Why can I blow up other potentials at the origin but not the one inside a dielectric sphere in an electric field?

Question: While solving the electrostatic field for a dielectric in an electric field, we take the potential at origin to not blow and thus, we eliminate the inverse powers of r in the expression for general potential for azimuthal symmetry. Why is that valid :- we can have bound charge inside the sphere at r = 0 and which can contribute to infinite potential at the origin by the usual electrostatic laws. Does this depend on whether the sphere is homogeneous or inhomogeneous ? There are potentials which we do blow up in our models at the origin, is there any specific constraint in blowing up the potentials inside the materials when we are considering only limited set of ideal conditions and nothing pertaining to ionization or maximum electric field for stability of the system has been mentioned.
Answer: The validity of setting the electrostatic potential to a certain value at a certain point comes from the fact that the electrostatic potential, like the potential, is defined up to a constant. The numerical value of this constant does not change the physics, but can sometimes make expressions nicer.
The potential should usually be finite everywhere, except in some limiting situations. The case of a point charge is one of such, because to have a point (of infinitesimal dimensions by definition) carrying a finite charge would require an infinite charge density, leading to an infinite energy and to an infinite electrostatic potential. You can avoid these pathologies defining the electrostatic potential by its value at infinity (instead that by its value at the origin where the charge is), and considering the resulting expression $-Q/r$ to be valid only outside the point charge, i.e. for $r \neq 0$. In this way we ignore the immediate proximities of the charge, meaning that we are not interested in probing its internal structure. When this is not true, and we care about how the charge is distributed in the body, the point charge model simply is no more suitable and something else should be used.
Answer: When you have a uniformly charged dielectric sphere, there is no finite charge at the origin. Instead, you have uniform charge density, so the total charge inside of a sphere of radius $r$ is
$$q = \frac43 \pi r^3 \rho$$
where $\rho$ is the volumetric charge density, $\rho = \frac{Q}{V}$ where $Q$ is total charge and $V = \frac43\pi R^3$ is total volume.
Now you can see that as you approach the origin, the equation does not "blow up" - the force that you would experience goes down linearly to zero, instead of going asymptotically to infinity.
As long as you don't treat charge as a finite point charge (where you have a number in the numerator that remains finite, and a denominator that goes to zero) but instead as a charge density (which doesn't have to be uniform as long as it doesn't have a delta function anywhere) the above analysis (or a variation) is valid.


Do cat / pet sanatoriums exist?

Question: I just discovered that my kitten has FeLV. Because this is no longer a death sentence for her, I decided to keep her. She is an indoor cat with no external feline contact, and she is otherwise healthy and happy. But the bias on FeLV is to euthanize around here. It got me thinking, are there places like the old sanatoriums for tuberculosis, consumption, etc., or even leper colonies, but for sick but happy cats or other pets? Where they can live out their lives without being a contagion threat, and with the needed extra care? Anywhere in the world, I ask out of pure interest...
Answer: In german speaking countries (Germany, Switzerland, Austria and probably the Netherlands, too) there is the concept of "Gnadenbrot" (literally "bread of mercy"). It stems from medieval times when old people who were too old to work but had worked well in younger times still received payment in the form of bread.
This concept is now applied to farm animals who are too old or ill to work and would usually be slaughtered. These farms are called "Gnadenhof" (farm of mercy) and are usually quite small, ran by environmental or animal rights activists and absolutely dependant on donations. There are extremely few of those farms in Germany.
And there is the fact that animal protection laws in Germany forbid any shelter to kill animals for any other reason than a severe, incurable illness or abnormal behavior that causes suffering for the animal.
Comment: Is there something similar for pets and strays? And with those laws, what usually happens to unwanted pets? The pressure on shelters must be huge....
Comment: @HenryStone Stray dogs are almost non-existent and stray cats mostly live in rural areas where no-one reports them. Every pet gets an ID chip implanted at their first visit to a vet, so you can find the owner if you find an animal. The pressure on shelters is huge indeed, especially after christmas and during summer holidays, when many pets are abandoned. But there are many people who want to adopt pets from shelters as an act of charity, because they're cheaper than puppies or because they want an older and calmer pet.
Comment: That just sounds amazing. Maybe my network should get German contacts, to help place a few dozen lovely (previously) homeless cats...

Do cat / pet sanatoriums exist?


Fluid Mechanic Buoyancy problem. How to determine the height?

Question:
I tried to solve it but I don't know how to bring the Area A in this equation. I used the area of the small vessel, but this one shouldn't be in the equation. Thanks a lot
Comment: The question text says density, mass and area are given - where?
Comment: Isn't the weight of the water displaced equal to the mass (added) of the floating object? In that case, the weight of water = density*H*(A-a), where a is the cross-sectional area of the floating vessel.
Comment: $\Delta H$ is not the increase in depth of the small cylinder (h3-h2 in your image). $\Delta H$ is how high the fluid in the large cylinder rises. Since the total volume of the fluid in the large cylinder remains constant, and the small cylinder is submerged an additional h3-h2, you can calculate how high the water rises in the large cylinder. Does that help?
Answer: This question is asking you to describe how buoyancy works using cylinders because their volumes are easy to calculate.
First, some things we know:

to be buoyant means an object has displaced a mass of fluid equal to it's own mass.
density is expressed as mass/volume, so if you know a fluid's mass and density then you can find its volume by dividing mass by density or $\rho = m / V$, therefore $V = m / \rho$
you know the mass of fluid being displaced - it's exactly the mass being added to the inner cylinder
the volume of a cylinder is its cross sectional area multiplied by its height, or $V = A \cdot H$

If you're with me so far then putting all this together for your question looks like this:
$$\begin{align}
V_{water} &= \dfrac{m}{\rho} \\
A_{total} &= A_{outer} - A_{inner}
\end{align}$$
And given #4 above:
$$V_{water} = A_{total} \cdot H_{\delta}$$
Dividing by area to solve for change in height:
$$H_{\delta} = \dfrac{V_{water}}{A_{total}}$$
And substituting known values:
$$H_{\delta} = \dfrac{\frac{m}{\rho}}{A_{outer} - A_{inner}}$$
Did that explanation help you understand where area fits in?
Answer: Since the mass of the fluid and its density are constant, the volume of the fluid is constant. Use that to find a formula that relates the change in height of the fluid on the side of the small cylinder to the change in height of the fluid on the side of the large cylinder.
If the small cylinder lowers by 1 unit, the fluid must rise on the big cylinder by (Aouter-Ainner)/A inner. So the level rises on the inner cylinder by 1 + (Aouter - Ainnier)/Ainner.
Thus the change in the level of the outer cylinder is [(Aouter-Ainner)/Ainner]/[1 + (Aouter-Ainner)/Ainner]. This can be further simplified with algebra.


Maintaining "Save to" folder for print-to-pdf files

Question: I need to document my work by printing files/pages to a PDF which creates an emailed link in my Microsoft Outlook inbox.  I open the newly created PDF file via clicking a link (not an attachment) and then save them to a shared drive.  The problem is, each time I go to save a new PDF, I have to navigate through all my subfolders to find the ultimate location that I wish to save to.  How can I make it so that it defaults to the last place I saved to?
Answer: Map a drive letter to the network location you save the files to.  That way instead of having to navigate through the sub folders you can just select the drive letter.


Small Windows Form Chat application ideas?

Question: I have a small application that I am building a Chat application into, so far I have the functionality to post messages!
I don't need a login as there will only be a small number of users and I will use their windows username for identification.
Has anyone done anything similiar? What else do I need to add? It doesn't need to be all singing and dancing as it is just for use within a small development team.
Any ideas would be great!
Thanks
Answer: Chat Histoty ;)
SSL enabled communication
Requirement : It should not add unnecessary vulnerabilities to the system, Since it a related with Windows Authentication + There are open ports So there is a good chance of going something wrong.
Answer: Couple Thoughts:

System Tray Icon, easily hidden/restored, etc.
Since you have the Windows Accounts, send them an email (if you have exchange/ad email integration), if they're offline.
Auto-start on windows startup
Click-once / Auto Update
Web-based and/or Mobile-based versions.
Copy/Paste (with ability to paste images)
Not sure if it is useful, but it'd be cool: sending a message to a given "bot" ... fake username... such as a Distribution Group Name, sends an email to the Distribution Group and then emails sent to that distribution group get posted to that conversation
"Show other user my screen" -> mini screen-cap, etc.
Source-Control / Bug-Tracking Aware.

Just some random thoughts, let me know how it turns out.
Answer: filesharing, webcam, auto away, timestamp!(this is where Live Messenger fails big time ;)
Answer:
searchable chat history - searchable from the desktop search
ability to chat to multiple people in the same conversation
status notifications for /away or /busy
Answer: It would be useful to bring new messages to your users attention without stealing focus, perhaps implementing a hook into the Snarl messaging program which is similar to Growl on Mac OS X?
This does mean that you would need to have the Snarl system installed on each of your workstations.


How do I run .NET programs?

Question: As .NET is OS independent how can I run programs built/compiled on .NET 4.0 ?
Can I develop software in Ubuntu for .NET 4.0 [if yes, what i need to download, link] ?
Comment: Right now, not every Feature of 3.5 and 4.0 is supported by mono. Hope you don't need WPF or stuff like that.
Answer: Overview
Mono provides the core compatibility API for the .Net framework.
The version in Natty (v2.6.7) has support for both Visual Basic & C# version 2,3 & partial API support for v4.
This also includes a VB .Net compiler & the runtime compiler.  The other members of the family, j# and f# are not supported in terms of development.  Obviously - their compiled variants are byte compatible with the rest of the .Net family.
Windows Forms up-to version 2 is supported - source
The next version of Ubuntu - 11.10 - will come with mono 2.10 which has virtually complete .Net v4 support
Mono Example Usage
An example of a .Net application which I use that runs without issue on Ubuntu is Keepass 2
You run the compiled .exe [it was created under windows] using mono i.e.
<code>mono Keepass.exe
</code>
Developing
To develop under Ubuntu, I would have a look at the Mono Develop project - an IDE based environment - search for MonoDevelop in the Software Center.
Installing
Take a closer look at the suggested Add-Ons - choose the Add-Ons in the area that you will be developing in.
Comment: It should be noted that .net 4.0 support was added in mono 2.8, and the current ubuntu has 2.6. So there is no .net 4.0 support in any released ubuntu version. mono 2.10 has however made it into ubuntu 11.10 which is not yet released.
Comment: @trampster - many thanks - have updated the answer.
Answer: The Mono project offers a compiler and class library for C Sharp compatible with .NET 4.0. The compatibility improves with later versions. Ubuntu contains 2.6.7, I believe, which is the latest long-term stable release. Mono 2.10.2 is the most recent release and has various improvements.
For development, install monodevelop. Ubuntu offers MonoDevelop 2.4. You can grab MonoDevelop 2.6 beta 3 if you're adventurous -- here's a script that installs both Mono 2.10.2 and MonoDevelop 2.6 beta 3.
Comment: what about VB.NET ?
Answer:
I installed using winetricks
<code>$ sudo apt-get install winetricks
$ winetricks dotnet40
</code>
Copy the file they say in dotnet40 to download to the folder they say
<code>$ cp gacutil-net40.tar.bz2 ~/.cache/winetricks/dotnet40/gacutil-net40.tar.bz2
</code>
Run winetricks again
<code>$ winetricks dotnet40
</code>
Then run your apps with wine
<code>$ cd .wine/drive_c/Program.../App
$ wine APP.exe
</code>

How do I run .NET programs?


Are Nassi-Shneiderman diagrams actually used?

Question: I learned about them in a Structured Programming course, but never saw them used thereafter either at the analysis phase or for documentation purposes. Not even for highly structured languages like Pascal (Delphi).
Does any of you actually use Nassi-Shneiderman diagrams? If yes, what tools do you use to create/maintain them?
edit:
Or have you never heard of them?
Comment: Never heard of 'em. Though I'm not formally trained in programming.
Answer: Heard of Nassi-Shneiderman diagrams, although I don't use them myself.
I can't help posting a link to the rejection letter that Nassi and Shneiderman received from Communications of the ACM when they first proposed the diagram:
http://www.cs.umd.edu/hcil/members/bshneiderman/nsd/rejection_letter.html
Comment: Indeed a most brutal rejection letter. Wonder what that Gries guy had been smoking?
Comment: I think the rejection letter was bashing flow diagrams in general, claiming that structured programming makes them obsolete.  To the author, this probably looked like yet another one of those flow-chart thingies.
Answer: We've never used them.
Edit
Yes, I (we) have heard of them.  Thanks for asking! :-)
Seriously, we just don't use them.  We ususally keep the diagraming to simple flow diagrams which are generally easier to read and understand.
Comment: But you (or your colleagues) have heard about them?
Answer: I've heard of them and read a few books that used them extensively. I quickly concluded that even assembly language (e.g., MIXAL in Knuth's books) was more understandable. I never had even the slightest urge to draw one (and can't recall anybody's ever having asked me to either).
Answer: I have used them. But more often I use some kind of pseudocode when designing an algorithm.
You can write pseudocode with any editor and pen/paper combination. Diagrams are often harder to edit and tend to get messy.
I still use UML diagrams for OO design. Mostly class, but sometimes state transition diagrams for classes with complex state.
Answer: I thought they were cool when I came across the notation in the early 80s. But it's so close to normal code, and cumbersome to maintain both the diagrams and the code that I decided just using indented code was close enough for my taste.
Comment: Indeed, for what the NS diagrams were intended for, pseudo-code is much better and practical.
Answer: Yes, I used them some years ago, but in times of UML they appear a little bit old fashioned. In my opinion the Nassi-Shneiderman-Diagram is still a good diagram type to visualize a structured code block, far better than an UML-Activity-Diagram.
On the other side it might be easier to simply have a direct look into the code...
You can find a commercial tool here: http://www.easycode.de/produkte.html?&L=1
Answer: When I was first in college, one lesson we had this compare-and-constrast-various-methods-and-diagrams thing. Nassi-Schneiderman was a winner but with certain issues highlighted. Conditionals subdividing the width of the page quickly became impractical, so it was proposed we might use a representation more like that for repetition. Also, the lines and boxes seemed redundant to some degree.
Think about it, and you'll realise that what we were leaning towards was basically indented structured pseudocode, but with some limited use of drawn lines down one side to highlight the indentation - or more accurately to reinforce the pretence that it was a diagram.
Answer: Many years ago, when I worked for the US DOT's Transportation System Center, I was given the task of developing a Nassi-Shneiderman editor which was then used to document software designs within the department.
Personally, I never used them. I prefer to look at code.


Several graphs in 1 loop, each iteration adds a line on every figure

Question: Trying to engineer the following in Matlab:
** loop start;
   y(:,i) = function of x;
   z(:,i) = function of x;
   plot(x,y(:,i))   on figure 1, hold all;
   plot(x,z(:,i))   on figure 2, hold all;
** loop end;
   add title, legend, etc for figure 1 (NB: we have multiple lines);
   add title, legend, ets for figure 2 (NB: same, have multiple lines for the legend);`

Tried multiple combinations without much luck. Managed to get 2 figures but only the 2-nd displays multiple lines, not the first. And can't figure how to add legends to these 2 correctly.
Answer: Save a handle to each figure, and to each axis object:
fh1 = figure;
hold all;
ah1 = gca;

fh2 = figure;
hold all;
ah2 = gca;

for i=1:N
    y(:,i) = function of x;
    z(:,i) = function of x;
    plot(ah1, x, y(:,i)); %# tell it which axis to use (ah1)
    plot(ah2, x, z(:,i)); %# (ah2)
end

legend(ah1, ...) %# legend options here
legend(ah2, ...) %# and the other legend

%# note: you can set figure properties for each using fh1, fh2 handles.

Comment: Works great. If you have a chance to answer -- why is it ok to put "hold all" in front of the loop? I did handles too, apparently 2-nd "hold all" was missing or in a wrong place.
Comment: "hold all" works on the current axes.  If an axes object doesn't exist, it creates one.  That's why it works before you plot anything.
Comment: How should one do "hold off"'s? Something like
figure(1);
hold off;
figure(2);
hold off;
Comment: From the [documentation](http://www.mathworks.com/help/techdoc/ref/hold.html): `hold off` resets hold state to the default behavior, in which MATLAB clears the existing graph and resets axes properties to their defaults before drawing new plots.
Comment: Well, not like I would want to get rid of all of my lines but theoretically once I have 'hold on' activated on both figures I "need" (or might want) to deactivate it on both too. I don't see how to use 'ah1' and 'ah2' to implement such de-activations => hence the question.
Answer: You can do this:
figHandle1 = figure(1);
figHandle2 = figure(2);

Then when you want to plot on that figure do this:
figure(figHandle1) %Plot on figure 1
%ie plot(someStuff)
figure(figHandle2) %Plot on figure 2

Also its the same for the title and stuff, you jjust need to identify which figure by doing:
figure(handle);

Hope this helps.
Comment: `figure(h)` will bring the handle `h` to the front, so new things are put into the figure - so, this will work.  However, it is comparatively slower than just specifying which handle to use by giving the handle to the plot command (i.e. what I did in my answer).  This can be important if you'd be switching figures often (as the OP would be doing, twice each time in a loop).
Comment: @tmpearce I didn't know that, but thanks, hopefully the OP isnt running that many loops.
Comment: I tried this before and tried again just now. The result -- 1 line on figure 1 and 10 lines on figure 2. I am reluctant to put all the code here as functions are very sophisticated. But everything works if I 'kill' figure 2 & ask to plot only figure 1, then all 10 lines are there.
Comment: Actually I just found the bug: there should be "hold all" used twice after every plot inside the loop.


If monochrome laser strikes a mirror perpendicularly, how would I know whether there is destructive interference?

Question: Suppose I have a setup where there are 2 mirrors parallel to each other and I let in a laser beam of a single wavelength of red color through a tiny little hole in one of the mirror so that the beam would strike the other mirror at right angle. Suppose the laser bounces back and forth and the mirrors remain perfectly still, how do I check if there is destructive interference or not?
Comment: I'm a little confused. Destructive interference where? It sounds like you're describing a Fabry-Perot cavity. What exactly is your question?
Comment: @Gilbert: I like to know suppose if I shift one of the mirror along the direction of the beam is there any way to tell whether it is destructive interference or not from the reflection on the mirrors?
Comment: This video, if you understand it, clarifies what happens with lasers and interference  https://www.youtube.com/watch?v=RRi4dv9KgCg
Answer: If you send a beam of atoms perpendicular to the laser bouncing back and forth such that these atoms have some absorption line in the frequency of the laser, then if the interference was destructive you would see the atoms coming out exactly as they have came in (because ther is no light to absorb). Otherwise, if the interference was not destructive at all, some atoms will end up in some excited level that you could measure.
Comment: Absorption is not interference
Answer: I will keep my answer general in hopes that it will be relevant to your question.  You should recognize that if you reflect a beam off of a mirror, you will get a standing wave, which is the result of the incoming wave interfering with the outgoing wave.  A standing wave has "nodes", which are points of destructive interference.  For instance, for a perfect metal mirror, the electric field at the surface is zero at all times due to perfect screening currents.  So you have destructive interference there, as well as at every half wavelength away from the surface (for normal incidence).  There is always destructive interference.
But you also asked about what seems like a Fabry-Perot cavity.   In this situation, you get optimal destructive interference (as well as constructive interference, at different positions) at the resonance condition.  How can you tell that you are at the resonance condition?  There are many ways, but the most obvious is to look at the light transmission through the (not quite perfect) mirror.  If you get a peak as a function of mirror position, you are at resonance.
Comment: if I understand correctly the light on the mirror surface becomes brighter when there is constructive interference and darker if it is destructive interference. I watch the video link suggested by annav but I don't have the lens to observe the fringesby the way the light on the mirror seems to appear consistent no matter how I adjust the mirror.
Comment: Ah, I see that you are confused about some concepts. The light scattered by the mirror surface is not an indication of destructive interference. Remember, there is *always* destructive interference in reflection, but the question is where (in space) is the interference destructive and where is it constructive? I still am unclear about your experimental setup. Explain it more carefully, and tell me *where* you care about the destructive interference occurring.
Answer: A schematic diagram of an arrangement which will enable you to view the interference fringes might look like this.

Please note that to see the fringes as described below the mirrors have to be optically flat, in other words flat to within fractions of the wavelength of the light emitted by the laser.
You will note that the inclined mirror reflects light from the laser down onto the parallel mirror and then allows light which is reflected from those parallel mirrors to pass through and hit the screen where the interference fringes can be viewed.
In the arrangement as shown you will see circular fringes whose radius will vary as the separation between the mirrors varies.
As the separation of the mirrors is decreased fringes seem to be "created" at the centre and appear to move outwards from the centre.
When the parallel mirrors are very close (a few wavelengths) to one another then the radius of the fringes becomes larger but you are unlikely to see a uniform illumination (or lack of it) across the whole screen.
The width/sharpness of the fringes will depend on the number of significant reflection there are between the mirrors and as the reflectivity $R$ of the mirrors goes up the fringes become sharper.

This increase in sharpness as there are more significant reflections between the parallel mirrors is equivalent to the increase in sharpness in the fringes produced from multiple slit interference as one goes from two slits to many thousands of slits which is called a diffraction grating.
A simple experimental set up to observe such fringes using two microscope slides is described here with the separation of the two reflecting surface (the sides of a microscope slide) changed by heating the microscope slide with a hair dryer.
The reflectivity of microscope slides is low so to achieve the very sharp slides you need to use a Fabry-Perot etalon which is described here.


Why do some applications ignore resize requests from finder

Question: I have an AppleScript which allows me to maximize a window (not fullscreen just maximize (keep top bar & task bar)). For some reason certain applications ignore the request and just stay the same size (Adobe products consistently ignore it). You can read the script here, I got this from somewhere else (a stackoverflow post). Why might the script not work for certain windows and how could I possibly fix it?
Comment: Can you link to the original stackoverflow post? There may be some clues there.
Comment: @DAVincent One of the most common program's I have trouble with is photoshop, just the general window. I can maximize it with the button but not script.
Comment: @Monomeeth I don't know exactly where I got the code from, but [here](https://apple.stackexchange.com/a/111557/209947) is a page which has the same code.
Comment: What apps are you having trouble with, and which windows in each app?  I gather it's not as simple as being unable to size a window that can't be sized by the mouse, such as the About Safari window.  Originally scriptability was optional for Mac app developers, and so would cost more.  As a sometime developer I can easily imagine how scriptability might not be the first priority of a product team.
Answer: Since you've referenced Adobe products (and Photoshop specifically) in your question/comments, I'll refer to those here.
There is really nothing inherently wrong with the script - it's just that some apps are designed to work differently. Below are some considerations using Adobe Photoshop as an example:

If you open an image, Photoshop will ensure it's initial state is at a zoom percentage that allows the user to see the whole image.
Now, if you manually click on the Zoom button (i.e. the little green circle at top left of the window) you will observe how Photoshop works with this button. It's typically not very helpful in the case of Photoshop, and it behaves differently to most other apps. This is because Photoshop has multiple windows (e.g. you want the script to work so that it maximises a window while keeping the top bar & task bar, but Photoshop has other windows on screen such as Tools) so the Zoom button only applies to the image window.
The only way to essentially change the view of the image in Photoshop is to use the View menu or shortcuts like ⌘0, ⌘1, ⌘+ or ⌘-.
Photoshop also uses a tabbed window view by default (i.e. as you open more images, Photoshop opens these in separate tabs within the same window).
You may be able to achieve the view you're wanting in Photoshop by changing the Screen Mode (go to View > Screen Mode).

So, I guess what I'm saying is that many applications use multiple windows (or panels). With Photoshop most users will have the Tools panel, the Layers panel, and so on open while using the app. It's the same for most of Adobe's apps and most other creative apps.
You may be able to achieve what you want with a specific script. And you may find the Adobe Photoshop CS6 Scripting Guide a handy reference.


Motorola Microphones

Question: Can a person use moto mic's on all their radios by rewiring the end plugs I have a few mic's and want to change the plugs to work on my Gn300's and Maxtracs and how to do this re  Colors and positions of wires in a cat6 plug
Comment: Have you found a technical reference for both the microphone and the radios in question>
Answer: There's a couple of things that have to happen to make mics work on a radio.  Obviously, correct pin out is important.  Also, the impedance has to be matched or you won't get good audio out of it, but the mic gain on the radio might cover that, and if not, it's not hard to match it externally.  Lastly, some mics need phantom power and some don't.
That last one is the killer -- if your radio is unable to supply or suppress the phantom power for a mic that needs or can't use it, you won't be able to use the mic on the radio without some external conversion circuitry.
Beyond that, the hard part of getting it to work is finding the correct mic plug that fits the radio, finding the pin out for the radio.
Answer: Older Motorola two way radios and microphones used a relatively standardized interface.
The connections to them are generally:
GND - ground
PTT - the PTT switch connects this to ground to key radio
MIC HI - microphone audio. The radio supplies a bias voltage (around 9V) to this pin to power an amplifier in the mic and in some, an electronic PTT switch.
HOOK - connects to ground through hang-up contact on rear of mic to put radio in monitor mode when the mic is taken out of it's holder.
You can find the wiring diagrams for various radios and mics online (try www.batlabs.com), and in most cases just matching up these connections will get a Moto radio to work with a Moto mic.
One thing to note on the 8 pin modular connectors is that Motorola reverses the pin numbers in their diagrams from what you would usually see for an "RJ-45" plug. So what's normally pin 1, Moto calls pin 8, 2 is 7, 3 is 6, etc., this trips a lot of people up.
Comment: Excellent this really helps I just have to track down the radio/mic schematics
Answer: My radio (mcHF) is a kit and didn't come with a microphone, so I had to match one up. I learnt that the biggest issue is whether the radio requires a dynamic microphone or an electret microphone (aka condenser). A radio is designed for one or the other.
Beyond that there are other differences, but the instructions to my kit imply that I don't need to worry about those differences. For example, the dynamic mics come in different impedances, but no one seems to worry about this when matching a mic to the mcHF radio. Maybe most mics come in an impedance range that the radio can tolerate.
Disclaimer: I am a newbie and just sharing my limited experience.

Motorola Microphones


File extensions and association with programs in linux

Question: In windows we can associate a file's extension with programs.
E.g. a file test.pl can be run by the installed Perl interpreter due to the pl extension.
In linux though it needs #!/usr/bin/perl as the first line.
Is this because there is no association between file extensions and programs in Linux?
Answer: No, it doesn't mean that.  If you have a text-file that has it's execute-permission set (e.g. chmod a+x somefile ), and the first line of the file is something like
"#!/path/to/executable"

It just tells Unix what program to use to execute the script.  If a text-file is marked as an executable (i.e. is a script), Unix will start whatever program is specified in this way, and send the rest of the  text-file (the script) to this program.  Typically the program specified will be a shell (/bin/sh, /bin/csh or /bin/bash), the interpreter for some programming-language (Perl, Python or Ruby) or some other program that executes scripts (like the text-manipulators Awk or Sed).
Usually the "#" specify a comment in many languages, it's only if the first line begins with "#!" it's something special.  If a file is marked as executable but doesn't start with a "#!", Unix will assume it's some kind of binary (e.g. an ELF-executable made by the C-compiler and linker).
In general Unix doesn't rely on the suffix of files.  Many programs neither needs nor automatically adds their typical suffixes, one exception being the compression-programs (like gzip and bzip2) which usually replaces the original file with a compressed one, adding a suffix to mark compression-type (these are one of the few programs that complains about incorrect suffix).
Instead the file is identified by it's content through a series of tests, looking for "magic-numbers" and other identifiers (you can try the command file on some files to test this).  This is also used by the file-browsers under GNOME and KDE to select icons and the list of programs to open/edit the file.  Here the MIME-type of the file is identified by such tests, then the suitable programs for viewing and editing is found from a list associated to the MIME-type - not the suffix as in Windows.
Since one of the tests would be to check if the first line of a text-file is "#!/something", and then look at what "something" is; you could say for example that #!/usr/bin/perl identified the file as a perl-script - but that is more of a side-effect.  Even if the file doesn't start with a "#!", the tests should be able to correctly identify the file.  In any case, it's the content of the file that is used to identify it, not an arbitrary suffix.  As such, endings like .pl (Perl) and .awk (Awk) is purely to help a human user to identify the type of file, it's not used by Unix to determent type (like the suffixes in Windows).
You can actually make a "script" without the "#!/something", but Unix wouldn't be able to automatically run it as an executable (it wouldn't know what program to run the script in).  Instead you'd have to "manually" start it with something like perl myscript or python myscript. Many scripts in larger Python and Perl applications will actually not start with "#!/something", as they're scripts for "internal use" and not intended to be invoked by the user directly.
Instead you'll start the main script (which does start with a "#!/something"), and then it will pass these other scripts to the interpreter as this script runs.
Comment: A script without a she-bang is meant to be interpreted by the system's shell (when execve() returns ENOEXEC and the file is not detected as being binary).
Answer:
In windows we can associate a file's extension with programs.
  E.g. a file test.pl can be run by the installed Perl interpreter due to the pl extension.

Right, but the extension is merely a hint about the type of file it is. A perl syntaxed script is still a perl syntaxed script irrespective of if it is called ".pl" or ".exe" or ".doc". You can still execute it by calling the interpreter directly for example; perl.exe thisfile.doc will work if the file is a perl syntaxed file. One can also associate '.blah' with being a perl file too if you wanted.

In linux though it needs #!/usr/bin/perl as the first line.

Again, this is merely a hint. You can still call a perl file without the shebang in it by directly invoking the script as an argument to the perl command.

Is this because there is no association between file extensions and programs in Linux

Technically, yes there is.
Linux actually has a number of ways to identify the executable format. The file command offers an illustration of this. It contains a database of "magic" (strings of certain lengths at certain offsets) to determine what type of file something is. It does this by inspecting the content of the file to work out what it is.
Another way of doing this is with the file extension.
You can actually register what behaviour you want using a system called 'binfmt_misc'. Wikipedia explains how this works (using 'magic' and file extensions).
A file is arbitrarily considered 'executable' in linux by the notion of the 'executuable' bit on the file permissions. When you attempt to run a application that has the executable bit, the kernel will read the first few bytes of the file to determine what to do.

If the file starts with #! then invoke the command in the path provided and make the following file the last argument of that command.
If the file starts with ELF, run /lib{64}/ld-linux.so (this is actually fetched from the section header of the ELF binary, its not a static path just used) followed by the command (this performs a series of static loading of libraries).
See what rules apply for binfmt_misc.

a.out formats I'm not too familiar with their invokation -- I assume the ld-linux.so loader still handles them.
Answer: No, it's not. Shebangs (#!) and associations between file types and applications serve different purposes, and you can find both in desktop Linux distributions.
It is customary to use the former in executable scripts that you call from the command line or from other scripts, and it does not depend on file extensions. Similar functionality is obtained in Windows through the PATHEXT environment variable, which does depend on file extensions.
The latter serves users who click at icons or file names on a desktop interface. In the Linux world, this comes from associating MIME types to applications, which, of course, is handled by specific tools and does not require reading the specification from typical desktop users.
Answer: The shebang line (#!) is used to indicate which Perl version you're using, and also to invoke the script directly from the shell
./myscript.pl

You can do this as well without the shebang line:
perl myscript.pl

Comment: But if I just have 1 version would I still need it?All tutorials and examples have this line.
Answer: You just can't make such comparisons, it's not that GNU/Linux does not have association between file extensions and programs, it's because GNU/Linux actually does not care about it.
You have file permissions, and if your file is executable, it will ALWAYS search for the shebang at the first line of any script to look on how to execute it.
Linux's ELF binaries, on the other hand, does not have exe extensions as windows binaries does, and they are always executables, even without ANY extension.
Comment: `if your file is executable, it will ALWAYS search for the shebang at the first line of any script to look on how to execute it` So without this line, Linux does not know how to execute the perl script?
Comment: Yes, and then you should `perl script.pl` to teach BASH actually, not Linux per se.
Comment: So unlike `Windows` I either do: `perl script.pl` **or** `./script.pl` with `#!/usr/bin/perl` as first line? So there is no real association between extensions and programs in Linux?
Comment: `Linux's ELF binaries, on the other hand, does not have exe extensions as windows binaaries does, and they are always executables, even without ANY extension.` This is not quite that accurate. ELF binaries are always ELF binaries, perl files are always perl files. But an ELF is not necessarily an executable directly (libraries and kernel modules are ELF binaries but not exectuable).
Comment: I'm able to execute libraries on linux. try this: `/lib/libc.so.6` and watch the **execution output** ;)
Comment: Thats because it has an init routine. There are only 2 libraries I know of that do this (libc and ld-linux). Feel free to go 'execute libraries' that are not one of these two.
Answer: Yes. There is no support for extensions in Linux. You are correct about first line i.e. shebang line, It is used to find which program to use to run the file, like in windows you have file extensions.
Even if you have seen something like .sh .pl at the end in linux, they are just given to understand the type of file simply by looking.
In windows, extensions mean everything and on basis of them correct application will be used to run it. For ex: .xls means Excel file, .doc means Word Doc, .exe is considered executable, for same in linux however you need to give execute permissions to file then only you can run as ./filename.
In windows, the match for extensions and program to use to run applications are generally stored in registry. So if there's no match stored for .pl, then that extension makes no sense similar to linux and you will have to select binary to run it explicitly.


Is there any evidence that subatomic particles are affected by gravity?

Question: If so what experiment has been done to show this?
Comment: Affected how? After all, I'm made up of subatomic particles, and I'm clearly affected by gravity.
Comment: The Milliken oil drop experiment.
Comment: Gravity is the curvature of spacetime due to matter and energy. Subatomic particles exist in spacetime, therefore they must be affected by gravity. I suppose the evidence is that gravity, by its very nature, affects everything in the universe
Comment: It's a stretch, but I think a fairly widespread impression that subatomic particles might not be affected by gravity stems from its role as the Heisenberg conjugate of time, together with the fact that memory functions through the use of electrons:  These two considerations may've been combined into the notion (popularized by Feynman) that antiparticles are particles traveling backward in time.
Answer: Yes. For the phrasing of this question, the neutron qualifies.
Neutrons have been slowed and collected, which are diverted from nuclear reactors via beamports. The methods for doing this are quite complicated, but in the final state, they are confined within a box where the "walls" present a nuclear barrier to the neutrons. The neutrons have a wavelength longer than the spacing between atoms in the wall, thus, they bounce off. An interesting fact about the design is that the containment area doesn't need a "top" because the neutrons are at such a low energy that thermal movement isn't enough for them to leap out. This phenomenon, alone, is a physical demonstration of gravitational affects on subatomic particles.
So thorough is our understanding and testing of these particles in gravity, that quantized levels of height have been observed for ultra-cold neutrons.


Can this type of workflow possible in Oozie?

Question: Can this type of workflow possible in Oozie?
or any suggestions on how to do this type of things in oozie
Please look at the picture attached
http://s7.postimg.org/yj30wb0mj/oozie_Workflow.jpg
Task3 should be run by only one work flow at a time other workflow needs to wait.
Task3 is ssh action only one needs to access that machine at a time.
Answer: This can be done.
Using the fork and join
Fork to create wf 1 & 2 and join then start task 3 , than fork again .
You may need to refactor your action to match this design and split your workflows, so the design will match what you want to achieve
Answer: To generalize the answer, any workflow that can be represented in the form of a Directed Acyclic Graph (DAG) is possible in Oozie. Ofcourse, this is bounded by the ecosytem and tools supported by Oozie.


Slow Kiln ( Tezos ) to Update Node from Snapshot

Question: First I installed Kiln on a VM. It was all perfect and updated the node in about 1 hour. Then I installed Ubuntu 18.04 on my computer and Kiln takes forever to update the node from the snapshot. Does anyone know why this is happening?
Answer: Thanks for posting your question.
First, you may already be aware that the OVA file is an alpha release, so you may run into issues or features that aren't quite as good as other releases, such as the .deb file. I have seen the OVA instance be resource constrained in the past, depending on the specifications of the VM you are running it in.
That said, I think there are two shortcomings of snapshot import with Kiln's OVA file effective v0.7.2.

There is no user feedback on the snapshot import, such as a progress bar. This can make the import process frustrating if it starts taking a while because you don't know if it is almost finished, if it hit an error, etc.
The snapshot import cannot be cancelled. So if it fails, you have to start Kiln from a fresh start. There's also no feedback to the user to inform them the import has failed. This can be frustrating.

I think #2 is the worse of these two, and v0.7.3 will allow imports to be cancelled. #1 is something we should address shortly after that.
For now, I would recommend giving the snapshot import a few hours for OVA. If you've already done that, unfortunately you may need to start fresh with a new installation. To maximize your chances of success, I also recommend making sure your machine doesn't fall asleep while doing the import and you've got ample resources allocated to the VM. v0.7.3 should be released by 2019-11-22.
Best of luck!
Comment: I have the Kiln v0.7.2 node updated on PC and would like to know if it is possible to copy the data to another PC that is not
Comment: It is possible to copy data, but you have to be careful of many things. Its always better to import a recent snapshot and let the node sync on its own.


Better way to handle HDF and GeoTIFF files

Question: I am using GDAL and MODIS reprojection tool in my python project and want to know better way for math new indices of HDF data (MODIS data).
Also I want to visualize them in web. Which format is more flexible for math and visualize my results?
Answer: As you can see from our website's Python example, you can directly access HDF with Python. Resolving lat/lon will be a little bit tricky for most MODIS products as illustrated in our NCL/IDL/MATLAB examples. However, you can use HDF-EOS2 dumper tool to dump lat/lon in ASCII files and read them back in Python.
Have you tried NCL? In my opinion, it's one of the easiest tool for MODIS data visualization and it can perform some math as well.
Comment: Welcome to our site!  I appreciate your honesty and transparency in identifying your connection to the solution you are recommending.  We are glad to have experts here who can recommend and explain their work.


How do I properly mince ginger?

Question: I often need to mince ginger to add to any number of dishes.  If I don't want to use a microplane, how do I mince ginger with a knife? What type of knife should I use and how should I cut it? I'm most interested in the techniques associated with efficiency and safety.
Answer: It's almost exactly like mincing garlic, but you want to make sure that the first cut is across the fibers of the ginger.  (you'll be cutting it into coin-like shapes)
... and then julienne what you can, and start the rocking to chop it up as fine as you want it.
Comment: @dpollitt It *is* a suggested improvement. It's just a slight modification to the technique.
Comment: Who is this julienne you speak of?
Comment: @dpollitt :  julienne == cutting into matchsticks.
Comment: You might also like cutting it in half down the middle first, so you have a flat side to put face down and it can't roll around as you cut the coins, especially if your knife isn't super sharp or you're not as comfortable yet.
Comment: @Jefromi Please use comments to ask for more information or to suggest improvements. Short answers as comments are generally discouraged at SE. Plus I'd like to vote on your contribution. Thx
Answer: First wash the ginger thoroughly and peel the skin off. You can use your regular peeler or knife too. Knife is better since much of the ginger is not wasted this way. Just work your knife with slight pressure on the skin of ginger in a upward motion and the skin will get scraped off. Now cut the ginger into chunks first, and then proceed with mincing. Hold a piece and start chopping it finely. Then club them together and work with your knife randomly on them if you want still finer bits. This method always works for me.
Comment: oh, right ... peeling.  You can also peel with a spoon -- using the same scraping method.  You want a fairy thin spoon -- if it's too heavy, the edge might be rounded over too much

How do I properly mince ginger?


Kitchen light switch malfunctioned

Question: I have a switch that only controls the kitchen light. It has a ground wire, two black wires and a red wire. The red wire connects to the kitchen light, but one of the black wires seems to sends power to the dining room light so a single pole switch doesn't allow for the light in the dining room to work. I also randomly attempted a 3 pole switch but you cannot have both the kitchen and dining room lights on at the same time.
Comment: Does the dining room light have it's own switch or does this switch control both(together)?  Imagine dining room black wire was connected to one of the black or red wire(hot or switch hot).
Comment: The dining room has its own dimmer switch.
Comment: Welcome. Please take the [tour]. "Thanks" comments are discouraged here.
Answer: Since you appear to have 2 cables rather than 3 (3 would be incoming power, light fixture, dining room), you likely had something like the following:

Power goes first to kitchen light - black hot, white neutral
At kitchen light, black incoming to black 3-wire cable, whites together (and to fixture neutral), red of 3-wire cable to fixture hot (aka switched-hot)
In switch box, 3-wire cable has black hot, red switched hot, white neutral
In switch box, 2-wire cable has black hot and white neutral for dining room

In this scenario, the blacks go together to one side of the switch, the red goes to the other side of the switch, whites go together (but don't connect to the switch unless it is a smart switch that needs neutral).
However, any color except white can be hot or switched hot. So there is no definitive answer to which wire is the actual unswitched hot wire. You need to figure that out. You may be able to do that with a non-contact voltage tester, and you can double-check and be sure with a multimeter. (Carefully!)
There are really only three possibilities:

Black of 3-wire cable. If that's the case, blacks together to switch, red to other side of the switch.
Black of 2-wire cable. Unlikely here, but technically possible. That would mean the feed to the dining room is actually through the 3-wire cable, with hot and neutral carried on from the fixture box to the dining room. And if that's the case, again blacks together to switch, red to other side of the switch.
Red of 3-wire cable. If that's the case, red goes together with the black that is not in the 3-wire cable on the switch and the black (in the 3-wire cable) goes to the other side of the switch.

On rereading the question, I see a key phrase: The red wire connects to the kitchen light
Assuming that is 100% definite, that would match either the first or second scenario. In either case, that means blacks together.
Blacks together (or any two wires together on a switch) can be done one of 4 ways:

Backstab + screw. Not recommended.
2 wires under one ordinary screw. Not allowed!
2 wires under a screw-to-clamp connection.
Pigtail.

The all-purpose works with any switch answer is a pigtail. That's three wires (more sometimes) in a wire nut, with one of those wires being a short piece (black or red or yellow or blue or any color not green or white if hot, white if neutral) of wire that then connects to a screw on a switch or receptacle. This makes life much easier when a switch is replaced.
If you, at some point, decide to put in a smart switch, timer, motion sensor, etc. that requires a neutral, then you will need to use a pigtail (add a white wire to the existing neutral bundle, might need a new wire nut).
Comment: For clarity: "might need a new wire nut" because the existing nut may be too small to handle an additional wire, not because they can't be reused.

Kitchen light switch malfunctioned


Chrome scroll is locked

Question: I'm using Google Chrome on Mac OS 10.9
When I opened a new webpage, I can't scroll it down.
Only after I moved to another tab and then come back, the scrolling works.
How can I solve this issue?
Answer: It also can happen on Ubuntu 13.10. The problem seems intermittent, and I know someone else on a Mac was having this problem. Fortunately Firefox doesn't have this problem, so that is one good solution. You could dig up an older Chrome version, but that would have security vulnerabilities.

Chrome scroll is locked


How to hide Map drive from remote domain users

Question: I am wondering how can i hide a map drive from remote domain (server2012r2) users, our mapped K drive is shared with everyone so any one login to domain they will see the K drive mapped on their computers so i want to know if is there any way i can hide K drive from some users, i cannot access deny to those users because our folder redirection is mapped to K drive if i setup access deny to those users they wouldn't be able to see any thing on their desktops, documents and appdata folders.
Thanks.
Comment: This feels like an [XY problem](https://meta.stackexchange.com/a/66378/271445). Why don't you want a drive mapped to a location users have permission to access? Is it because there's something there you don't want to see? If so, you're going about this wrong.
Answer: You can setup 2 groups. Show-Mapped-Drive and Dont-Show-Mapped-Drive. Put the respective users in the two groups. Now in Group Policy when you map the drive create two mappings for K:. With each targeting a group. In the mapping for Dont-Show-Mapped-Drive you can choose to hide the drive.
Please note this of course depends on how you map drives. Also note, that if Show Hidden Drives is turned on it will show them the drive.
Comment: @user313162 that's why it's important to configure the item-level targeting to only affect the users you don't want to have the drive.
Comment: Thanks for responding i gave already created huge map drive gpo and linked to contractors group but it's not hidding the drive so temporarily I am running tasks scheduler on those machines to unmap the drive at user login.
Comment: We had this issue when we first attempted a hide drive mapping. We had to push out an unmapping first. Then we had to use Update instead of Create.
Comment: Okay let try that but I don't want to un map drive from everyone though.
Comment: I have tried with item level targeting by using groups but nothing helped I guess I have to unmap k drive some how first then I might be able to hide it using item level targeting.


Reviews: recommend deletion with "no comment needed"

Question: Is the no comment needed option acceptable if the comment for recommending deletion is already there, or should I choose the reason for recommending deletion even when there already is a similar comment?
Comment: What do you mean by *" if the comment for recommending deletion is already there"*? If there is already a comment recommending deletion for one of the other reasons (like, say, too broad)?
Comment: i.e. if someone comments *this should be a comment* - should I choose *This is commentary on another post, not an answer* option OR is the *no comment needed* acceptable in that case?
Comment: @JanTuroň I'd upvote the comment that said "this should be a comment" and not leave another one. Especially if the user should have known better.
Comment: @MarcoBonelli "Recommend Deletion" is an option for low-quality _answers_ only. "Too broad" is not really applicable to those.
Comment: @pizza thank you for the clarification.. it wasn't clear
Answer: If a comment isn't needed, then don't add one.
Comments might not be needed for all sorts of reasons. A few I can think of off the top of my head:

An existing comment adequately describes the problem
The author's account was deleted
The post being deleted is clearly abuse/nonsense/cat-pictures and doesn't deserve a response.
Comment: Cat pictures are *always* appropriate.
Comment: [cat-pictures!](http://programmers.stackexchange.com/q/46835/31260)
Comment: another reason I see: revenge downvotes :)
Answer: It doesn't hurt to choose the same delete option that someone else already has.
If you see the same auto-generated comment that you think is appropriate (e.g. "This does not provide an answer to the question. To critique or request clarification...") already there, feel free to upvote that comment, but whether you choose the "No comment needed" or the other option, it will only generate the comment once for each type instead of having the same comment from multiple people. (Just confirmed while reviewing low quality posts.)

As Shog9 said, the comment might not be needed at all, but if you're worried about duplicating generated comments, SO is clever enough to handle that.
Comment: Yes, the comment will only be auto-generated once. But are you sure the other instance was actually auto-generated, and not a helpful user outside the queue putting it there? It happens.
Comment: I used to type out *possible duplicate of * because I thought that was the standard comment to post, not knowing that they are actually auto generated when you VTC as dupe
Comment: @Deduplicator: If a _similar_ comment exists, I guess having the "official" version there (including links to reputation help, etc.) as well, or even the same comment twice (if pasted manually) can't really hurt, but you're correct; I was referring to the case where the _exact same_ comment is already there.
Comment: @TimCastelijns: Yeah, I guess there's no way to know how those comments get there until you get access to the queues that put them there :-)


Does cancellation property imply invertibility?

Question: I have no problem seeing how having an inverse implies cancellation property, but I've problems proving (or even seeing why) the converse must be also true. Can you please suggest a way to approach this?
Answer: No. Cancellation property does not imply existence of inverse element. Consider the additive monoid $(\Bbb{N},+)$. (We wiil assume that $\Bbb{N}$ contains 0.) Then $(\Bbb{N},+)$ satisfies associativity and cancellation property but all elements except 0 have no inverse element.
Comment: So is it accurate to say that cancellation works because there is an inverse, but the inverse element is not nessesarily part of the set and that's what invertability is?
Comment: @BenjaminThoburn I do not think your explanation is correct in full detail. Every non-zero element of N has no inverse, and so we do not know how to talk about what the inverse of 1 is, for example. In my example, it should be −1, but it may not be obvious for more complex structure.
Comment: However, I *guess* every monoid with cancellation property can be extended to a group. It really holds if we assume the monoid is abelian. I do not know my guess really holds and I have no time to check it.
Answer: However, if you require the set to be finite, then you do get invertibility (this is a common way to show that $\mathbb{Z} / p \mathbb{Z}$ is a field).
Comment: Brilliant! I just knew there had to be a way to do this [without the concepts of 'injective' or 'surjective'](https://math.stackexchange.com/a/2830620/140308).


Which is this plant with purple flowers and elongated, opposite leaves?

Question: Found this plant in a nursery around Central India. Not sure whether this is an indoor or outdoor plant. Want to know the species and tips about its upkeep
Answer: It looks like Melastoma malabathricum, commonly known as the Indian Rhododendron, or Tibouchina is another name used mostly in the West, though it is synonymous with Melastoma. It's considered a tropical plant because it won't tolerate temperatures below about 5 deg C very well, though it copes with a bit of light frost. Does well outside provided temperatures are suitable. Prefers not to be in full, very hot sun, morning sun is fine, though it will adapt gradually to full sun if it has to - it is evergreen. This particular example looks as if its suffered from heat/direct sun or a bit of drought in its pot. USDA hardiness zones are listed as 8-12, but in zone 8, they may die back and regrow in spring. Even so, in my experience in the UK (mostly Zone 8) these are easily killed in winter outside, particularly in pots. Needs fertile, moist soil that drains freely, but doesn't constantly dry out.
As far as I can tell, Central India, if that's where you are rather than just where you found the plant, does not experience temperatures below 5 deg C, so this plant should be fine planted outside.


Sci-fi story about a utopia about some technologically advanced people living in space stations and some on planets

Question: I read this sci-fi story at least 15 years ago. The story is about a far future utopia. There were two segments of people, technologically advanced  people living in stations or ships, and not so advanced ones living on planets. The station/ship people were living a life of 1000 years, the reproduction is completely controlled and in vitro, the concepts of family, parents, siblings and such are long lost their meaning.
Comment: I've edited out the second story as questions should be focused to one work. Please ask the second identification question as a new question. Also should you remember anything else about the story, plot wise, please [edit] your question to add in information.
Comment: You could improve this question by going through the checklists [here](https://scifi.meta.stackexchange.com/questions/9335/how-to-ask-a-good-story-id-question) and editing in any relevant info you can think to add.
Comment: What drove the plot, were there conflicts between the space-dwellers and the planet-dwellers?
Comment: possibly the same as https://scifi.stackexchange.com/questions/97099/help-remembering-a-story-where-partial-stasis-is-taking-place
Answer: The question thus far is extremely light on plot details, but the dichotomy between long-lived space-dwellers and short-lived (and technologically less-advanced) planet dwellers makes me think of Between the Strokes of Night (1985) by Charles Sheffield.
It fits with the "hard SF" tag because there is no FTL in the universe; the space-dwellers' long life-spans is the only thing that enables a society to exist given the decades-long (or longer) time spans required for interstellar travel.
The novel opens with the initial expansion off of Earth and the investigation of hibernation as a means of surviving long space journeys.  This leads to the discovery of a stable metabolic state that allows the perception of time at an extremely slow rate, and a concomitant extension of lifespan.
The story then skips thousands of years into the future and to a planetary society where high-scoring individuals are recruited into the space-dwelling society.
You can read more details in the plot synopsis on Wikipedia.
Comment: "Between the Strokes of Night" was my initial thoughts but I don't recall invitro fertilisation being a major plot point?
Comment: @Alith In vitro _per se_ was not, but it _was_ a plot point that people living in the slow state couldn't have kids, so they needed another way to replenish their population.  I figured this was very close and worth proposing as an answer.
Comment: you're right, I'd forgotten that was one of the problems with the slow state :)


Riemann Zeta Function, Stirling's Numbers, and Infinite Series

Question: A while back I was able to prove the following identity,
$$\sum_{k=1}^{\infty}\frac{\Gamma(k+r)}{\Gamma(k)(k+r)^s}=\sum_{k=0}^{r}s(r+1,r+1-k)\zeta(s-r+k)$$
where $s(k,n)$ are the Sterling numbers of the first kind. To my satisfaction (and dismay) I was able to type this infinite series into WolframAlpha and it gave me my exact same finite summation to the point, meaning one I was correct but two it had already been done. After some googling I was seeing quite a few papers that tied finite sums involving Sterlings numbers and the Riemann Zeta function to infinite sums involving the Pochhammer symbol, $(x)_k$,
$$(x)_k=\frac{\Gamma(x+k)}{\Gamma(x)}$$
I was wondering if anyone knew whether these results are well known and/or what central theorem/idea they stem from.
Comment: Are you using the gamma function because you expect $r\in\mathbb{R}$?  If so, I will point out that defining the Stirling numbers for fractional values is a pain, and the RHS summation has issues with its upper bound as well.  If not, why not use the factorial?
Answer: Suppose we seek to simplify the sum
$$\sum_{k=1}^\infty \frac{\Gamma(k+r)}{\Gamma(k) (k+r)^s}.$$
with $r$ a positive integer. This is
$$\sum_{k=1}^\infty \frac{(k+r-1)!}{(k-1)! (k+r)^s}
= \sum_{k=1}^\infty \frac{1}{(k+r)^s} (k+r-1)^{\underline{r}}
\\ = \sum_{k=1}^\infty
\frac{1}{(k+r)^{s+1}} (k+r)^{\underline{r+1}}.$$
Now recall that
$$x^{\underline{r}} = \sum_{q=0}^r
(-1)^{r-q} \left[r\atop q\right] x^q.$$
We thus obtain
$$\sum_{k=1}^\infty
\frac{1}{(k+r)^{s+1}} \sum_{q=0}^{r+1}
(-1)^{r+1-q} \left[r+1\atop q\right] (k+r)^q
\\ = \sum_{q=0}^{r+1}
(-1)^{r+1-q} \left[r+1\atop q\right]
\left(\zeta(s+1-q) - \sum_{p=1}^r \frac{1}{p^{s+1-q}}\right).$$
Observe however that
$$ \sum_{q=0}^{r+1}
(-1)^{r+1-q} \left[r+1\atop q\right]
\sum_{p=1}^r \frac{1}{p^{s+1-q}}
\\ = \sum_{p=1}^r \frac{1}{p^{s+1}}
\sum_{q=0}^{r+1}
(-1)^{r+1-q} \left[r+1\atop q\right] p^q
\\ = \sum_{p=1}^r \frac{1}{p^{s+1}} p^{\underline{r+1}}.$$
However with  $1\le p\le r$ the term  $p^{\underline{r+1}}$ includes a
zero factor and hence the latter sum vanishes, leaving just
$$\sum_{q=0}^{r+1}
(-1)^{r+1-q} \left[r+1\atop q\right] \zeta(s+1-q).$$
To ensure convergence we must have $s-r\gt 1.$
Wikipedia lists the
Stirling number identity.
Comment: Is that the rising factorial you have going over there?  ($r$ in second line)


Non-proper surjective holomorphic map from the unit disk to itself

Question: Let $\mathbb{D}$ denote the open unit disk of the complex plane. Does there exist a  non-proper surjective holomorphic map $f \colon \mathbb{D} \rightarrow \mathbb{D}$? In other words, does every surjective holomorphic map $f \colon \mathbb{D} \rightarrow \mathbb{D}$ satisfy $\lim\limits_{\lvert z \rvert \rightarrow 1} \lvert f(z) \rvert = 1$?
Comment: Corrected. Thank you.
Comment: Using the fact that $f$ is proper iff $f$ is a (ramified) covering, the question is equivalent to find a surjective holomorphic map which is not a ramified covering... Don't know if it helps.
Comment: The proper holomorphic maps $\mathbb{D} \rightarrow \mathbb{D}$ are exactly the finite Blaschke products. Therefore, my question is equivalent to asking whether every surjective holomorphic map $\mathbb{D} \rightarrow \mathbb{D}$ is a finite Blaschke product.
Answer: A counter-example:
Let $g$ be a conformal map from the unit disk $\Bbb D$ to the semi-disk
$$
 G = \{ z : |z| < 1, \operatorname{Im}(z) > 0 \}
$$
which exists according to the Riemann mapping theorem, and has a continuous one-to-one extension to the closed disk (Carathéodory's theorem). It is not difficult to find an explicit expression for this mapping, but that is not needed here.
Now let $f(z) = g(z)^3$. $f$ is a surjective mapping from the unit disk onto itself, but for $z \to g^{-1}(\frac 12) \in \partial \Bbb D$ we have $f(z) \to \frac 18 \notin \partial \Bbb D$ .


Printing pdf files from firefox

Question: Got a brain teaser for everyone. :) I recently needed to turn in paperwork for insurance reasons. I found the pdf file for the form I needed online, however it would not let me print the file. When viewing the file in Firefox which is using the default viewer, I click the print button and click ok/print. The printer will just push out a blank page. If I save the file on my system and pull it up in any other pdf file viewer I can print the file without fail. Not sure what is happening. My printer is connected to my pc and the ubuntu is connected to a wireless network which shares the printer on the pc. The printer should be setup properly because I can print from any other application and any other file type. Not sure if this is normal or I am having a small issue. Have tried changing a few settings but no help. I have put everything back to the default settings the way it was. Anyone have any ideas? Not sure what else to do....
Comment: You should write your own answer in a separate answer instead of a comment and then accept your own answer. The answer you have accepted is not what worked for you as per your comment below.
Comment: It seems like [a bug inFirefox](https://support.mozilla.org/en-US/questions/997170)
Comment: Understood about the answer. My appologies, new to linux and the ask ubuntu website. Sincere thanks for the comment :) and the input on my issue.
Answer: I had this exact problem, but instead of installing the Adobe plug-in, I went into Firefox -> edit preferences -> Applications and changed the PDF action from "preview in Firefox" to "Use Document Viewer".  This launches a separate Evince window with the document which prints nicely.  I never liked the idea of pdf's being viewed in the browser window anyway, so I'm happy.  There is, however, something nasty wrong with Firefox for printing blank pages from it's preview window.
Comment: Thanks for the input. Happy to hear someone else had this issue. Wasnt sure if something was wrong with my setup or if this was a known problem. Starting to beleive its a problem with firefox/ubuntu. Thanks again for the input.
Answer:
Visit Firefox official printing help.
Reinstall Firefox.
Comment: I went to the printing help and could not find anything that dealt with my issue. I removed and reinstalled firefox, no help. What I ended up doing was installing adobe reader add-on to firefox and used it as the default viewer. This has solved my issue. I wanted to set this up anyway, was curious if anyone else ran into this issue though. Thanks for the help anyway.

Printing pdf files from firefox


How to default the working directory for JUnit launch configurations in Eclipse?

Question: Our Java code (not the test code) reads files from the current directory, which means the working directory needs to be set properly whenever we run the code.
When launching a JUnit test from within Eclipse, a launch configuration automatically gets created. The problem is, that the working directory in that launch configuration is always by default the root project directory which is always wrong, the test fails, I have to open the launch configuration dialog, change the working directory and relaunch the test. This is very annoying. The same thing happens when I run a single test method.
I've already considered these:

Changing the current directory from the test code - not possible by design.
When opening a file, pass a parent directory parameter - too difficult, since this would affect lots of places.
Use the Copy launch configuration feature of Eclipse to create new launch configurations from existing ones that already have a correct working directory set. This doesn't really makes sense here, since I would like to launch a test or a test method quickly, just by invoking "run this test / method as JUnit test".

All in all, it looks like it's responsibility of Eclipse, not the code.
Is there a way to set the default working directory for all future, newly created JUnit launch configurations?
Answer: This is a subjective answer:
I believe you're doing your tests wrong, you shouldn't be loading the files from the JUnit using  relative or complete paths, but instead, have them as resources in the project (add them to the build path) and load them as resources in the JUnit tests. This way if something changes on the filesystem, someone uses a different filesystem or IDE but still has them in the build path (as source folders) you're not going to have a problem.
I'm unsure if this is what you mean but if you really want to change it go to the Run Configuration options -> Your JUnit -> Arguments tab, at the bottom (Eclipse 3.4) you see Working Directory, click 'other' and change it from there.
Comment: What if you want to set the CWD as `target/my-test-loc` but `target` doesn't exist yet?
Answer: As far as I can tell, there's no way of changing the default working directory for future JUnit launch configurations in Eclipse 3.4. This issue has also been reported as a bug in the Eclipse bug database.
However, it is possible in IDEA. There's the Edit Defaults button for setting all kinds of defaults for each launch config type separately.
Comment: It might be possible for moreunit to work around this also. I've request a feature request for that here - https://sourceforge.net/tracker/?group_id=156007&atid=798059.
Answer: With Eclipse 3 you can set you working directory. Go to you run/debug configuration -> Arguments tab. In the Working directory select "other" and enter the root of your test
Answer: I haven't found a possibility to do this, but what you can do is to use:
getClass().getResourceAsStream(filename);
getClass().getClassLoader().getResourceAsStream(filename);
This methods locates a resource on the classpath.
The first one is relative to the location of the class, the second one is relative to any classpath "root entry". You can then for example add the project root directory to the classpath.
This does not work however if you want to write to a file as well.
Answer: If your tests depend on the current working directory, I think it is responsability of your tests to setup correctly that working directory, and to configure your classes to be tested to point that directory.
If you have a superclass for most of your tests, write a constant within it.
Or: if you have a superclass for most of your tests, write a @Before setup method.
Or: if you have not a superclass for most of your tests, write a constant in some class of the testing codebase.


Making Transparent Image Layer not effect top layer image

Question: I have an indesign image, that has a Transparency effect placed upon in via Objects->Effects. However, I would like the image that is on the top most layer above the transparent layer to not be effected by the Opacity of the bottom image. Is there a way to do this? I have tried the Knockout and Isolate options shown in the attached image to no success.
Answer: I'm not certain I understand.
I assume you are wanting the logo to be 100% opacity while the photo behind it is 60% opacity.
In order to do that you need the logo image placed as a separate object. InDesign does not allow you to pick and choose object layers for its effects. The InDesign effects apply to the entire object. They essentially alter the frame, not the contents within the frame.
If the photo/logo is a layered Photoshop document, you can adjust opacity in Photoshop, then replace the linked image rather than using InDesign for opacity.
Comment: Ah! I thought the logo was a subject object, but it wasn't!


dlls on windows and dylibs on macintosh

Question: I have joined a project that has been written in xcode using objective-C that is using some frameworks. This project is aimed at adding Wii-mote functionality to both the windows and Macintosh versions of Unity3d.
My question is how should I go about getting a windows dll that mirrors the functionality of the Macintosh .bundle/.dylib?
Do I need to start translating the code from Objective-C to C#?
or,
Do I need to find/code my own windows .dll from scratch?
or,
Is there a library type that is compatible with both windows and Macintosh?
Thanks in advance for your help!
Answer: There is not a library type that is compatible with both systems.  It might be possible to use C# libraries on both systems if you're using Mono on the Mac to run C# code, but you already stated that it's an Objective-C project, so I'm guessing that option is out.
So yes, you'll need to write your own .dll on the Windows side in your language of choice.
Comment: You may want to explain to the OP why that is, on the more basic levels (assembly, architectures, etc.)

dlls on windows and dylibs on macintosh


Is a Replacing GFCI difficult

Question: I have a GFCI outlet in my powder room that needs to be replaced.  Is it significantly more difficult to replace a GFCI than an ordinary outlet?
Comment: https://diy.stackexchange.com/q/28379/46271.  This possible duplicate has your answer.
Comment: The `LOAD` terminals have warning tape on them saying "Do not use - for wizards only" - not exactly, but that's what it means.  If you heed that advice, super easy.  If you want to play "Sorcerer's Apprentice", fairly hard and easy to botch.
Answer: No, but you do need to identify the "line" (supply) and "load" (additionally-protected portion of the circuit, i.e. downstream outlets) cables. If they're connected to the wrong terminals the downstream section of the circuit won't be protected.
With a standard outlet, connection location isn't important other than maintaining polarity.
Comment: For example, in our tract house both bathrooms are on one circuit and were protected by a single GFCI receptacle installed in one bathroom, the one closer to the electric panel. In that case I used the "load" connection to go to the farther bathroom. The deficiency in this arrangement is that if there would be a GF trip (we never had one) due to activity in the farther bathroom someone would have to go to the first bathroom to reset the GFCI receptacle. For some reason I later took out the GFCI receptacle and put in a GFCI breaker. A trip now would mean someone going to the garage.

Is a Replacing GFCI difficult


Hide authoring information programmatically in a content type

Question: How do I programmatically hide the Authored by part in a content type using hooks?
Comment: You know you can disable this manually by editing the article content type and unchecking `Display author and date information.` in the **Display settings** section?
Answer: I assume you need to hide the Authored by part basing on some conditions you check at run-time. If you need to hide that only for a content type, for example for the Article content type, then you can change settings in admin/structure/types/manage/article.

If you want to hide that information basing on the user ID of the user who created the node, then you could use the following code.
function mymodule_preprocess_node(&$vars) {
  $node = $vars['node'];

  // Remove that information if the node has been created from the user #1.
  if ($node->uid == 1) {
    $vars['display_submitted'] = FALSE;
    $vars['submitted'] = '';
    $vars['user_picture'] = '';
  }
}

Notice I am setting the same variables set by template_preprocess_node(), which uses the following code.
  // Display post information only on certain node types.
  if (variable_get('node_submitted_' . $node->type, TRUE)) {
    $variables['display_submitted'] = TRUE;
    $variables['submitted'] = t('Submitted by !username on !datetime', array('!username' => $variables['name'], '!datetime' => $variables['date']));
    $variables['user_picture'] = theme_get_setting('toggle_node_user_picture') ? theme('user_picture', array('account' => $node)) : '';
  }
  else {
    $variables['display_submitted'] = FALSE;
    $variables['submitted'] = '';
    $variables['user_picture'] = '';
  }

The template files used by Drupal core check $display_submitted before rendering $submitted and $user_picture. Probably Drupal core sets also $variables['submitted'] and $variables['user_picture'] for those template files that render them without checking first $display_submitted.
Comment: thx that's exactly what i'm looking for :)
Answer: This variable is added in template_preprocess_node, which is the second step in template processing.
These processors pass the variables by reference, so in any of the lower hooks (you can use a theme or a module to implement hook_preprocess_HOOK), you can alter this variable.
So in your module or theme (change MYMODULE with theme name), use this code:
function MYMODULE_preprocess_node(&$variables) {
  $variables['display_submitted'] = FALSE;
}


Feature importance ranking in random forest: intuitive explanation

Question: How the ranking of feature importance is generated in Random Forests? I'm searching for a less formal explanation, that is, an intuitive way to explain it, for instance in a nutshell.
Answer: There are at least three ways in which one can formally compute the relevance of each feature in a dataset once the forest has been built. However, rationale behind all of the procedures that I am aware of, is almost the same in all three cases.
When you are building a tree, you have some candidate features for the best split in a given node you want to split. If a feature is very important intuition tells us that it should produce a very good split, i.e., reduce the variability measure significantly.
Thus, the relevance of a feature can be defined as a sum of variability measure reductions over the nodes where feature appears.


How can I listen to new receive transfer?

Question: I want to listen to new transfers and get the hash and amount.
How can I?
Also, Can I get list transfers of account like this url?
https://westend.subscan.io/account/5D8JvKv1yL8ZPN9xxNbQiRxX14KvpiP8UJcDys1ouDnKpczx
Thank you
Answer: I would suggest splitting your questions into two - it is somewhat difficult to answer 2 completely different questions coherently in one.
First off, you mention "hash" - be aware that transaction hashes in Substrate are non-unique. Here are some older questions that deal with this. You also mentioned JS in your tags, so the FAQ entry could be useful, along with the wiki entry for unique identifiers
To listen for transfers, your best approach is to listen to events or indeed scan the blocks as the answer suggests.
For retrieving a list of transactions by account, here is a previous question that answers this in detail with specific tools that you can use.


Can someone take money out of my account with just my Account & Routing number?

Question: I had recently given someone my account number and routing number to deposit money into my checking account. They owed me money I had lent and said this would be the easiest way for them to do it. I had recently asked if they followed through and they blocked my phone number. I have a bad feeling they're trying to do more than deposit money. They don't know where I live, my real name nor do they have my card number. Is there anything they can really do with just my account and routing number? Should I get a new account? Will I be fine?
Comment: Yes; They can..
Comment: "Should I get a new account?" Yes you should
Comment: My bank has an option for a deposit-only account. No money can be withdrawn, only deposited. Only way to get money out is to transfer it to another account owned by me. So the account/routing numbers for my new account are theoretically safe since the bank will reject any withdrawal requests. That hasn't been tested yet. I started using this account since someone a year ago accidentally transposed the account numbers and made a withdrawal instead of deposit in a similar situation.
Answer: In theory they can, just like companies can directly take money (called Direct Debit here in the UK) from your account using those values.
However to be able to do direct debits you'd need an established business account with a major bank which won't do any good for a fraudster as it's pretty much impossible to open one anonymously.
Finally all transactions are logged and banks can reverse them if there is suspicion of fraud, so there's no need to worry. Account and routing numbers are fairly low entropy so if this was a good fraud opportunity we would already have crooks bruteforcing those numbers.
Comment: Account numbers are indeed low-entropy, and new accounts are often sequential, making them even more guessable. Routing numbers are public knowledge.
Answer: Disclaimer: I an not a lawyer...
Technically they can. More exactly, they can ask your bank to transfer money from your account to theirs, since they have enough information to uniquely identify both accounts.
The real question then is whether the bank will accept. From a legal point of view, as you did not explicetly allow the operation the bank should reject it. In a real world, banks do not always control everything, and some could simply accept the transfer. But as you did not allow it, they must refund your account if you ask for it - this may depend on local laws but at least in France it is clear: when a bank has validated a transfer that was not explictely allowed by the account owner, either with a signed order or with a numerically authentified operation, the bank is in fault and must refund the account if the owner asks for it. If the owner contests an operation, the bank must prove that it has been regularly approved - but if you contest an operation that you did allow, the bank can charge you with the research costs.
Answer: Do you know this person?  It's hard to tell if you have been victimized by a professional con artist or if this is just a bad friend/family member/acquaintance?  Do you know where this person lives?
I had an experience over a decade ago where I paid with a check and the cashier typed my account and routing number into a credit card machine and the bank processed it.  Then when I got the statement, they had also deposited the paper check.  It was a simple error on the part of the store and I worked it out with them.
What was really disconcerting to me was that when I called the bank and asked why they would process the same check number, for the same amount twice for the same retailer, the service rep got snippy with me and said that they didn't know whether it was my mistake or the retailers and said I needed to talk to the retailer.
To bring this back to your question, that experience makes me believe that this is probably easier to do than most people think.  Yes, an account would be needed with the bank but if it's a sophisticated operation, they may have gotten control over one the bank trusts.  Whether you should worry depends on a lot of things one being the country you live in.  If this is a personal account in the US under 100K, you are insured by the FDIC.  It might take a while to sort out if they drain your account and you probably have bills to pay so it will at least be a major annoyance.  If this is a business account, the bank is under no obligation to refund your money and there is no federal insurance.  If you are in this situation you should get an account with 2-factor security.
Regardless of what kind of account it is, you should watch it like a hawk and if there are any odd transactions, contact the bank immediately.  I would also call the bank and tell them of your concerns.  Ask them to not allow any transfers without your explicit authorization.  If they say they can't, you might want to look for a new bank.
Comment: Reversing the transaction only works if the other party is in a country that has a federal agency that cares about fraud.  Yes; the bank might reverse the check, even if they don't get the cash back, but we live in a digital world.  1 minute is enough to transfer the cash to an account which cash is withdrawn from.  You can with an American BoA, withdraw Euros, while in Italy only paying for the differences in actual value ( no actual fees ).  You can process a paper check digitally easily enough, even as a criminal, what do you care if it's illegal (your trying to steal money...)
Comment: @Ramhound I hear you but if the account is FDIC insured, it doesn't matter whether the transaction can be reversed.  Business accounts are another matter and more precautions are strongly recommended.  Chances are that if this person planned to steal, it would have already happened.


What is the best practice to resize game views-assets in WPF/silverlight?

Question: I am working on simple game using WPF/C#, and I am wondering what is the best practice to resize the game (views, assets…) when user re-sizes the game window? Is it good practice to use the ViewBox control for this job?
Comment: Could you add some information about how your assets are currently represented and handled?
Comment: @josh The assets are very simple, user controls contain star-shape(s) with some animations –using storyboards- and I use canvas to contain these assets, during the game-play the assets move inside the canvas and do some actions/animations. Also, and as usual, I have other canvas for game option and game score and so on...
Answer: This depends on the size you designed you application at and what resolutions you want to support, I've played with using a ScaleTransform to try getting my 800x600 app down to a smaller size for the windows phone 7 (I ran into other problems while converting my silverlight app to the windows phone 7 sdk so I gave up, but the scaling worked pretty nicely).
Mike Snow has a very good article on how to do this along with hooking up to the Content_Resized event in your Page:
Silverlight Tip of the Day #33: How to Scale your entire App and its Elements to your Browsers Size
You do want to be careful since scaling your app down too small will make certain elements like text too small to read and unusable.


Is ${G}/\ker(\phi)$ uniquely isomorphic to $\phi(G)$?

Question: Let $\phi:G \rightarrow G'$ be a group homomorphism. Then, by the First Isomorphism Theorem for groups, $\frac{G}{\ker(\phi)}$ is isomorphic to $\phi(G)$. For ease of notation, denote $\ker(\phi)$ by $K$. Is the "usual" isomorphism, $\psi: \frac{G}{K}\rightarrow\phi(G)$ defined as $\psi(aK)=\phi(a)$ the only isomorphism?
Comment: You can compose $\psi$ with an automorphism of $G/K$.
Comment: The only automorphism need not be the identity. Counterexamples abound.
Comment: Right, of course...maybe a better question would be if the only automorphism would be the identity map. I'll edit.
Comment: And every group with order more than two has a nontrivial automorphism? So that means we can have more than one isomorphism in all cases except when G itself is the trivial group or the group of order 2.
Answer: While it is true that the isomorphism $\psi: G/ \ker(\phi) \to \text{im}(\phi)$ is not the only one (as mentioned above), the reason $\psi$ is often called "usual" or "canonical" is that it is the unique isomorphism (even homomorphism) that is compatible with the projection $p : G \to G / \ker(\phi)$ and inclusion $j : \text{im}(\phi) \to G'$.
Explicitly, given the homomorphism $\phi : G \to G'$, one can form the following commutative diagram.
$ \require{AMScd}
\begin{CD}
\ker(\phi) @>{i}>> G @>{\phi}>> G' \\
& & @V{p}VV & @AA{j}A \\
 & & G / \ker{\phi} @>{\psi}>> \text{im}(\phi)
\end{CD}
$
If $f: G / \ker(\phi) \to \text{im}(\phi)$ is any another homomorphism making the diagram commute (i.e. such that $j \circ f \circ p = \phi$), because $p$ is surjective and $j$ is injective we can right-cancel the former and left-cancel the latter from the equation $j \circ f \circ p = j \circ \psi \circ p$ to otain that $f = \psi$.
Answer: As mentioned in the comments above, one can simply (right-)compose $\psi$ with an automorphism of $\frac{G}{K}$ (call this $\pi$) and obtain an isomorphism, $\psi \circ \pi: \frac{G}{K} \rightarrow G'$. Every group with more than two elements will have a non-trivial automorphism, and we can use that as $\pi$, thereby obtaining a "new" isomorphism $\psi\circ\pi$. For the groups of orders one and two, the only automorphism is the trivial/identity automorphism.
In such a case, $\psi$ is the only isomorphism.


If my iPhone was stolen and I decide to cancel my phone plan, would it still show up on my iCloud?

Question: If the thief uses the device with another sim or cellphone service would it still be tied to my iCloud?
Answer: There is no relation between the carrier's plan for cellular services (which is tied to your carrier) and iCloud (which is a service from Apple). So even if you cancel your phone plan after it's stolen, the iCloud linkage that was established when you setup your phone (or later modified it) would continue to remain with Apple.
If your iPhone is running iOS 7 and you have Find my iPhone turned on (Settings > Privacy > Location Services > Find my iPhone), then the person who has your phone cannot erase it or use it without your iCloud account and password.
If your iPhone is lost, you can visit http://www.icloud.com, login with your iCloud account (the one setup on your iPhone) and use Lost Mode (which prevents activation of the phone without your iCloud account credentials and also allows you to send a message to the device) or Erase iPhone (which erases your iPhone remotely).
Also see:
iOS 7: Activation Lock + Find My iPhone
How To Enable Activation Lock On Your iPhone And iPad (iOS Tips)
Answer: No, depending.
They can't necessarily access your data and account if it's logged out on the phone. I suggest changing passwords you think might be saved so that it forces them to be re-entered.
On the iCloud website, you might be able to disable your iPhone permanently until they use your password.
You can access all iCloud and Apple services through the various websites or on other devices like a Mac or iPod/iPad. Maybe you have Find My iPhone and can access that app through the iCloud website?

iCloud
AppleID


Use Django-Storages with IAM Instance Profiles

Question: Django-Storages provides an S3 file storage backend for Django.  It lists
AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY as required settings.  If I am using an AWS Instance Profile to provide S3 access instead of a key pair, how do I configure Django-Storages?
Answer: You simply omit these parameters from your settings.
The Django-Storages documentation now explains this:

If AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY are not set, boto3 internally looks up IAM credentials.

The way this works under the hood is that if you do not provide them, Django-Storages passes None to boto3, which uses the machine's privileges instead of a key pair.  If the machine has an associated Instance Profile, this is what gets used.  (See the boto3 docs for more on boto3's credential hierarchy)

Thanks to @ChrisShenton for pointing out that the Django-Storages docs had been updated.  The Django-Storages docs previously listed these configuration parameters as required, which was incorrect.
Answer: The docs now explain this:

If AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY are not set, boto3 internally looks up IAM credentials.


How do I sanitize a javascript text?

Question: I have a textarea that will receive a js snippet(Google Analytics). Is there a way to sanitize  that? Since I cannot use functions like wp_filter_nohtml_kse(), what should I use?
Answer: One simple way to do it would be to have the user enter just his or her Google Analytics Property ID, instead of having them input the entire JavaScript code.  Then you generate the snippet yourself, using their Property ID.
According to this Google help page, here is the current Analytics tracking code:

The 'UA-XXXXX-Y' is the Property ID.  It looks like the first two characters are always 'UA'.  'XXXXX' represents some number of numeric digits (not necessarily five digits, since one of the Property IDs for my sites has eight digits). 'Y' is an integer that might have more than one digit.
One way you could validate the Property ID would be with a regular expression, like this:
preg_match('/^UA-[0-9]+-[0-9]+$/', $input);

This will return 1 if $input is a valid Property ID format, and false if it is not.
Comment: Yeah, I know this method. But I prefer to give the user a textarea, so advanced users can tweak their codes.
Answer: No, there is no function for that. You would need a complete JavaScript parser. This is not part of the WordPress core.
Comment: Yes, there is no solution like that. I also stuck on JSON string sanitization.
Answer: If you want to allow the user to be able to input their own Javascript code, but you don't want to allow them to enter any HTML, you could use something like this:
preg_match( '/\A\Z/', trim($input) );

The trim function removes whitespace from the beginning and end, and the regex pattern will only match the string if it begins with '', and doesn't have any '<' characters immediately followed by a letter.  That should effectively keep any stray HTML out, while allowing the user to put some Javascript code using a less than operator, if they need to for some reason.
The other thing I would do is make sure that only an administrator can set this option, and if he or she sneaks some HTML into there and breaks their own website, it's their own fault.  Of course, if this is going into the database, make sure it is sanitized for SQL.

How do I sanitize a javascript text?


Wire tag vs. wires tag

Question: Wire is useful. "Wires" as a tag is not. I've edited the question that has wires and removed it (admittedly its replacement, wiring, is almost as bad a choice) but I still can't attach wire to a question about Wire. I believe someone may need to nuke wires directly, but is there any way to prevent this from happening in the future?
Answer: No one really can nuke wires. There is a system background process that is ran daily that cleans up tags without any questions.

Tag it wire-library so you can get it past the system (and it's much less ambiguous)

As far as what to do to prevent this from happening, I did this this:

Synonymize wires ← wiring, because someone wouldn't type "wiring" when referring to the library. The system will automatically rename everything tagged "wiring" to "wires."

This way, if someone types wires or wiring, they will get wiring. If they type wire, it will have an option for "wire-library" and "wiring." This will fix the problem.
We should leave wire alone. It's way to ambiguous to be synonymized to one of the two types of wire.
Comment: I have tagged the question [tag:wire-library] and have edited the tag to provide some useful information.

Wire tag vs. wires tag