the load balancer to handle the request.
The backend Tomcat server sets JSESSIONID but does not note the actual value being returned.
The next request comes in, Apache notes that there's no backend server noted for the given JSESSIONID so it selects the next available, which in this case the other one as served the first request
Tomcat notices that the value of JSESSIONID is invalid so it creates a new one.
Apache does not take a note that the JSESSIONID has changed to pin it down to that backend server.
Back to pt. 3

Is there a way to convince Apache to note the value returned by Tomcat?
Answer: maybe if you try with tomcat session replication. I found this interesting post:

http://www.bradchen.com/blog/2012/12/tomcat-auto-failover-using-apache-memcached

.
You could try too with redis:

http://shivganesh.com/2013/08/15/setup-redis-session-store-apache-tomcat-7/

Let me know your experience please.


Is Surya Namaskar Considered Dynamic Stretching

Question: They say do Dynamic Stretching before run. Will train for Couch to 5K.
Is Surya Namaskar considered Dynamic Stretching?
Or are there other Dynamic Stretches to do in addition to Surya Namaskar?
Comment: If you are doing a stretch movement quickly and moving it is considered dynamic, static stretches are the stretches are those that require you to hold yourself in a specific position for ~20 seconds. Looking at those pictures I would say it best qualifies as static
Comment: @muntasiralam Very well then .... will find dynamic stretches
Answer: Yes, Surya Namaskar is considered to be dynamic stretching the poses are not held longer than 10 seconds and there is constant movement while stretching. If you are new to the sun salutations, do your first two rounds with cobra pose instead of upward facing dog.
You can include other dynamic stretches into your sun salutations as well to make it more effective for running. Banarasana (runner's lunge) is a great pose to include. Modify it with the back knee down for a deeper stretch in the quad or include a twist as well to warm up the spine further.
There are three different variations of Surya Namaskar (A, B, and C). This video on the different variations is fairly good.
Once you're comfortable with them, mix them up a bit to suit your stretching needs.  For a good warm up, try doing 2-3 sets of each type.
Answer: Surya Namaskar is considered static as well as dynamic.
It becomes static when you are holding any one position in Surya Namaskar for 30 seconds and when you are getting perfection and comfort in that particular position. If you are following fast rounds in surya namaskar before static positions, it does not make your mind to concentrate on body parts. Before dynamism, anyone should aware of what is exact position of hand, neck and leg. Only following this yoga practice for stretching without deep knowledge is not the good way for getting excellent outcomes. Surya Namaskar has huge importance in Indian Country. I am doing Surya Namaskar daily when sun arises (at least 21 rounds). Within 15 minutes I do 33 rounds. But at very initial days I followed it slowly.
In India, there is one 'Sanskrit' language shlok (sentence)--
"Sthir Sukham Asanam"
Meaning - The Asana position (yoga position) in which you body feels comfort and concentration (without getting pain) then that is perfection in that position.
After perfection in each step, dynamism can be followed. Fast Surya Namaskar helps in all the way and to all the body parts. It activates and boost your energy for any work. It strengthens your abs muscles, it generates flexibility in your legs and joints. Cobra position generates flexibility in your back. It reduces your tummy fat. It is excellent stretching because it contains 7 asanas (posturse), 12 positions (few asanas are repeated) in each round. It activates each and every body part. It generates cardio strength also. It is good stretching practice as compared to all.


Image texture showing up in texture mode but not render

Question: I'm fairly new to texturing in blender and I was trying to make an american flag and set up a whole scene for it, but when I went into render mode the flag appeared black. I've gone into the nodes and made sure that I have an image texture connected to the output and the correct image chosen. I've uv wrapped the flag texture to make sure it shows up correctly and I have a texture with the flag set up but when I go to render mode I only see black
Answer: You have connected the image texture directly to the surface output, but it needs to be connected to the Color input of a shader, for example a diffuse, like this.

World map courtesy of NASA, downloadable in several pixel resolutions from NASA Visible Earth
Below is the same material as it appears in the node editor, to make it clearer how to connect them.

In case your texture has an alpha channel, that doesn't get passed through the Color sockets. Instead it's passed separately through the Alpha socket. This can be used to mix multiple shaders, depending on the alpha channel, but to simply use it for transparency, connect it like this.

The logic here is, that when the alpha is 1.0 (i.e. fully opaque), the lower Shader socket to the Mix shader is passed to the material, and when the alpha is 0.0 (i.e. fully transparent), the upper Shader socket gets passed to the material, and anything in between, causes a corrsesponding mix of the two to get passed to the material.
Comment: @C.Pendergrass Glad I could help. You may also find these two posts on how to connect node sockets to each other useful: [What is the meaning of the color of the node sockets in the node editor?](http://blender.stackexchange.com/questions/33915/what-is-the-meaning-of-the-color-of-the-node-sockets-in-the-node-editor) and [How are color images used as input values in nodes?](http://blender.stackexchange.com/questions/32236/how-are-color-images-used-as-input-values-in-nodes).


Can patents be featured/explained in a youtube video i.e. are patent descriptions/images in public domain?

Question: Is it legally allowed to use images, which are obtained from patent forms, possibly with modifying/coloring them or left as is, inside youtube videos? In the same vein with images how about the text i.e., are patent description texts in the public domain so that the patent description can be contained in a video?
Answer: The government's position is that material in the patent documents is generally in the public domain
See Public Domain Copyright Trademark & Patent Information Schedule:

As part of the terms of granting the patent to the inventor, patents are published into the public domain.

And slightly more specific, see Terms of Use for USPTO websites: Patents:

Subject to limited exceptions reflected in 37 CFR 1.71(d) & (e) and 1.84(s), the text and drawings of a patent are typically not subject to copyright restrictions.

1.71(d) allows for the patent author to specifically indicate that some of the material is protected by copyright.
But that position has not actually been tested in courts
However, there is no actual statute explicitly exempting material in the patent documents from copyright nor any case law holding that in the absence of a notice, the material in fact becomes public domain. See Alderucci, "The surprising consequences of exempting patents from copyright protection (2016), at p. 13 and footnote 62.
Comment: @b.g. A shorthand is everyone treats it as public domain and there has not been enough pushback for it to ever be 100% settled.
Comment: @b.g. But if you don't credit the source, someone is just going to copy it without knowing that it is from a patent. You can do some good with minimal effort by stating the source to stop people wasting time.
Comment: So, unless explicitly stated in the patent description, by default, text and drawings are re-useable by others without even having to credit its source(as a consequence of being in the public domain)? Thank you
Comment: I would think the sensible approach would be to say that, for purpose of fair use determinations, all of the material within a patent may be deemed "necessary" for the purpose of discussing the patent within which it appears, but publication in a patent shouldn't imply that the material is public domain for arbitrary other purposes.


Does spanning tree make sense for DAG?

Question: Why cannot I find any information about spanning tree for DAG ?  I must be wrong somewhere.
Comment: Please flesh out the question a bit. Why do you think the usual definition does not make sense? Of course, you will have to decide wether the spanning three should have directed paths from a root to all nodes or not; that might make a difference.
Comment: Whether such a thing exists depends crucially on your definitions.
Answer: Why focusing on dags and not general directed graphs? I think you should have a look at the directed minimum spanning tree problem, also known as the arborescence problem (PDF). The problem can be solved using the Chu-Liu/Edmonds algorithm. The wikipedia entry is not as clear as I was expecting, but it does have links to the original papers.
Comment: Maybe you should edit the Wiki article? :)
Comment: First link is broken
Comment: The Chu-Liu/Edmonds algorithm requires specifying a single root whereas spanning trees do not specify a root.  There is nothing about directed graphs, whether cyclic or acyclic, that inherently constrains one to minimum solution with a single root, let alone that the root(s) is(are) givens.
Comment: @wanderingmathematician I checked the Wayback machine and that link has unfortunately been broken since it was first archived way back in 2015. I found some PDF lecture notes to link instead.
Answer: One generalization of a tree in a directed graph is an arborescence. It is a directed tree with all edges directed from parent to child.
Comment: What about some sort of Minimum Spanning Forest? I don't see a problem with making an MST for each root.
Answer: The case for DAGs is trivial, which might be the reason you cannot find any dedicated information about them.
First, there must be a unique vertex $r$ of zero incoming degree, which is the root of the directed spanning tree (arborescence). (Otherwise no spanning tree can exist.)
Next, for each vertex $v \neq r$ choose a parent vertex $P(v)$ such that the $w(v, P(v))$ is minimal.
We chose $|V| - 1$ edges in a graph of $|V|$ vertices with no cycles, so thus chosen subset of edges forms a spanning tree $T$. Any other spanning tree, for each vertex $v \neq r$, must have a unique incoming edge $(v, Q(v))$. By our construction $w(v, P(v)) \leq w(v, Q(v))$. Summing over all $v \neq r$ we get that $T$ is minimal.
Comment: What about some sort of Minimum Spanning Forest? I don't see a problem with making an MST for each vertex of zero incoming degree.

Does spanning tree make sense for DAG?


OS X: menu naming (File Edit View ... Windows Help)

Question: Apple suggests to stick with the known menu naming: File Edit View ... Windows Help. What to do if our application does not work on files directly, but rather on "projects" (a project may work on multiple directory structures)? Should I rename "File" to "Project" or use "File" though the menu items operate on projects?
Answer: I think you should use 'Project' because it more accurately reflects your user's mental model while they use your app.
A nice rule of thumb is to ask yourself how easy this is to document. If it's hard to document it is probably hard to use. Saying "to work with a project, first open the 'File' menu..." is clunky and, for me, falls into the "hard to document" category.
Answer: I would rename File to project then. Other Mac OS X applications do the same, however I have none on my mind right now.


Content Type - Folders

Question: I added a custom content type with the Folder parent, then added columns to display when adding that custom content type; however, the custom content type shows up under "New Document" and none of the columns I added show up. I'm very confused. Can you help?
Answer: add the columns to that view then it will visible to you,listsettings-->scrolldown you will find "Views"

Content Type - Folders


Literary term for "I picked up my suitcase and convictions and walked out the door"? (intangible and tangible items together in a list)

Question: All! I've been stumped with this question for a while now. What do we call it when you use an intangible object and represent it as though it is tangible, usually coupled with tangible objects? Example: "Both my bike and my pride lay crumpled on the ground." I know there's a specific name for this literary device. Any ideas?
Answer: You may be thinking of syllepsis. Meriam Webster defines this as:

the use of a word in the same grammatical relation to two adjacent
  words in the context with one literal and the other metaphorical in
  sense.

https://www.merriam-webster.com/dictionary/syllepsis
This construct may also be referred to as zeugma. Here is the opening sentence of Wikipedia's article on Zeugma and syllepsis:

In rhetoric, zeugma and syllepsis are figures of speech in which one
  single phrase or word joins different parts of a sentence.

https://en.wikipedia.org/wiki/Zeugma_and_syllepsis
There are several examples of syllepsis in ThoughtCo's article on the topic. For example:

When I address Fred I never have to raise either my voice or my hopes.

https://www.thoughtco.com/syllepsis-rhetoric-1692166
Answer: There are better and worse uses of this device (syllepsis). If you stick with complement phrases that are regularly used with the verb, you'll be ok - pride can be crumpled or crushed figuratively just as a bike can be crushed literally - it's the same with raise - we can raise our hopes or expectations, or raise our voice, but with other words like 'pick up' we can pick up the pace (figurative) or pick up an object, these are common expressions, but we don't  pick up convictions, it's just not standard use, though we might pick up an idea or an attitude. It doesn't mean it's wrong or even unacceptable, but it stands out as unusual use-case, to my ears anyway. You could test this on corpus sites like Sketch Engine. They take a little getting used to but you can quickly find common usage patterns using this tool.
Comment: Thanks for the tip about Sketch Engine. This is the first I've heard of it.


Boost not working on pages with ?_utma Google Analytics cookie in url

Question: Boost works on every page, but it doesn't work on pages with Google Analytics variable in the URL. For example, http://stratton.wanderlustfestival.com/?__utma=1.485948833.1348167330.1348796515.1348837057.31&__utmb=220.127.116.118837058695&__utmc=1&__utmx=-&__utmz=1.1348167330.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)&__utmv=-&__utmk=172983075 doesn't show  while http://stratton.wanderlustfestival.com does.
Answer: Boost saves the page cache in a file named after the page + the query string, the limit on the length of a filename is 255 in Linux so for pages with very long name/query string combinations caching will fail.
This should show up as an error in the Drupal log and if it does the solution is edit the .htaccess: Find the comment "Caching for anonymous users", after this you'll find a line "RewriteRule .* - [S=4]", replace this line with the following code:
RewriteRule .* - [S=6]

# If the page/query string combination is not over 255 characters, skip the following 2 rewrites
RewriteCond %{QUERY_STRING} "utm_campaign"
RewriteCond %{QUERY_STRING} "utm_medium"
RewriteCond %{REQUEST_URI}_%{QUERY_STRING} ^(.*\/)([^\/\n]{0,255})$
RewriteRule .* - [S=2]

# Custom Campaign Query
# Create file if not already boosted
# One campaign item is lost for file creation.
ReWriteCond %{QUERY_STRING} "utm_campaign"
ReWriteCond %{QUERY_STRING} "utm_medium"
RewriteCond %{DOCUMENT_ROOT}/cache/normal/%{HTTP_HOST}%{REQUEST_URI}_\.html !-s
ReWriteRule ^(.*)$ http://%{HTTP_HOST}%{REQUEST_URI}? [L,T=text/html]

# Boosted file for campaign (now with query string).
ReWriteCond %{QUERY_STRING} "utm_campaign"
ReWriteCond %{QUERY_STRING} "utm_medium"
RewriteCond %{DOCUMENT_ROOT}/cache/normal/%{HTTP_HOST}%{REQUEST_URI}_\.html -s
RewriteRule .* cache/normal/%{HTTP_HOST}%{REQUEST_URI}_\.html [L,T=text/html]

This makes sure that if the page/analytics query string combination is too long it will cache the page without query string instead, the first request will be redirected to the page without the Analytics query string, the next request will be served from cache and the query string will stay intact.
Note that this code is aimed at long url's with Google Analytics tracking variables only but can be (and possibly should be) rewritten so it can be used universally.
Answer: I ran into this issue in June 2016 after installing Boost on fairly busy site. Note that there are two related issues when requests exceed 255 characters: (1) Boost generates an error in the log and doesn't finish creating a cache file and (2) "orphaned" temporary files will appear in the boost cache (files beginning with "boost").
After studying the nice answer by @midnightMotion a while, I decided to try a fix in .htaccess
Using Boost 7.x-1.1, this is what I have added:
# If the page/query string <= 255 characters, skip following 2 rewrites
RewriteCond %{REQUEST_URI}_%{QUERY_STRING} ^(.*\/)([^\/\n]{0,255})$
RewriteRule .* - [S=2]

# No cache file - strip query and redirect to create
# One campaign item is lost for file creation.
RewriteCond %{DOCUMENT_ROOT}/cache/%{ENV:boostpath}/%{HTTP_HOST}%{REQUEST_URI}_\.html !-s
RewriteRule ^(.*)$ http%{ENV:protossl}://%{HTTP_HOST}%{REQUEST_URI}? [L,T=text/html]

# Cache file exists - use it
RewriteCond %{DOCUMENT_ROOT}/cache/%{ENV:boostpath}/%{HTTP_HOST}%{REQUEST_URI}_\.html -s
RewriteRule .* cache/%{ENV:boostpath}/%{HTTP_HOST}%{REQUEST_URI}_\.html [L,T=text/html]

Note syntax is adjusted slightly for a newer version of boost. I also adjusted the "skip" after the intial boost rule to s=4 to skip the new stuff above when boost isn't triggered.
This appears to have fixed the errors in log, the orphaned boost temp files, and the general problem of long uri + queries not getting a cache.
I'm not an htaccess expert my any means, so please jump in with suggestions.


Let's get thrashing

Question: I am looking for a correct form of the word: thrashing(?).
You say "let's get thrashing" when you move your hands violently while do finger painting. It the word correct?
Comment: It sounds very unusual to me, but I'm not very familiar with the kind of vocabulary a "finger-painting teacher" might use. More generically, *Let's get **cracking*** (and BrE *Let's get **stuck in***) are commonly used to mean *Let's **start/get started***. In the specific context, ***daubing*** might be an acceptable (if slightly "whimsical") choice.
Answer: Two minor issues with the choice of word thrash:

To thrash means to whip about uncontrollably and with significant or great force.  It usually implies both A) the thrasher isn't being careful or cautious and B) the thrasher might damage something.  Someone might think you are implying that the canvas could be damaged or you literally don't care if paint gets everywhere except the canvas.
When a person thrashes, if no instrument (e.g. specifying you are thrashing with something) or target/object is specified or implied, the default is the entire body.  So when you say "Let's get thrashing" it sounds weird.  As fingers/painting is not typically a violent activity, it can fail to provide enough context for someone to assume you mean just their fingers without thinking about it for a second.

There isn't a good word that's less stronger than thrash that really works for finger painting.  Whip, wipe, smear, etc. all won't sound right.
Finger-painting is a messy activity though, since you actually touch the paint, so something like "Let's get messy and start painting" may be better to say.

Let's get thrashing


Fourier transform and momenta flow in Feynman diagram

Question: Imagine that it is necessary to evaluate some (general) Feynman diagram in a coordinate representation (simply $x$-representation) and the only $x$-dependent part is given by some function $F(x)$. Then, having to re-express it in the momentum representation (simply $p$-representation), one has to perform the Fourier transform, i.e. to calculate $F(p)=\int\mathrm{d}^{4}x\,F(x)e^{-i p\cdot x}$. It is of course assumed that in this particular case there is no choice other than start in the $x$-representation and then move to the $p$-representation. The question is the following. It has been pointed out to me that the minus sign in the exponential corresponds to the convention that the momentum $p$ in the diagram is incoming into the vertex. Is it really true? Because I have never realized it and took the Fourier transform in my calculations to be of the form $F(p)=\int\mathrm{d}^{4}x\,F(x)e^{+i p\cdot x}$ (i.e. with the plus sign) although I have always considered the momentum to be incoming. Therefore - is there really a connection between the sign in the exponential and a convention of the momenta flow? If so, is my convention with the minus sign correct?
Answer: Short answer
you can choose whatever convention you want as long as you use it consistently.
What does it mean consistently? It means that, chosen a convention, you have to use it to compute ALL the quantities of your theory as well as ALL the solution of the differential equation you may encounter (like the ones you solve to compute propagators).
Long answer
Take a two-point function, where on the external legs the momentum is the same(because of momenta conservation) and we call it $p^\mu$.
Let us consider the following cases:
First case
The convention is the following :

All momenta are incoming with + sign.
The exponential of the Fourier Transform is with - sign for all incoming momenta.

The two point function in the coordinate space is a function $K(x,y)$ and assuming translation invariance, we can Fourier transform it as
$$
K(x,y)= K(|x-y|)=\int d^4 p \, d^4 q e^{-ipx}e^{-iqy} \, K(p,q) \delta^{(4)}(p+q) = \int d^4 p e^{-i p (x-y)}\,K(p)
$$
where $p$ is the momentum flowing from $x$ and $q$ the momenta flowing from $y$. Both are incoming momenta by convention and then they come with a minus sign in the exponential.
What about the case you cited?
Second case
The convention is the following :

All momenta are incoming with + sign.
The exponential of the Fourier Transform is with + sign for all incoming momenta.

The two point function then can be trasformed as
$$
K(x,y)= K(|x-y|)=\int d^4 p \, d^4 q e^{+ipx}e^{+iqy} \, K(p,q) \delta^{(4)}(p+q) = \int d^4 p e^{i p (x-y)}\,K'(p)
$$
Then, you can transform $p \rightarrow -p$ and you get
$$
K(x,y)= \int d^4 p e^{-i p (x-y)}\,K'(-p)
$$
and if $K'=K$, you see that the result may be different wrt the previous cases! What is happening?!
In general, $K' \neq K$ and this is because in deriving $K(x,y)$ you solved the differential equation
$$
-\int d^4 \,y\, \partial^2 \delta^{(4)}(x-y) K(y,z) = \delta^{(4)}(x-z)
$$
by Fourier transforming and you have to use the same convention also here!
In the first case, $K(p)$ is obtained by transforming with a minus sign in the exponential whereas in the second case $K'(p)$ is obtained using the plus sign. For scalar theory there is no problem since K(p) is an even function(the operator to invert is $\partial^2$ and this cancel any sign difference), so maybe this is why your results have been always correct.
The lesson to be learnt is then : you can choose whatever convention you want as long as you use it consistently.
Comment: Thank you for a very nice answer! My summary: if I would like to stick to my convention of the Fourier transform from x- to p-representation in the form $F(p)=\int\mathrm{d}^{4}x\,F(x)e^{+i p\cdot x}$, then the reverse one would read $F(x)=\int\frac{\mathrm{d}^{4}p}{(2\pi)^{4}}\,F(p)e^{-i p\cdot x}$, and then I am obligated to use the same convention also for the propagators, like the fermion one, which would in the x-representation read $S(x-y)=\int\frac{\mathrm{d}^{4}p}{(2\pi)^{4}}\,\frac{i}{\slashed{p}} e^{-i p\cdot (x-y)}$. Is all that correct, please?
Comment: Yes, exactly! and Welcome in StackExchange! You can mark the answer with the check if you are satisfied


SMTP Address re-use in Office 365

Question: About 6 hours ago I removed an SMTP address from one of our users using the Office365 admin portal. I want to attach this address to a new shared mailbox.
Trying to create the new mailbox with the address is just giving me
Email addresses need to be unique and this one is already being used by the pgreen.

I understand that there is a time period for the removal of the address from the original recipient to propagate but I thought that was measured in minutes, not hours. Certainly after 6 hours I'd expect it to be clear, so now I'm wondering if I've done something wrong.
Checking the recipient from the O365 powershell module show the address is no longer associated.
The address removed was firstname.lastname@example.com from the recipient pgreen.
PS> (get-recipient pgreen).EmailAddresses    smtp:firstname.lastname@example.com
SMTP:firstname.lastname@example.com
SIP:firstname.lastname@example.com
smtp:email@example.com
smtp:firstname.lastname@example.com
smtp:firstname.lastname@example.com

Email sent to the removed address [email@example.com] is returned undeliverable pgreen wasn't found at somethinggroup.com
I can't think of any other way to identify if/what/where is going on. Is it just a case of waiting it out?
Answer: Please add the alias email@example.com as an alias of the shared mailbox. Don't try to create the shared mailbox with that alias directly
Your default address policy must come block you
Once the shared mailbox is created with a dummy email address, add the one you want to use as an alias, then set that as the primary address and delete the dummy address.

SMTP Address re-use in Office 365


When I try to send a text on Mac, it highlights the message rather than sending it

Question: When I try to text on Mac, and I hit the return button to send it, it will highlight the message instead of sending it (as if I wanted to copy the message). Does anybody know how to help with this?
Comment: Welcome to Ask Different. :) Could you please edit your question to include the version of macOS you're running?
Answer: That is a strange, because pressing the return button in Messages on macOS 10.14.6 sends the message as you expected it to do.
Are you using the return key on a numeric keypad? That might be the reason?
The other option is to press ⌘+return which should send the message. Other than that, you can try signing in and out of Messages app on the Mac, and ensure message forwarding is enabled in the iPhone's messaging settings. I'll keep you posted if I find out any other setting to change it.
Edit
I just found that you can potentially make the return key act as send message by creating an app specific shortcut in Keyboard Settings. I'm not 100% sure it will work but give it a try?
Go to System Preferences->Keyboard->Shortcuts->App Shortcuts then add a new shortcut and select the Messages App from the Application Dropdown menu. Enter the exact menu command name Send Message. Now in the keyboard shortcut box it won't let you just press the return key, so you will need to press fn+return but it should display just return in the box.
Now go back to the Messages app and see if it will require you to press the fn key before pressing return or without the fn key? If not you can just remove that app shortcut from the keyboard settings.
I've attached an image what the shortcut setting looks like.


How do you change the default Pear/PECL build folder from /var/tmp?

Question: I'm trying to install a PECL package on Linux, but the installer never gets past the configure stage.
My hosting provider mounted a file system at /var/tmp that prevents files from executing, and that's causing this error:
root@host [/usr/local/apache/conf/includes]# pecl install pdo
downloading PDO-1.0.3.tgz ...
Starting to download PDO-1.0.3.tgz (52,613 bytes)
.............done: 52,613 bytes
12 source files, building
running: phpize
Configuring for:
PHP Api Version:         20041225
Zend Module Api No:      20060613
Zend Extension Api No:   220060519
building in /var/tmp/pear-build-root/PDO-1.0.3
running: /root/tmp/pear/PDO/configure
checking for egrep... grep -E
checking for a sed that does not truncate output... /bin/sed
checking for cc... cc
checking for C compiler default output file name... a.out
checking whether the C compiler works... configure: error: cannot run C compiled programs.
If you meant to cross compile, use `--host'.
See `config.log' for more details.
ERROR: `/root/tmp/pear/PDO/configure' failed

I'm 99% sure the problem is that you can't execute files in /var/tmp (I copied an executable file over here and tried it to verify).  I'm sure I can get PECL to complete if I have it do its build work somewhere else, but I can't find the appropriate configuration setting.
How do you tell PECL to build in a non-default folder?
Comment: Mounting /tmp (or that which links to it) as noexec is a false sense of security. `cd ~ && /bin/sh -c /tmp/badcode`.  .. too many hosts hire admins that do whatever Google says.
Answer: Whoa, why are you installing PDO from PECL?  PDO has been built in to PHP since 5.1.  Are you running 5.0?
If you're running 5.1 or higher and are missing PDO, chances are that:

Your copy of PHP was compiled without it, and must be recompiled; or
Your copy of PHP was installed from your OS's repository without the PDO module

If 1, you'll need to either recompile PHP from scratch, or compile just the shared module and copy it to the correct location.
If 2, you'll just need to install your OS's PDO extension.  Under RHEL and CentOS, try yum search php-pdo.  I don't know the proper syntax for calling the package installer for other distros, but the package will almost certainly include the string "php-pdo".  (Tip: If you got your PHP from your OS, they also might provide common PECL packages.  Try searching for "pecl" in their repositories.)
Finally, if you're running 5.0... for the sake of all that is good in this world, upgrade!  Read through the upgrade notes in the PHP manual first, just in case you're using old behavior by accident.
(Also, to reconfigure pecl, try pecl config-show to see what settings are available, followed by a pecl config-set ... to change a setting.  Do not try to install the PDO extension for a version of PHP newer than 5.0.x, it will break.  There's even a big fat box at the top of the PDO page on the PECL site.)
Comment: Thanks!  I ran phpinfo() and saw "./configure' '--disable-mbregex' '--disable-pdo'".  No idea why they'd have PDO disable...
Comment: If it's your host, they're insane.  If it's your OS distro, it's so they could compile it as a separate module for whatever insane reason they have.
Answer: The answers above may help the user, but not those that run into the "noexec" problem with the /tmp dir.
Here's how to pecl install [program] when it fails with "configure error cannot run c compiled programs" because you cannot execute stuff in the /tmp directory:
First remount the /tmp directory with the exec option turned on:
mount -o,remount,rw,exec /tmp
Second, install your pecl extention:
pecl install [extention name]
Third, once you are done you should set it back to 'noexec' using:
mount -o,remount,rw,noexec /tmp
Comment: I have this problem while installing memcached.  I've remounted the tmp and var/tmp directory but the error persists and I can't get the extension installed. CentOS 5.9. Should I open a question for this?
Answer: In my case /tmp is noexec, /var is executable.
main10:~# mv /tmp/pear /var/tmp

main10:~# ln -s /var/tmp/pear /tmp/pear

It solved the problem.


How to decrypt the username in Jmeter

Question: I am having an issue with an application, when am recording the flow in jmeter, username and password is coming in HTTP header manager in encoded format ("TWFuYWdlcjE6cGFzczEyMw=="), I tried to decrypt and parameterize the value by adding Authorization Manger which didn't resolve my issue.
Comment: this value is Base64 encoded, which is normally used in Basic authentication. If you use a Base64 decoder you can see the values you used. What is it you're actually trying to achieve with this?
Answer: If you want to see username and password in plain text you can use __base64Decode() function available via Custom JMeter Functions bundle which can be installed using JMeter Plugins Manager:

__base64Encode() function does the opposite thing:

Check out How to Use the Custom JMeter Functions Plugin for more information on the above and other custom JMeter functions.
So in order to parameterize the credentials you could:

Add HTTP Header Manager and configure it to send Authorization header with the value of Basic ${__base64Encode(Manager1:pass123,)} where Manager1 and pass123 could be JMeter Variables
Add HTTP Authorization Manager and provide base URL, and username and password in plain text - the Authorization Manager will automatically generate and add the proper Authorization Header to all HTTP Request samplers it its scope.

How to decrypt the username in Jmeter


Can there be resonance in electromagnetic waves?

Question: So, resonance is created in sound waves when a body is made to vibrate by an external force at its natural frequency, the result is a build-up amplitude and higher intensity of sound. Similarly can we create resonance in electromagnetic waves, to build up their intensity by applying external force.
Comment: Like a laser cavity?
Comment: You can also look up terms like "optical cavity", "optical resonator", "dielectric resonator", "Fabry-Perot", ...
Comment: @JonCuster I am completely unaware of this term?
Comment: see  http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascon.html
Comment: Look up 'cavity magnetron'.
Comment: microwave oven? (klystron).
Answer: Electromagnetic waves can be brought to resonate in a cavity (in both optical and Microwave/RF regimes) much in the same way sound waves resonate in an acoustic cavity. Subject to boundary conditions the wave (electromagnetic) has to end at the cavity wall and hence the cavity can only support integer number wavelengths of the frequency at which the EM wave oscillates. For example, a 1 dimensional cavity supports a mode described by the following expression: $$E(x) = E_0 sin(\frac{n \pi x}{L})$$

Image source: http://pcwww.liv.ac.uk/~awolski/Teaching/Liverpool/PHYS370/AdvancedElectromagnetism-Part5.pdf
You can see that only a half integer number of wavelengths fit in between the cavity walls in much the same way an acoustic cavity would have the wave terminate on either side if it were completely closed on both ends. The EM wave can be externally driven in a multitude of ways but if the cavity is lossless then the wave will oscillate to infinity without needing an external driving force. This is of course the same in acoustics. By continuously driving the system by say an external source at the resonant frequency the amplitudes of the cavity wave and the driving wave will add constructively and result in an increase in intensity withing the cavity walls.
Comment: great!! so I can relate the cavity for sound waves like that of a guitar, in which a string is plucked and the vibrations are set through the cavity into the air box and resonance is created...thanks for the answer!!
Answer: Plasma supports a rich variety of both electrostatic and electromagnetic resonances. There is a nice, succinct Wikipedia page listing the ones most commonly encountered in the field. Resonances in plasma occur not only due to applied electromagnetic fields, but also from fields produced by the charges making up the plasma itself. The possibility for feedback between the plasma and external fields means certain resonant modes in plasma are unstable, meaning that the amplitude of the wave grows in time. The is an electromagnetic analogue to shattering solid materials acoustically.


how to add html elements inside block and get data when saving

Question: i want to add few html elements call first name and last name,company name in top of user/register page and save that data when user click create account button..,how can i achieve that,i add html elements  in block as normal html tags like
but dont't kow how to get data...thanks
Answer: Does it have to be in a block?
If you want to add fields to the registration form and do something with the values when they submit, you really have to add them to the registration form itself, not in a block somewhere nearby.
To do this you can create a custom module that implements a form alter.
For example:
<code>/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function MODULENAME_form_user_register_form_alter(&$form, &$form_state) {
  // Add your fields.
  $form['firstname'] = array(
    '#type' => 'textfield',
    '#title' => t('First name'),
    '#description' => t("This is help text that will display with this field. you can leave it off if you don't need it."),
    '#required' => TRUE, // Leave this line out if you don't want a required field.
    '#weight' => -10, // This controls the field order. Smaller number is higher up the form.
  );
  $form['lastname'] = array(
    '#type' => 'textfield',
    '#title' => t('Last name'),
  );
  $form['company'] = array(
    '#type' => 'textfield',
    '#title' => t('Company'),
  );

  // Add your own submit handler to save the values.
  $form['#submit'][] = 'MODULENAME_custom_submit_handler';
}

/**
 * Custom submit handler for user register form.
 */
function MODULENAME_custom_submit_handler($form, &$form_state) {
  // Do whatever you need to save the values here.
  // Submitted values are in $form_state['values'] like this.
  $fname = $form_state['values']['first_name'];
  $lname = $form_state['values']['last_name'];
  $company = $form_state['values']['company'];
}
</code>
Now having said all that, is there some reason you want to do this custom?
It is possible to use a module that lets you add fields to the register form, like the profile2 module.
That module lets you add profiles (essentially a group of fields, similar to a node) to users, which can also be displayed on the registration form for the user to fill out at that point.
The module handles saving the data for you, along with a lot of other features, like views support.
Answer: Just go to the people category on your configuration page , click on account settings and select the the manage fields tab, and create your new fields. You don't need html elements to do that


Why do some Japanese verbs have different spellings?

Question:
I have been wondering, why some japanese verb have a variety of ways to spell. Does it matter which spelling I used or does the variety of spelling serve a purpose. And if it doesn't matter, then why do they have it because I don't think I have see something like this before in other language
Comment: Not your question but I don't think ひきうける means "to contract a disease"...
Comment: Related https://japanese.stackexchange.com/q/8315/45489
Answer: I think much of that is due to the shortage of standardization. That said, it serves some purpose.
送り仮名の付け方 tried to standardize it and succeeded to some extent for long. Many large newspapers and publishers have adhered to it. Before then, I believe there was no official rule and things were more chaotic.
The standard way, like 引き受けます, basically boils down to having kanji for the stem (the constant part in conjugation) and hiragana for the suffix (the variable part).

ひかない, ひく, ... → 引(ひ) is the constant part
うかる, うける, ... → 受(う) is the constant part

This distinction helps readers, because when the variable part is "hidden" in a kanji, it can create ambiguity and the reader may not know what is the reading the writer intends. If you write おりる as 降る, the reader might confuse it with ふる. (Although おり in おりる is the constant part now, the verb had forms like おるる historically.)
The non-standard way to write ひきうけます as 引受ます is economical in terms of the space it occupies. So it may be seen on an advertisement where you are charged per character or per square meter.
Answer: It doesn't matter.
Indeed, multiple patterns can be found in the Japanese language in the kun-yomi transcription of kanji, as in this problem.
This problem is caused by the combination of kanji (imported from China) and hiragana (invented in Japan).
Although many people are familiar with the kanji themselves, the use of the Okurigana is not precisely defined, and the Okurigana is used in various patterns.
No matter which pattern is used, the meaning is understood without confusion thanks to the kanji, so the word survives and has become a part of the Japanese language.


Algorithm for document retrieval in QA system

Question: I am working with question answering and machine reading comprehension system. I want to match questions and documents (around 100,000 docs) in database. I've used tf-idf but it accuracy is about 55% and I need to be at least 80%. Could you give me some advice?
Answer: I am still not quite sure how you are solving in the problem with tf-idf for a QA system. However, there have been lots of improvements that has been done in the QA domain over the years, with the usage of deep learning for natural language processing.
I would urge you to look at the following approaches that might help you to reach the accuracies you are looking for:

Bidaf Model that was used to solve SQuAD dataset.
BERT model that was used to solve the SQuAD dataset.

You have multiple open source implementations to train the network and use it for prediction. You could  also learn about them in the papers published by the creators of these networks.


Is it possible to make desktop visible behind drawn object using SDL2?

Question: I would like to draw an avatar over the desktop, something similar to those word 97 assistants, my idea so far is to make only the background transparent but I haven't had any luck, any suggestion? is it possible to make with SDL2?
Answer:
You could try SDL_SetWindowShape which is implemented but is not officially documented, probably because it does not work well on all supported platforms. It may work for major platforms though (Windows, macOS, X11/Mir/DirectFB)
Try this Windows-only approach: https://stackoverflow.com/q/4052940/2038264 The caveat is that graphics performance will be very slow since it involves coping GPU data to RAM every frame
Answer: Not with SDL directly. These sorts of things are generally accomplished by getting the DC of the desktop window and drawing to it using GDI. I'm not sure how well that technique works with the window composition manager of modern Windows.
You could also look at using SetWindowRgn to define a non-rectangular window. This imposes some pretty strict limits on the overall outline of your little avatar though.


When can one extend a flat family from a subscheme to the whole scheme?

Question: Is there a nice condition on a closed subscheme $Y$ of $X$ such that for every flat family $Z\to Y$, there is a flat family $W\to X$ whose restriction to $Y$ is $Z$?  In particular, I'm interested in the case when the closed subscheme is two lines in $\mathbb{P}^2$, or three planes in $\mathbb{P}^3$, or generally $n$ hyperplanes in $\mathbb{P}^n$.
Answer: I'm pretty sure the answer is not in general.
Take a Hilbert scheme which is reducible, for example, that of the Hilbert polynomial $3t + 1$ in $\mathbb{P}^3$.  This Hilbert scheme has two irreducible components - the one corresponding to twisted cubics and the one corresponding to degree 3 plane curves union a point.  The intersection of the these two components corresponds to degree 3 nodal plane curves with an embedded point "pointing out of the plane".  It turns out that both components are rational (actually this is pretty easy to see) and even that the intersection is rational.
Anyhow, take a line in the twisted cubic component that meets the other component (see III.9.8.4 in Hartshorne's Algebraic Geometry) at a point $[C]$ and a line through $[C]$ contained in the other component.  This gives you a flat family over the union of two intersecting lines in $\mathbb{P}^2$, but you will not be able to "fill it in" because the Hilbert scheme in question is not irreducible.
EDIT:  I realized that, in this example, the family extended to $\mathbb{P}^2$ doesn't have to be as a family of closed subschemes in $\mathbb{P}^3$.  Still, it cannot extend to a flat family in this example - just for a different reason.  The general fiber of the extension would be a rational curve, but over a line in $\mathbb{P}^2$ you would have a disconnected family.  This is impossible.
Answer: This is not going to help too much in your case, but we do have the following I think.
Suppose that we have a map $f : X \to Y$ which has a section (whose image we call $Y$ as well, as above), in other words $Y \subseteq X \to Y$ is an isomorphism.
Given a flat family $Z \to Y$, we can form $Z \times_Y X \to X$.  I think this should do what you want.  Of course, it won't help with $Y = $ two lines in $\mathbb{P}^2$.
Comment: Right, that's in particular how it works for one point in $\mathbb{P}^1$.  But just to elaborate on why it doesn't work for two lines in $\mathbb{P}^2$: there can't be a surjective map from $\mathbb{P}^2$ to the two lines, because $\mathbb{P}^2$ is irreducible and so must be its image.
Comment: Indeed, although people do sometimes do contractions of this sort in the analytic setting (I recall seeing this in Hodge theoretic contexts) if you are willing to work analytically and sufficiently locally.


CW Tags: need for them?

Question: In this question, the question of having a CW tag so that community wikis can easily be browsed through cropped up. Could this be implemented?
Answer:
This would be a meta tag. Which are strongly discouraged across all sites of the network.
There is no need for a tag to begin with. Use the Advanced Super Ninja Search Options:

All CW questions
All CW posts (questions and answers)
Closed CW questions
CW questions tagged "meaning"
All non-CW questions
etc.
Comment: +1 Ah! Great. [Silly dummy text as a filler, full of nothingness and nullity.]
Answer: Essentially, that tag would be considered a meta tag; even in the case it would be considered a meta tag, it would still be a bad tag. A tag describes what is the topic of a question:

etymology marks the question as being about the etymology of a phrase
adjectives is for questions related to adjectives
meaning is for questions related to the meaning of a phrase

If I would see a question tagged cw, I would get it's a question about Community Wikis, which would not be on-topic on EL&U.
Answer: That's just what I was looking for! I was sure there was such a tag when I was going to browse all wikis on the website, but then it turned out there wasn't. I say we make such a tag. It would be a bit of work, though, to do it by hand, though we could get started with that if our Overlords haven't the time to implement this ...

CW Tags: need for them?


Negative norm for bosons and fermions

Question: How comes negative norm is such a big thing for fermions that Dirac equation was chosen over KG, while the same kind of problem for photons is just ok? In fact, many books in QED do not even mention that the norm has to be indefinite to preserve covariance.
Answer: Those are two very different situations.
Dirac equation was chosen over KG only in the early days of QFT, when people haven't discovered second quantization yet. The problem was the apparent negative probability of observing a KG particle. Nowadays it is generally accepted that both KG and Dirac give consistent QFTs of particles with spins $0$ and $1/2$ respectively. All states in the 2nd-quantized theory are positive norm.
The negative norm in gauge theories is entirely different. It is a norm of a 2nd-quantized state, and cannot be given a physical interpretation. It is also not real, meaning that we don't actually get negative-norm states in the gauge theory where we have correctly accounted for the constraint equations (you don't need to justify Gupta-Bleuler by getting rid of the negative-norm states, it is just a quantum-mechanical way of imposing constraint equations. Those constraints are real, physical, exist in both classical and quantum theories and are crucial to the theory's dynamics. Negative-norm states are purely an artifact which shows up before the constraints are imposed. They don't show up in the actual theory.)
Comment: Interesting. Do you in general agree with the redefinition of the inner product in order to preserve covariance (e.g. Akhiezer, and Berestetskii. Quantum electrodynamics. No. AEC-TR-2876. 1986., Ch: "quantization of EM field")? The second quantized state, from what I understand, does not come out with indefinite norm, if one follows the original prescription. The quantization procedure had to be modified (solely for photon fields) to fit the Maxwell's equations. I am sorry if I am wrong, I am just testing my understanding.
Comment: I feel like they hit the topological effect in quantization procedure and didn't find a better way out rather than to hide it into the norm... I am sorry for my ignorance-related skepticism...
Comment: @MsTais Could you provide either a link to the pdf or a screenshot of the relevant page please?
Comment: I have added the link, but my version is russian. If it would be helpful, let me know and I will make a screenshot.

Negative norm for bosons and fermions


Resetting Menu / Desktop

Question: I installed Loki over an older installation (prior version of Elementary) where I only saved my home partition (everything else was wiped clean).  What is the best way to reset the desktop so it is back to the default?  I tried copying in the files from /etc/skel, but that didn't seem to work (menu sort of appeared but standard apps in the launch bar did not).
Answer: Do you want to reset only desktop settings or all programs settings?(including desktop)
If you want to reset all settings run this command and reboot:
<code>sudo rm -rf .*
</code>
Be sure to run this in your home directory
If you want to reset only desktop you should delete only some of this files, but i do not know which, you should wait a reply from a developer.

Resetting Menu / Desktop


Direct selection tool on multiple vectors

Question: Is there a way to select multiple vectors with the direct selection tool.
I have a bunch of buttons with rounded corners and I want to select all 4 points on one side of each vector and scale at the same time. I can do it by using the shift on one vector but not on multiple so far.
Answer: Sorry but there is no way to change different vector elements at the same time.
The only way to do this is to merge them into one vector shape layer.
Hope this helps.


What do the remaining columns (after column 2) in a Z table mean

Question: In the Z table , I can understand the first 2 columns. The first column is the Z value, the second column is Prob(X<=Z). But what do the remaining columns mean?
Answer: All of the columns are probabilities of the sort you mean.
The table is organized so that you can look up your z-values to two decimal places.
For example, to look up the value 1.42, you look down to the "1.4" row, and then across to the "0.02" column, and the value at the intersection is 0.9222, which is the area to the left of 1.42 in a standard normal density.
You can approximately work out the values for an additional figure beyond the third (i.e. beyond the second place after the decimal point) if you use interpolation.
For example, the area to the left of 1.428 is about
0.9222 + 0.8 x (0.9236-0.9222) = 0.9233
If you're old enough to have used log-tables, they're often organized the same way (and frequently with explicit additional information to speed up interpolation).
See the examples here and here which solve slightly different problems but illustrate the use of tables.
Answer: Look in the third column, fourth row.  The column header says $0.01$, and the row header says $0.2$.  That means $Z = 0.2 + 0.01 = 0.21$.  The value in that cell corresponds to $P(X <= Z)$, as you said, just at a finer level of $Z$s.


.NET wrapper for JIRA api?

Question: I'm working on a JIRA implementation and need to make use of the API.
Does anyone know of an existing .NET wrapper for the JIRA SOAP API?
Answer: In a Visual Studio .NET project, right click the project references and choose 'Add Service Reference', enter the URL of JIRA's WSDL descriptor (http://your_installation/rpc/soap/jiraservice-v1.wsdl), and Visual Studio will auto-generate a .NET class for accessing the JIRA SOAP API.
The parameter names aren't particularly meaningful so you'll need to refer back to the documentation quite a bit at first.
Comment: For anyone reading this, there is now a v2: http://your_installation/rpc/soap/jirasoapservice-v2?wsdl
Comment: And the docs mentioned are at:

http://confluence.atlassian.com/display/JIRA/Creating+a+SOAP+Client

http://docs.atlassian.com/software/jira/docs/api/rpc-jira-plugin/latest/com/atlassian/jira/rpc/soap/JiraSoapService.html
Comment: I needed ?wsdl not .wsdl. +1 though. I'd +2 you if you'd linked to the docs ;)
Answer: In Visual Studio .Net 2010 choose menu Project and Add Service Reference. In the dialogue choose advanced option. Then in the new dialogue choose Add Web Reference.
http://localhost:8080/rpc/soap/jirasoapservice-v2?wsdl where localhost:8080 is your installation of jira.
I did not get it to work in VS2010 with just Add Service Ref. It then only created methods without any parameters. For example the login method claims to have no arguments althought the [documentation](http://docs.atlassian.com/software/jira/docs/api/rpc-jira-plugin/latest/com/atlassian/jira/rpc/soap/JiraSoapService.html#login(java.lang.String, java.lang.String)) says there should be a user and a password parameter.  See this jira forum for more info in the matter.
I did not need to use the old program Wsdl.exe. But I do not get any intellisense of the parameters more then what type it is like:
this.JiraSrvTalker.createIssue(string In0 and,RemoteIssue In1)

To know that the first parameter should be a token from
this.LoginToken = this.JiraSrvTalker.login(JiraAdmin, JiraAdminPwd);

you have to check documentation.
Comment: agree that plain "add service reference" left me with useless parameter-less methods.
Answer: I was also unable to get parameters on the methods when using a service reference.  What I had to do was a strange 'add web reference' trick.

Add Service Reference
Click 'Advanced...', in the lower left corner
Click 'Add Web Reference...', also in the lower left corner

http://<servername>/rpc/soap/jirasoapservice-v2?wsdl

You will now have a 'Web Reference' folder under service references, with the <servername> as the name.
var jira = new com.myserver.JiraSoapServiceService();
var loginToken = jira.login("user", "pass");
var issue = new com.myserver.RemoteIssue();
jira.createIssue(loginToken, issue);

Comment: Alternative solution if you cannot use the 'Add Web Reference':
http://stackoverflow.com/a/22379843/766872
Answer: JIRA SOAP API has been deprecated. So use REST API instead.
Here is JIRA REST API .NET wrapper: Atlassian.Net SDK
Answer: As per this page https://developer.atlassian.com/jiradev/support/archive/jira-rpc-services/creating-a-jira-soap-client/remote-api-soap-examples, JIRA SOAP API has been deprecated, and as per this page https://developer.atlassian.com/jiradev/latest-updates/soap-and-xml-rpc-api-deprecation-notice, completely removed from JIRA 7.0+.
I would recommend to go with JIRA REST API.

.NET wrapper for JIRA api?


When to add a new table or new column

Question: I have looked at previous questions dealing with New table or new column and When should I use a new table? and a few others. I'm a little confused, not sure if my problem has a different need/solution; so here goes:
I have a table for users with a userid, name, email, etc.  This is used for both employees and for customers (at around 800 entries, it is split nearly 50-50).
I have a need to incorporate about 4-6 more columns for just the employees; such as manager, citizenship, office, etc.  These would only be used for the employees and not for the customers.
My question is: do I create another table for the employees with those extra columns or do I add those columns to the 'users' table and default to NULL (for customers) in each one?
I thought this was a different situation than the other questions, which is why I posted it.  If not, please let me know.
Thank you
Comment: At just 800 entries you could get either way in my opinion.
Answer: For proper normalization, I would split them up into two tables. It makes for a more logical design, and easier to maintain down the line if you want to make additional modifications.
Answer: If you follow a traditional entity relationship diagram customers and employees would typically be considered as two different entities and therefore would have thier own tables.
This would make sense to me in this case if a person could be both an employee and a customer at the same time. How would you handle that?

When to add a new table or new column


Prove that the transpose of a symplectic matrix is also symplectic

Question: I worked out with symplectic matrices that the transpose is also symplectic for the $2\times 2$ case since the algebra was easy and the determinant of the matrix just needed to equal $1$. [The expression for determinant is easy for $2\times 2$ matrices.] and the transpose has equal determinant. But what about the $n\times n$ case. Is it also true?
Answer: If $S$ is symplectic of size $2n\times 2n$, i.e., satisfies $S^TJS=J$, so is $S^T$. To see this, take the inverse of the equality $(S^{−1})^T JS^{-1}= J$ (because if $S$ is symplectic, then $S^{-1}$ is symplectic too).
Comment: Here $J$ is supposed to be the matrix of some non-degenerate skew-symmetric bilinear form with respect to some basis of a $2n$-dimensional real vector space. Then how can you even conclude that $J^{-1} = J^{\top}\ $? I have no idea.
Comment: What do you mean by take the inverse of the equality?
Comment: I mean, taking the inverse on both sides, e.g., if the equation is $C=DE$ then taking the inverse gives $C^{-1}=E^{-1}D^{-1}$, right ?
Comment: Ah I understand, thank you.
Comment: Thank you! You helped me get unstuck today!


Could there be a closer star to Earth than the Alpha Centauri triple star system, excluding the Sun?

Question: Do we know for certain that the nearest star to Earth, excluding the Sun and the theorized Nemesis, is the Alpha Centauri multiple star system, at +/-4.4 light-years away? Have we been able to identify every star that is <5 light-years from Earth or is there a possibilty a faint star could be nearer?  Are there stars that we haven't looked at yet that could be closer?
Comment: Or even better ;)  http://astronomy.stackexchange.com/questions/10124/is-it-possible-that-the-sun-has-a-binary-partner-the-nemesis-theory-that-has-e
Comment: @RobJeffries Close, but not totally a duplicate. I think this question doesn't rule out stars other than Nemesis.
Comment: HDE226868 Just add 2.5 magnitudes.
Answer: It's begining to look like there aren't as many brown dwarfs as was once thought:

NASA's Wide-field Infrared Survey Explorer, or WISE, has been turning up a new crowd of stars close to home:
...
Previous estimates had predicted as many brown dwarfs as typical
  stars, but the new initial tally from WISE shows just one brown dwarf
  for every six stars. It's the cosmic equivalent to finally being able
  to see down a mysterious, gated block and finding only a few homes.

Reasonably current data puts the current closest brown dwarf at 6.5LY (2 parsec) and less than 0.08 solar masses.
A nearer one could show up, but it's not looking likely.
Answer: It's pretty easy to put upper limits on how bright any nearer stars could be. As a rough guide, consider a survey like Hipparcos, which is complete to something like 9th magnitude. If there's another star closer than Alpha Centauri, the brightest possibility is if it's at the same distance.  Wolfram|Alpha tells us that a magnitude 9 star at Alpha Centauri's distance would have a luminosity of about 0.0004 times solar. Using the rough main-sequence mass-luminosity relation, $L\propto M^{3.5}$, that would give a mass of about 0.11 times solar. So already, you can see that if there's anything out there it's barely a star even if it's as distant as Alpha Centauri, and otherwise almost certainly a brown dwarf. I'm sure more recent surveys will place even stronger limits.
Admittedly, the catch here is that the brightness will be concentrated at longer and longer wavelengths as the mass decreases, so things could be a bit bigger on the ground that we may have only looked in the wrong band so far. But even so, I think the sum of present observations make it nearly impossible that there are any stars closer to us than Alpha Centauri.
Answer: In this answer you can find a calculation for how bright "Nemesis" would be at near-infrared wavelengths. This calculation assumed we were looking for a 20 Jupiter mass object with a similar age to the Sun at a distance of 1.4 light years (to fit in with the Nemesis hypothesis). The calculated magnitudes were H=14 and W2=8 (in the WISE infrared satellite system).
If we relax the assumptions and let the object be 4.4 light years away, we have to add 2.5 magnitudes to these numbers. i.e. H=16.5 and W2=10.5. Whilst the former is right on the edge of detectability in the 2MASS survey, the former is comfortably detectable in the WISE survey (limit is about W2=15.6 see http://wise2.ipac.caltech.edu/docs/release/allsky/ ) and the object would have a large parallax/proper motion. Whether this would be detectable might depend on the presence of a good "first epoch" image. In the case of the brown dwarf Luhman 16 at 6.5 light years (but which is maybe 40-50 Jupiter masses), well it showed up easily in first epoch 2MASS images (H=9.6).
Now, your question asks whether a star may have been missed. Such an object would be much brighter than the hypothesised brown dwarf above, or Luhman 16. The "industry-standard" models of Baraffe et al. (1998) suggest that a $0.075\ M_{\odot}$ minimum mass stellar object at 4.4 light years would have H=7.5. It is hard to imagine how such an object would have been missed, unless it has such a small proper motion that it has not moved significantly between the 2MASS survey in the 90s to the WISE survey in ~2010. This is unlikely (but not impossible).
Once Gaia results are published in 2017, it will have complete parallax data for all stars down to about V=19. This should include even the lowest mass M-dwarf stars out to around 10 parsecs (30 light years).
Comment: Sorry, I was assuming that a "brown dwarf" is a kind of star. I just checked Wikipedia, and it says that brown dwarfs are "substellar objects". Still, finding a brown dwarf closer than Alpha Centauri would be quite interesting.
Comment: The hypothetical Nemesis would be a brown dwarf in orbit around the Sun. A nearby brown dwarf that's *not* in orbit around the Sun would meet the question's criteria, and would not necessarily be "many magnitudes brighter" than Nemesis.
Comment: @KeithThompson If it was at 4.4 light years away it would be 2.5 magnitudes fainter than the hypothesised Nemesis object (which would be at 1.4 ly for a 20 Mjup object) as I stated. However, the OP asks about whether a *star* could have eluded detection - which on checking, as it turns out you are right, would not be "many magnitudes brighter", it would be about 2.1 magnitudes brighter at H. I have edited to give chapter and verse.


What is a good wide-angle prime lens for Nikon DX (APS-C) format?

Question: I am looking for a wide angle lens for my D90, between 12 and 18mm focal length, preferably a prime, because I like fast(er) and cheap(er) lenses.
Any recommendations ?
Comment: @Reid - I don't find this particularly open-ended.  Choices are fairly narrow in this range, and his constraints (fast, cheap) are stated pretty clearly. In fact, I think this is a better "what should I do" than the typical "I've tried X, Y, Z, prefer Q photography, what is the best?" as all those letters tends to be quite extraneous.
Comment: I think you need to give us more to go on. What sort of photography do you do? Why are you unsatisfied with your existing gear? What lenses have you considered? Open-ended equipment recommendation requests like this are generally considered off-topic.
Comment: OK, that's fair. I guess I'm underwhelmed by the question because the right answer seems like it would be a list of F-mount lenses between 12 and 18mm, which isn't a very interesting answer (and doesn't have good archival value, as that stuff changes).
Comment: Are you asking for a 12-18mm FF FOV equivalent?
Comment: @Alan - 12mm is extremely wide even on APS-C, so probably not.
Comment: I have to agree with Matt, I think this is a perfectly acceptable question with a fairly narrow range of possible answers.
Comment: @Reid yeah, I see both points. But I think it's just part and parcel of equipment-recommendation questions. Seasoning them with personal details doesn't necessarily change that.
Comment: @Alan, I am asking for a 12-18mm real focal length, aproximately 20 to 28mm FF equivalent.
Comment: You may want to consider renting a couple different lenses to find the one you like best.. Then go buy that one.
Answer: While I generally encourage people to consider primes over zooms, you're definitely in the territory here where zooms start to become more plausible.  Before getting into that, a recommendation:
Your best bet is probably a DX zoom in the 10-24 range. You'll lose a stop of speed compared to the 14mm prime (most are f/4), but they will be much cheaper (~50%) and a little wider.  I've never tried it personally, but the Tokina 11-16mm has a good reputation.
On to the comparison.  Primes, like you say, are typically cheaper, faster, and of higher quality.  But when it comes to ultra-wides, the situation changes a little, as ultra-wide primes have never been at the level of simplicity and well-corrected design that's been possible in the 35mm+ range, and zooms are rapidly improving.  Sticking to Nikon only, here's the interesting comparison:

 Zooms:
   Nikkor 14-24mm f/2.8    $1800
   Nikkor 12-24mm f/4 DX   $950

 Primes:
   Nikkor 24mm f/2.8       $425
   Nikkor 20mm f/2.8       $600
   Nikkor 14mm f/2.8       $1700

 Fast Primes:
   Nikkor 24mm f/1.4       $2100

So even considering only the 14mm area, the fancy, fast zoom is the same speed as the 14mm prime, and only barely more expensive (one downside: much larger).  Throw in even one other prime in the range it covers, and it's cheaper in absolute terms.  The DX zoom is half the price, only one stop slower.  Quality is harder to show in a chart, but Nikon says the 14-24 is as sharp as the primes in that range; pretty bold, but going on the review/testimonials I've read, they seem to be living up to that particular claim rather well.
Comment: The 14-24 is a stellar lens, the envy of a lot of Canon shooters (like the 85 f/1.2L is for Nikon shooters and the 24 f/1.4 until Nikon released their version).
Answer: Late to the party, but the obvious choice is the Tokina f2.8 11-16mm.
Cheaper, faster, and sharper (really!) than anything in that focal length except for the Nikon and Canon 14mm primes (which are rather expensive for most people).  Ken Rockwell did a fairly detailed review of it.  I certainly like mine.
Comment: I own one of it and it is a very good choice. I especially like the f/2.8 which allows to make pictures inside whitout a tripod (and just increasing a little the ISO if needed). I will soon complete the focal range with the 16-85mm from Nikon.
Answer: I believe there are no wide-angle primes specifically for APS-C cameras. There are zooms that start at 10 or 12 mm. The widest available primes are 14mm and those are full-frame and very expensive. I suggest you look into Nikon or third-party ultra-wide DX zooms. Some reviews can be found at photozone.de.
Comment: Suggest clarifying "no wide-angle primes for APS-C cameras" to "no dedicated DX primes". There are wide-angle primes for APS-C cameras; they just happen to have full-frame coverage, but will happily work on the smaller formats as well.
Comment: List of wide-angle DX zooms for Nikon can be found here: http://photo.stackexchange.com/questions/2231/sigma-10-20mm-f5-6-alternatives/2233#2233
Comment: @matt: True, fixed.
Comment: Well, there's also wide-angle primes designed for APS-C cameras. Pentax has three: DA 21mm f/3.2 Limited since 2006, the DA 14mm f/2.8 since 2004, and the DA 15mm f/4 Limited since 2009. Of course, that's no good for Nikon users. (There's the Samyang 8mm f/3.5 fisheye, but that might not count.)
Answer: Seems like a 14mm f/2.8 is a great way to go.

Nikon 14mm f/2.8D ED AF
Pro-Optic 14mm f/2.8 Aspherical

The Pro-Optic version is a whole lot cheaper if you're on a tight budget, but I would have to believe that the quality is a lot higher with the Nikon.
Answer: As of April 2, 2013, the answer is a Tokina AF AT-X 124 PRO DX II 12-24mm. (Tokina will introduce a newer lens this month) The DX II WILL autofocus with the Nikon D3100 and D5000, both are adequate cameras. Keeping in mind that an APS-C censor is already "cropped" from a full 35mm equivalent, wide is not as wide in real values, it's an equivalent in DX. ALL the reviews of UWA lenses I've read point to this lens for quality of build, and images. There really isn't an (affordable) prime lens for an APS-C (Nikon DX) other then a 10mm fisheye. Tokina 11-16 will not autofocus, Tamron is not quality in 10-24mm, and the 10-24mm Nikon is twice the price. Most reviews I've read by owners, and Ken Rockwell, who gives the best advice for me, the prosumer/better than average photographer but not able to spend like a pro type, say that their wide angle lens stays at the 10mm range on their camera with the Nikon 10-24, and they REALLY like having the 2 extra mm on the wide end, and image barreling (distortion at the edges) is normal and within acceptable limits, especially if you are able to correct for this in camera or on a pc. I, after much research will go for the Nikon 10-24mm, cause I wanna shoot WIDE and can afford the extra $400. Best for the money hands down the Tokina AF AT-X 124 PRO DX II 12-24mm. Found one used on craigslist for $350. I gave up my search for a prime.
Answer: If you do end up going with a zoom, look at the older Sigma 10-20mm and the Tokina 12-24mm.
Answer: I also looked into this issue for quite a few years, since my D70s. If for automatic focusing prime lens, the only option seems to fit this question in a reasonable price would be the old Sigma 18/3.5 or 18/2.8 lens. I did used the 18/3.5-af for a few times and it's not a bad stuff for its price, which usually around $150. The Sigma 18/2.8, however, I only saw MF
version once and I cannot confirm whether it has a af version or not. Yet the Sigma 18/2.8 seems to be even older and cheaper then the /3.5 lens.
Also if you have $200~250 budget then you can go and buy the Tokina ATX Pro-17/3.5 ASPH. It is a very fine lens and in fact I still keep the Tokina 17 now because it's good for full frame as well -- and it is the last 17 or 18mm AF prime lens in production until 2006.
If you expand you option a little bit then you can go for the Sigma 20/1.8, which you still have equivalent 30mm on aps-c camera, and its a HSM lens so it's faster. But it'll double the price up to $500 or more.
The only 14mm AF prime lens beside the legendary Nikon AF 14/2.8D-ED would be the Tamron SP-14/2.8 af lens. In fact the the Nikon 14/2.8 is acturally oem of the tamron 14/2.8 so they're the same optic and performance.. and also the same price.. It would be no necessary to spend $2000 on a APS-C camera prime lens.
Answer: I am in the same position now where I am looking to replace my Tokina ATX 116 - 11-16mm f/2,8 and despite you said you prefer primes you also said because it is cheaper.
The answer of ex-ms still applies and you can see the cheapest are the zoom and not the primes. Finding a (fast) prime 11mm for a low budget is even impossible. There is a prime Zeiss 18mm f/3.5 for around 1'300 $ but this a manual focus lens.
In 2015 Tokina released an upgrade of the ATX 116: Tokina AT-X 11-20mm f/2.8 PRO DX which seems to fit the best your requirements:

Very wide angle: fit your needs for 11mm and 18mm
Fast: f/2.8 is quite fast for such a lens.
Price: ~600 USD, not very cheap but not very expensive either

And to finish, the performances chart seems to be very good and sharp.
In the second hand market you can perhaps find some Nikkor like

Fisheye AI-s 8mm f/2,8
Fisheye Pre-AI 10mm f/5,6
AI-s 13mm f/5.6
AF-D 14mm f/2,8 ED
AI-s 15mm f/3,5

But I doubt the quality fits the current digital cameras (perhaps still OK for a D90) and due to the rarity of such lenses the price would not be so cheap. And as you can notice, there's nothing faster than f/2,8 for the range 12mm-18mm, then I think the best option is the Tokina.
You will save a bit of money for the 11-16mm, but you have a shorter focal range, but as I owned it previously I can say this was a very good lens for a DX cameras.


How to create background image for site

Question: I have just made background image for my web-site. But I'm confused - if user have resolution no similar mine, then he will see wrong image right? How can I fix this problem and what resolution is normal for browser?
Answer: They will see the correct image, but it will repeat itself.
You have two options:
Create an image, which looks nice when it's repeated. Then you just have to exchange the image.
otherwise you have to disable image repeating, so that the image is just displayed once.
see: http://www.w3schools.com/cssref/pr_background-repeat.asp
It usually looks nice if you define an appropriate background-color after the image ends.
Otherwise there are more advanced ways for full scale background images: http://css-tricks.com/3458-perfect-full-page-background-image/
The "normal" Screen resolution on webpages range from the Smartphone screen (320x480) to 30" screens (2.560 x 1.600). Those are the both (not so uncommon!) extremes. The average is between 1024x768 and 1920x1200.

How to create background image for site


Why is it that every fusion in Dragon Ball Z and Super has never won?

Question: Fusions like Vegito, Kefla, Zamasu (fused), etc. never won. The only fusions that have ever won were the ones in the movies, that's because they're supposed to win because it's a movie, but what about the show?
Answer: Not sure if you are talking about the fusions of saiyans only, because you mention Zamasu, who is the fusion of a saiyan body with a supreme kai. If you are talking about fusions in general, Buu fusions have won battles (the original fusion against Vegeta, super buu against Gotenks Buutenks against Gohan, etc.). Piccolo is a fusion and he has won battles (against Android 20 for example). Cell is a fusion and he has won battles.
Regarding saiyan only fusions, Vegito had totally the upper hand against Buutenks, he just let be absorbed for trying to save Gohan and the others. I think, and this is only my opinion, that they don't let saiyan fusion win for narrative purposes, because if they do, the original non fused characters would be useless I guess.
Answer: Mostly because the villains in the show are more powerful, and if they use more power while in fusion like against Zamasu, they ran out of time.
Even with Buu, Vegito was stronger, but being stronger than someone and beating them are two entirely different things. If both Buu and Vegito were to fight, the battle would initially be lopsided in Vegito's favour with Vegito easily overpowering Kid Buu. However, the problem arises when the battle is prolonged. The problem with Buu is that he is extremely resilient. You just can't take him down easily and the biggest flaw of Vegito is that he likes to toy with the opponents.


Why are most sounds dubbed in afterwards for movies, instead of being recorded live?

Question: I've heard that sounds tend to be dubbed in afterwards, rather than being recorded live. Everything from walking to opening and shutting doors. What is the reason for doing this?
Comment: I'm not an expert but the logical reason would be the editing of the video, which would have destroyed the sound synchronization of the sound was pre-recorded.
Comment: It's so obvious,If it recorded live, how the sound engineer and musician could do the audio effects?
Answer: From the wikipedia page on Foley Artists.

Foley artists look to recreate the realistic ambient sounds that the film portrays. The props and sets of a film do not react the same way acoustically as their real life counterparts. Foley sounds are used to enhance the auditory experience of the movie. Foley can also be used to cover up unwanted sounds captured on the set of a movie during filming that might take away from the scene at hand, such as overflying airplanes or passing traffic.

Here is a great website on how it's done.
Comment: I was going to say the same thing. Good answer +1
Answer: The main reason is really cost, plus you can get far better results in a Foley stage.
Recording sounds live is very expensive and difficult to get each of the sounds independently so that they can be properly mixed later. Say that you want to record the sounds you describe live where there are footsteps and a door shutting. You would have to record two separate sounds for each to allow for later flexibility in mixing the sounds. Furthermore you have the entire crew (who would need to be very quiet) waiting around while the sound guys do their work. Alternatively, they= sound crew could come back after the main crew leave and record the sounds, but this require access to the location for a longer period of time (and again results increasing the cost).
Or consider the case of a restaurant scene where the main characters are engaged in a conversation. When shooting such scenes, all the background people pretend to be chatting and eating but they have to refrain from making any background noise such are knives hitting plates. These sounds are added later which ensures that they are at the appropriate mixing level and the appropriate time so as to not distract from the dialog
Furthermore, a lot of sounds are added that were not part of the scene that was shot, for example outdoor ambiance: The sounds of crickets are added to indicate that this scene is a t night -- even in cases where such sounds would not exist as is the case for highrise in New York for instance.  Or African jungle sounds are added to indicate that one is in the wild, even though the scene is in Vietnam for instance.
There is one notable except to this and that is when recording background ambient noise or "room tone".  When shooting on location, there is always some background noise. In every day life we get used to hearing that and don't even notice it. At some point during the shooting of a scene the background noise will be recorded in case it needs to be mixed in later. This will involve all the normal sounds emanating from the film crew and the background noise. So for instance if the air conditioner went on in an adjacent room for one take of a scene, but was off in separate take of the same scene, this would be very noticeable when the scene was cut together.  This room tone recording would be used to smooth out the background noise so that the ambient noise would not be too jarring.
If you really want to get a feel of how much work goes into the sound design, you need to close you eyes and not look at the picture.  One of the best examples is the opening scene in Apocalypse now.  Try it for yourself and see. Well, not see, but you know what I mean.
Comment: Another thing to note from a technical standpoint is that the boom mics used on sets are highly directional in order to get a clean feed of the actors; by design they have a tight pickup pattern (super- or hyper-cardioid) meaning they have a high level of off-axis rejection (essentially, any sound not approaching from directly in front of the microphone will be reduced in volume a LOT). This means you can get great speech, but on-set, live diegetic sound recordings are virtually ruled out by this. In some blockbusters, set vocals are only used as a guide and the entire film is ADRed & foleyed.
Answer: The typical big budget movie has 50–150 people standing just outside but close to the shot:

actors not in the shot
stand ins
stunt performers
extras
cinematographer, camera crew, dolly crew
script supervision
lighting crew, grips, electricians, light controller
hair and makeup
wardrobe and assistants
props
greenery
set supervisor
continuity
capturing sound (!)
stunt supervision
assistant directors:  1st, 2nd, and 3rd
production assistants
behind-the-scenes photographers
still photographer
runners
producers and studio execs—usually by the director at the video tap monitors
etc., etc.

Further away are tradesman who do

set construction:  carpenters, finish, set construction supervision
set painting
riggers
equipment movers
more electricians and a generator operator
plumbers
drivers
and a huge set of electrical generators

Someway into calling the roll, there will be a call for quiet—possibly including briefly ringing bells and/or flashing red lights coming on around the sound stage—at which time all hammering, drilling, pounding, sawing, etc., air compressors, and air conditioning are silenced.
The transition is profound—from total bedlam to near silence—but there are still dozens and dozens of people breathing, drinking, chewing, or moving—much more slowly than normal—and further away talking quietly (if they dare).  All that activity adds up to background noise that even the most directional microphone can't help but capture.
Then there is the inevitable unavoidable sounds:  actors (or the camera) on turntables for revolving shots:  the turntable always makes some creaking and grinding noises.  Or fake elevator doors have the wrong sound.  Or the subfloor on a sound stage is always two layers of 3/4 inch plywood over a frame 4 feet above the floor.  Even if the flooring is granite, an actor walking over that just doesn't sound right.
The general acoustics of a sound stage also aren't conducive.  A conference room won't really have a ceiling:  it is open or missing for lighting and perhaps so the boom folks can reach in to get a good microphone angle on an actor.  The windows aren't really there—to avoid troublesome reflections, and so the acoustic ambiance of almost any onset room will sound slightly wrong.
For comedy, most directors and actors prefer capturing the sound live for its more spontaneous and natural effect.  But for drama, it is easier and more compelling to ADR and/or foley in a soundproof room with only one or two people present to eliminate all the subliminal distractions.
Also, films which are dubbed into other languages need to have the set's apparent acoustic ambiance separate so that its sound can be layered with the dubbed audio.
Answer: An issue not particularly mentioned in other answers is that films are often edited to change the camera POV without changing the audio POV, and this is often done by combining pieces of material that were shot at different times (and sometimes different places).  If background sound is added later, it can be played smoothly across the camera cuts, making it seem as though the film is cutting between material that was captured by multiple cameras simultaneously.  If the film had instead used whatever background sounds happened to be present during each take, it would be hard to avoid jarring changes in that sound.
In cases where few of the ambient sounds tie in closely with any actions in scene, it may be practical to shoot a master shot with live audio, and then insert imagery from other shots while using the master-shot audio throughout the scene.  Any time something in the master shot audio needs to coincide with on-screen action, an editor can cut to the master shot.  This approach really limits what an editor can do, however, and it doesn't work very well for dialogue.  Unless an actor's timing in a close-up matches the timing in the master shot, it will be hard to make the actor's lip motions match the sound of their voice.  Using after-dubbed background sound gives the editor much less flexibility and makes it much easier to ensure that everything lines up smoothly.
If a scene features a pile driver in the background that is continuously pounding once per second, but it isn't visible in most shots, dubbing in the sound in post-production would mean that the editor could insert and remove arbitrary amounts of time before/after shots where the pile driver isn't visible, provided that only whole numbers of seconds are inserted or removed between shots where it is is visible (and even that might not be needed if there are two or more master shots available, and the pile driver hits occur at different times relative to the dialogue).  An editor may be able to start with one pile-driver shot, cut away to a close-up, and then cut to a different pile-driver shot, without creating any apparent discontinuity.


Can I change all tires to use different sidewall ratio?

Question: So I'm needing new tires, and I'm looking at some times that might better suit my needs, and while they're same width and rim specs, the sidewall ratio is different. Current is 235/50 r18 and the one I'm looking at is 235/60 r18. I'm in an AWD Volvo V60 CC. I understand I don't want to run a tire with a different ratios mixed in, but if I change them all at once, could that be OK?
Comment: What range of sizes do Volvo specify for that car & rim?
Answer: I would advise against doing this. This will remove 6.4% from your speedometer reading. Usually speedometers show 5% above your current speed with the stock tire size. By removing 6.4% from your speedometer reading, your speedometer will become 100% illegal, because it shows a speed below your actual speed.
Not only that, but the tires are 6.8% larger. There may not be enough room for the larger tires.
Usually you'll find that you can change the aspect ratio if changing the width too. For example, 235/50R18 is nearly the same as 195/60R18 when considering the tire diameter. Similarly, 215/55R18 might be fine too. When doing this change, you need to be certain that the rim can accommodate the tire width.
Comment: In all places I'm familiar with, it's illegal to drive above the posted speed limit.  the law doesn't care about the speedometer.
Comment: Thanks, that's what I was afraid of. And the car manual only has alternatives based on the different rim sizes they offered, so no go for this. *sigh*  Ah well.


No resonance in Anti Aromatic compounds

Question: My teacher commented on Anti aromatic compounds as being less stable compounds as it doesn't undergo resonance inspite of having π-bonds/electron pairs in conjugation. I didn't understand why so. Can you help?
Answer: As according to Molecular Orbital theory , when anti- aromatic compound is formed, the electron goes to anti bonding, and is unpaired even, which reduces it's stability.,
Or we can say that in anti- aromatic,the plane of electron cloud twists there bonds between carbons, making it unstable, thus resonance is not possible.
Antiaromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n electrons in it. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions.
Greater the delocalisation energy(resonance energy) of a compound, more stable it is. The resonance energy can be calculated using Huckel Molecular Orbital Theory(HMOT). Antiaromatic compounds have zero resonance energy hence are unstable.
According to HMOT,
Delocalisation energy=( Pi electron binding energy) - (total energy of isolated double bonds in a classical structure)

No resonance in Anti Aromatic compounds


Do number of reflections depend on frequency?

Question: Consider there is a transmitter at A and receiver at B and transmitter is transmitting radio waves. Further there are obstacles of different sizes in between A and B. Now if we increase the frequency of operation will there be more reflections in B. This assumption comes form the fact that for smaller frequencies some obstacles may not reflect to B.
Comment: obstacles... and apertures formed by the obstacles.
Comment: @rackandboneman sorry dd not understand what you meant? what is your conclusion?
Answer: Many aspects of a transmitter to receiver path change with frequency.
The first and most obvious is the size of the Fresnel Zone between them. This will affect how many obstacles need to be considered as being 'near' the transmission path.
For objects whose size is comparable to the wavelength, there may be resonance effects which change the reflection of that object strongly with change of frequency.
The path difference between off-axis reflectors and the on-axis main beam will vary with changing frequency.
The situation is so complex when there are obstacles that links are usually set up so the Fresnel zone is clear of them. When there is significant obstruction, measurement is necessary to assess the range of link conditions that will be experienced, no operator would accept such a link based on modelling alone.
Comment: Thank you for your answer, but it did not answer the specific question about reflections. I understand to see the difference in number of reflection the frequency difference may be more? Any comments?