Why do Sarah and Abraham doubt they will have children in Genesis 17 and 18 when their forefathers conceived at older ages?

Question: We are told in Genesis 21:5 that Abraham was 100 when Isaac was born. In Genesis 17:17 Abraham doubts that a child could be born to someone of 100 years old. But we are also told that Adam was 130 when he fathered Seth, Seth was 105 when he fathered Enosh, Methuselah was 175 when he fathered Lamech, Lamech was 182 when he fathered Noah, and Noah was 500 when he fathered his three sons. Abraham's own father was 70 when Abraham was born, which would still be considered old be modern standards. So it appears that having a child in one's old age was not an uncommon thing. Why, then, would Abraham and Sarah doubt their ability to birth a son at 100?
Comment: Longevity was rapidly reducing. Abraham's father begetting at 70 would indicate that the next generation (looking statistically at the declines) would be less. It seems to have bottomed out at about that age, since then. Some claims of more are doubtful. But Sarah had 'ceased' as is the way with women. That is permanent. The reversal of it, is supernatural. I don't think you have properly considered the details of your question, myself.
Comment: @NigelJ well spotted. Initially I thought it to be fine as it is was but the question assumes that age was the only issue at stake and that's not true like you also referred. Hence I agree the question needs clarity in why should we ignore Sarah's state and progress with looking only at age.
Answer: Earlier, God pronounced a reduction on human longevity in

Genesis:6:1 When human beings began to increase in number on the earth and daughters were born to them, 2the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 3Then the Lord said, "My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years."

By the time of Abraham, it was extremely unusual to father a child at 100 years old.
Genesis 17:17

Abraham fell facedown; he laughed and said to himself, "Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?"

The word "doubt" is not found here. He laughed.
Comment: I'd like your opinion on something: I have been told that "their days will be a hundred and twenty years" was referring the flood, which occurred 120 years after this pronouncement, and not necessarily to the age of men. Do you find this to be a valid possibility? Thanks for any insight.
Comment: Even if you accept this interpretation, you still cannot deny the other interpretation. I can accommodate both.
Comment: Please understand, I'm not denying it, I'm only seeking clarity. Do you think it's reasonable to accommodate both? I don't want to read something into scripture that may not be there, whether it's implying the lifespan of man, the time until the flood, or both.
Comment: The lifespan-of-man interpretation actually implies the time-until-the-flood interpretation. It is reasonable to accept both.
Answer: The answer here is as simple as the recognition of the biological fact that Sarah was menopausal and past the age of child-bearing as recorded in Gen 18:11

Abraham and Sarah were already very old, and Sarah was past the age of
childbearing.

The ancients understood that when women reached menopause, no conception was possible under normal conditions.  If Sarah was to have children, it would be a miracle - it was such a possibility of a miracle to which both Sarah (Gen 18:12, 13) and Abraham (gen 17:17) laughed.  They we both rebuked by the LORD for this lack of faith.
I often think that perhaps God deliberately waited until Sarah was wellr past the age of child-bearing to make the miracle very obvious, and thus to ultimately strengthen their faith.
Answer: Besides the fact of Sarai's menopause, The simple answer is that Abram would have looked back, not at the earliest of his ancestors, but at his most recent ones.
Take a look at his forefathers, and keep in mind that Abraham had not had a single child yet:

Shem, the last to be born prior to the flood, fathered Arphaxad at 100

But then, for everyone born after the flood, everything changed:

Arphaxad fathered Salah at 35
Salah fathered Eber at 30
Eber fathered Peleg at 34
Peleg fathered Reu at 30
Reu fathered Serug at 32
Serug fathered Nahor at 30
Nahor fathered Terah at 29
Terah fathered Haran at 70 before he had Abraham
Abraham fathered Isaac at 100

Almost all of them began having children in their early 30's.
Why Abraham's father Terah started so late is up for speculation, but it's not unreasonable to think that Abraham would have been concerned about not having any children by this point, even far past his father, who was already quite old before he had children.
Answer: I'm late to the thread.  For the sake of others reading in the future I would challenge the assertion that Abraham doubted a child could be born to someone who is 100 years old, in the sense that he was commenting on male fertility in general.  I think he was specifically commenting on his own age-related lack of performance.  My reasons are:

Scripture requires us to understand Terah fathered Abraham at the age of 130.  I know it is more popular to believe Abraham was born to Terah when he was in his 70s.  The problem is Stephen in Acts 7:4 says Abraham left Haran after his father died.  We know from Genesis that Terah lived to be 205 and that Abraham left Haran at age 75.  If all three scriptures are correct we are compelled to believe Terah fathered Abraham at 130.
This fact alone invalidates the assumption Abraham was commenting on his understanding of the history of male fertility because he would have known his father's more advanced age.  Based on the limited post-flood genealogy it may appear on the surface Abraham's birth would have been a very unique occurrence.  But all we know from those few ages stated are that the post-flood fathers typically began fathering children in their 30s.  We don't know the age when each ceased function.

Late male fertility was not surprising in the case of Ishmael.  I note that neither Abraham nor Sarah seemed surprised that Abraham was able to father Ishmael at age 86.  I think that Ishmael represented what seemed possible to Abraham and Sarah at the time.  But age 86 isn't all that far from age 100 given the extended lifespans at that time.  It seems that in the intervening 14 years between Ishmael and Isaac that Abraham must have ceased performing.

I believe the assertion that Abraham was commenting on male fertility, in general, is a translation issue due to the translator's interpretation.  I note that various English translations have significant differences.  The KJV makes it clear that Abraham was referring to his own body and not male fertility in general.  It says:  "Shall a child be born unto him that is a hundred years old".  Note the word "him" and not "a man" as in many modern translations.  The sense in the KJV is that Abraham was commenting on his own body's lack of performance and not his understanding of the history of male fertility.
Comment: Welcome to Bible Hermeneutics and thank you for your contribution. When you get a chance, please take the [tour] to understand how the site works and [how it is different than others](https://hermeneutics.meta.stackexchange.com/questions/653/how-can-we-educate-new-users-about-our-site-distinctives/803#803).


Mac: How to log in as different user when Mac requires password to wake from sleep?

Question: If your Mac is set up to require a password when being awoke from sleep or the screen saver is it possible to log in as a different user? If so how?
Answer: If you enable "Show fast user switching menu" in System Preferences → Accounts, then the lock screen gains a new button "Switch User..." which will take you to the login screen.
On Mac OS X Snow Leopard and previous, the Switch User button appears on the password dialog:

On Mac OS X Lion, the button appears at the bottom of the screen:
Comment: FYI- there is a [related bug in Mountain Lion](http://superuser.com/questions/465441/switch-user-from-locked-screensaver) if there is only one *Local* user account and a *Network* user wants to log in from the lock screen.
Comment: Thanks! That's what I thought but had to look a bit harder for it in Lion. It's no longer in the dialog as you've shown. It's now at the bottom of the lock screen.
Comment: Ah, so you're right. I don't have Lion so I didn't know they moved it. I added that detail to my answer, thanks!


Permission upload/overwrite

Question: we need to set up a new permission level for external invited users in sharepoint online:
They should overwrite existing files, but not create or upload any new content ( file and folder ).
Comment: SharePoint mirrors the NTFS permission model. To do this in NTFS, it requires Delete and Create rights, because an overwrite is a combination of those actions. Thus, this isn't possible in SharePoint in a permission model -- you'd have to build another business process to do so, i.e. use another library where they can drop a document and Power Automate to match a file name in a destination library and copy over, if not, email the submitter that the file is invalid, etc.
Answer: Mostly is like Trevor said, overwrite needs Delete and create permission, as long as the external user can overwrite, he can create new file.
Here is more information about SharePoint permission level: https://docs.microsoft.com/en-us/sharepoint/sites/user-permissions-and-permission-levels

Permission upload/overwrite


Why is a gradient field a special case of a vector field?

Question: My calculus manual suggests a gradient field is just a special case of a vector field. That implies that there are vector fields that there are not gradient fields. The gradient field is composted of a vector and each $\mathbf{i}$, $\mathbf{j}$, $\mathbf{k}$ component (using 3 dimensions) is multiplied by a scalar that is a partial derivative. Is this because the scalar may be of the form that there is no antiderivative of? Perhaps of a function that can not be differentiated? Maybe a vector field that has sharp turns but I can't come up with any that I can't find a derivative for. Does anybody have any so I can get some intuition of the problem?
Comment: Consider a "rotational" vector field, for example $(-y,x)$ in 2D. Suppose it was a gradient of a scalar field; apply the gradient theorem on a circle around the origin and arrive at a contradiction.
Comment: See also [Helmholtz decomposition theorem](https://en.wikipedia.org/wiki/Helmholtz_decomposition). Roughly speaking any vector field in $\mathbb{R}^3$ can be decomposed as ${\bf V} = \nabla \phi + \nabla\times {\bf A}$ for some scalar field $\phi$ and vector field ${\bf A}$.
Answer: If you have a gradient field $(X=\partial_xf,Y=\partial_yf,Z=\partial_zf)$,
by using the relation ${\partial^2f\over{\partial x\partial y}}={{\partial^2f}\over{\partial y\partial x}}$ you obtain $\partial_yX=\partial_xY$. This is not true for every vector field.
For example $(x,x^2y^2,z)$ is not a gradient vector field.
Comment: And working out the converse so that you can decide when a given field is a gradient field is important in physics and mathematics.
Answer: A vector field is just a function $\mathbf{F}:\mathbb{R}^n\to\mathbb{R}^n$. (Or the domain can be some subset $D$ of $\mathbb{R}^n$, not all of it, of course. To avoid making this remark repetitively, I'll just stick with $\mathbb{R}^n$, as an example). From an intuitive/geometrical point of view, it is best visualized as having a vector attached to (or starting from) each point of the domain $D$. Given a multivariable function $f:\mathbb{R}^n\to\mathbb{R}$, its gradient $\nabla f$ is precisely that kind of a function, as it acts from $\mathbb{R}^n$ to $\mathbb{R}^n$, so it is a vector field.
What the book says is that not all vector fields can be obtained in this way. Intuitively, it should make sense: the fact that the components of $\nabla f$ are derived from the same original function $f$ should result in some relationships among them. If you put random functions as components, most likely they wouldn't be related in that special way. It's like siblings: being the descendants of the same parents they have some things in common, something that random people wouldn't.
In more mathematical terms, one of such special relationships is the condition of equality of mixed partial derivatives. Say, for a function of two variables $f:\mathbb{R}^2\to\mathbb{R}$, we must have $\displaystyle \frac{\partial^2f}{\partial y\partial x}=\frac{\partial^2f}{\partial x\partial y}$, assuming these derivatives are continuous. Therefore, if $\displaystyle \mathbf{F}=\nabla f=\left\langle\frac{\partial f}{\partial x},\frac{\partial f}{\partial y}\right\rangle=\langle M,N\rangle$ is its gradient, then we have to have
$$\color{blue}{\frac{\partial M}{\partial y}}=\frac{\partial^2f}{\partial y\partial x}=\frac{\partial^2f}{\partial x\partial y}=\color{blue}{\frac{\partial N}{\partial x}}.$$
So being the gradient is very special! This condition allows you to construct vector fields that are not gradients. Pretty much anything you would put into a vector field at random almost surely wouldn't be a gradient. For example, $\mathbf{F}(x,y)=\langle 2x+3y,4x+5y\rangle$ isn't a gradient, because
$$\frac{\partial M}{\partial y}=3\neq4=\frac{\partial N}{\partial x}.$$
Comment: @Sedumjoy : The gradient is irrotational.  For an example of something that looks like a rotational gradient, see Escher's [Ascending and Descending](https://en.wikipedia.org/wiki/Ascending_and_Descending).  Note that there is no way for that staircase to exist as it appears to.  (Going around the circuit once, you descend always.  You must be lower when you return to your starting point.  But this is absurd, the function whose gradient you are taking only has one value at that point.)
Comment: @IvanGonzalez: See [Clairaut's Theorem](http://mathonline.wikidot.com/clairaut-s-theorem-on-higher-order-partial-derivatives).
Comment: That is amazing....but from an intuitive geometrically visual point of view what does the geometry of the second partials being equal do to the vector field!  After all when you map that vector field that happens to be a gradient you draw the First derivative vectors , correct??? the second derivative vectors are the change of slope of the First derivative vectors, correct ?  so if the order does not matter and you get the same change what does this mean visually and geometrically...maybe you can tell me?
Comment: Why we must have $$\frac{\partial^2f}{\partial y\partial x}=\frac{\partial^2f}{\partial x\partial y}$$?
Answer: General vector fields can generate flows and circulations. A gradient fields and only gradient fields (under some additional regularities) always generate circulations that amount to zero. Try to compute the circulation of a gradient field in a plane around an elementary rectangle centered at any point whose sides are parallel to the coordinate axes. You will get an elementary circulation that is zero simply from the fact that mixed second order partial derivatives of the scalar field does not depend on the order of derivation. Notice that the density of such elementary circulations (that is, the curl of the gradient field in every point) is also null.
So gradient fields and only gradient fields (under additional regularities) have curl identically equals to zero.
You can also see that there are fields whose flows (and elementary flow density in every point, that is their divergence) always amount to zero.
Comment: @Sedumjoy: A [Google image search](https://www.google.com/search?q=gradient+vector+field&tbm=isch) turns up plenty of examples. Although a few of those are examples of vector fields that are specifically *not* gradient fields, so do click through to the actual web pages and read the image descriptions there.
Comment: @Sedumjoy i found this helpful https://malarney.github.io/vector-calc-visualization/
Comment: You wouldn't happen to  know of any two or three dimensional pictures of graphs that show a gradient field that you can share with me do you....I mean so I can compare that to a vector field that goes in a circle?
Comment: @llmari Karonen  Your Google...brings up some fine examples. . Thank you
Comment: @Koon W . thank you i will check it out


Why did flying come so difficult for Clark?

Question: Other Kryptonians seemed to perfect this power rather quickly but it took Clark all ten seasons- was there ever an explanation for why he struggled so much with flying?
Comment: out-of-universe, it was an explicit decision by the producers to have "no flights/no tights" for Clark until the very end.
Answer: Because he was burdened with worry and self-doubt, figuratively weighing him down.
Or, as the Smallville Wikia puts it:

Clark has been able to fly since he was 14, but cannot consciously use it. This is apparently due to his own psychological barriers.

To a large degree this was the overarching message of the entire show!
Notice how meeting real family (Kara) got him close [7x15 "Veritas"], and being a super-confident madman ("Kal-El" under Jor-El's control) took away the problem too [4x01 "Crusade"] … but as Clark under normal circumstances, it took getting comfortable with Lois and seeing fragments of a future in which he completely trusted her, to really begin the journey towards control over his flying ability [10x04 "Homecoming"].
Sadly, the finale had Jor-El just magically turning the powers on completely, rather than showing us some moment in which Clark finally realised he was "ready to go". Oh well.
Comment: I don't think Jor-El "magically" did it. The first time he flew intentionally was when fighting Darkseid in Lionel's body: he had a "flashback sequence" of some sort showing all the heroic things he'd done and when he snapped out of it, he was flying.
Comment: @MichaelEdenfield: The "flashback sequence" included Jor-El telling him that his time had come and that he was proud of him etc etc. Then suddenly he's flying? Not a "leap" to interpret this as Jor-El unlocking the power.
Comment: @LightnessRacesinOrbit maybe the speech from Jor-El is what kicked Clark's self-confidence into overdrive?
Comment: @Omegacron: Maybe but it's hardly the first time Jor-El gave a speech like that.


GRUB for Android taking too long to install

Question: I have this Digital2 10 inch Windows 8.1 tablet and I want to install Android Lollipop as the main operating system. So I properly load the android installer image to a usb, boot the tablet from usb, and it loads and even runs live just fine. The problem is that when I select install grub bootloader, it seems to hang on the screen. The confirmation window stays there, but a black bar with a blinking MS-DOS like cursor for more than 10 minutes. Is the installation of grub in this manor supposed to take this long, or should I try some other images?
Answer: It appears that I just needed to go into the bios and enable some kind of linux mode. That's something I didn't expect to find on a Windows tablet.


Auto Revoke Shared Document Permissions

Question: Is it possible to auto revoke permissions on a shared folder or document after a period of time? E.g., if I share "Document A" or "Folder A" with a user for 4 days, after 4 days the users permission should automatically expire.
Answer: There is no OOTB feature which will allow you this, but you can create a workflow to achieve this feature.

Create extra columns SharedWith- person or group  and Duration - Number.
Create a designer list workflow which assign permission to the user and wait for n days and revoke the user permission.
Run the workflow manually when you need to Share the document

Auto Revoke Shared Document Permissions


Subsequent NSURLConnections not firing delegate methods

Question: I've looked all over for answers to this question and have seen a lot of similar issues but the solutions did not seem to pertain to my situation. I'm very new to iOS development so any help would be appreciated.
The current app is looking to display events happening in a city based on an XML feed being generated by their website. I'm storing the events in a plist and checking for new events explicitly on app load. The current process is as follows:

Application launches and makes a NSURLConnection to check the server for a timestamp and download it.
Checks the newly downloaded timestamp vs the local timestamp to see if it is greater. If it is, it rewrites the file containing the plist and fires a notification that new events are needed.

This is where the second NSURLConnection does not seem to fire. The callback method itself fires but it does not seem like the delegate methods fire after.
The strange thing is that if I call both connections at the same time, as opposed to one as a result of the other, they both will fire and call the delegate methods.
Here is the code for some of the classes with some of the guts removed for clarity:
Application Delegate:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

// Set up analytics account

// Set the tab bar controller as the window's root view controller and display.
self.window.rootViewController = self.tabBarController;
[self.window makeKeyAndVisible];

//Set up listeners for event handling
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(timeStampResponse:) name:@"TimeStampResponse" object:nil];

//Set app loading screen

//Create new OperationQueue, set ActivityIndicator and run TimeStamp Check
parseQueue = [NSOperationQueue new];
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
[self timeStampCheck];
return YES;
}
-(void)timeStampCheck {
NSLog(@"Time Stamp Check, Creating Connection");
NSURLRequest *timeStampURLRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:kTimeStampURL]];
self.timeStampConnection = [[NSURLConnectionWithTag alloc] initWithRequest:timeStampURLRequest delegate:self startImmediately:YES tag:1];
}

-(void)newEventsNeeded {
NSLog(@"Event File Doesn't Exists or New File Needed, Creating Connection");
NSURLRequest *feedURLRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:kEventFeedURL]];
self.eventsFeedConnection = [[NSURLConnectionWithTag alloc] initWithRequest:feedURLRequest delegate:self startImmediately:YES tag:0];
}

//Callback from DateStampParser
-(void)timeStampResponse:(NSNotification *)notif {
NSString *response = [[notif userInfo] objectForKey:@"TimeStampResponse"];
if ([response isEqualToString:@"NewEvents"]) {
    [self newEventsNeeded];
    self.appLoadingViewController.loadingLabel.text = @"New Events Found...";
} else if ([response isEqualToString:@"NoNewEvents"]){
    [self allEventsLoaded];
}
}

And the NSURLConnection Delegate methods
-(void)connection:(NSURLConnectionWithTag *)connection didFailWithError:(NSError *)error {
if (connection.tag == 0) {
    NSLog(@"Connection failed on Event Feed");
} else if (connection.tag == 1) {
    NSLog(@"Connection failed on Time Stamp Check");
}
}

-(void)connection:(NSURLConnectionWithTag *)connection didReceiveResponse:(NSURLResponse *)response{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
NSLog(@"connection:didRecieveResponse:NSHTTPURLResponse");
if ((([httpResponse statusCode]/100) == 2)) {
    //Handling for eventsFeedConnection
    if (connection.tag == 0) {

        NSLog(@"Connection tag = 0");
        self.eventsData = [NSMutableData data];

    } else if (connection.tag == 1) {

        NSLog(@"Connection tag = 1");
        //NSLog(@"Timestamp connection received response");
        self.timeStampData = [NSMutableData data];

    }

} else {
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Network Error" message:@"No Response from the Server" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
    [alert show];
    [alert release];
}
}

-(void)connection:(NSURLConnectionWithTag *)connection didReceiveData:(NSData *)data{
//Handling for eventsFeedConnection
if (connection.tag == 0) {

    self.appLoadingViewController.loadingLabel.text = @"Downloading Events...";
    [eventsData appendData:data];

} else if (connection.tag == 1) {

    self.appLoadingViewController.loadingLabel.text =@"Checking for New Events...";
    [timeStampData appendData:data];

}
}

-(void)connectionDidFinishLoading:(NSURLConnectionWithTag *)connection{
//Handling for eventsFeedConnection
if (connection.tag == 0) {

    NSLog(@"EventFeed Connection Finished");
    self.eventsFeedConnection = nil;
    [eventsFeedConnection release];
    ParserOperation *parser = [[ParserOperation alloc] initWithData:self.eventsData];
    [parseQueue addOperation:parser];
    [parser release];
    self.eventsData = nil;
    //[eventCategoryListViewController reenableRefreshButton];

} else if (connection.tag ==1){

    NSLog(@"TimeStamp Connection Finished");
    self.timeStampConnection = nil;
    [timeStampConnection release];
    DateStampParser *parser = [[DateStampParser alloc] initWithData:self.timeStampData];
    [parseQueue addOperation:parser];
    [parser release];
    self.timeStampData = nil;

}

}

Not sure if it may be obvious from just this code or not what the problem is but if any additional code is needed/clarification on what I'm trying to accomplish is needed please let me know.
Thanks
edit for clarity
Just to clarify, the NSLog from newEventsNeeded fires, but just the delegate methods don't get fired.
Comment: I can't see where you fire the notification. Is it in some code that isn't shown in your question?
Comment: The notif is at the end of the DateStampParser operation. It fires a notif that's recieved by timeStampResponse:.
Comment: I know this is a real old post, but I wanted to just mention the way I ended up fixing it was just by circumventing the real issue by using the ASI HTTP library.
Answer: Make sure the thread starting the NSURLConnections has a runloop. NSURLConnection's asynchronous callbacks are dispatched onto the calling threads runloop. If the calling thread (in this case, it seems to be an NSOperation queue) doesn't have a runloop, or if it's not operating in the default run loop mode, you won't see any delegate callbacks.  Your first call to [self timeStampCheck] is on the main thread, so that's fine. Your later calls are from the operation queue, which possibly has no runloop.
The relevant snippet from the documentation:

delegate
The delegate object for the connection. The delegate will receive
  delegate messages as the load progresses. Messages to the delegate
  will be sent on the thread that calls this method. For the connection
  to work correctly, the calling thread's run loop must be operating in
  the default run loop mode.


Identify Science Fiction Story about underwater city and liquid breathers (Not "The Abyss")

Question: I read a story in the 1970s, probably older, the only part I remember was the person had his gag reflex cut so he could live in the undersea city.  It was separated from the rest of the ocean only by a membrane to keep the waters separate.  It had oxygenated water that you breathed.  It mentioned he could not go back to land because he might choke since no more gag reflex.  He might have been captive.
Comment: I don't know the book you mean, but you might enjoy "Crisis on Conshelf Ten" by Monica Hughes. There were under sea cities, and humans surgically adapted to living under water.
Comment: Gratuitous comment: I loved "The Abyss", the oxygenated water breathing, the happy ending, all of it. Even Bruce Willis was tolerable.
Comment: @FeralOink Bruce Willis wasn't in The Abyss... Perhaps you're thinking of Ed Harris?
Comment: @PhilPursglove Yes, I checked IMDB, you're right! That's why I liked him most likely. Because he wasn't Bruce Willis ;o) Thank you.
Comment: One of these might be what you are looking for http://io9.com/5560901/the-11-greatest-underwater-cities-of-science-fiction
Answer: Could it be "Ocean on Top"  (1973) by Hal Clement? The membrane separating the ocean from the breathable fluid matches.
Edit: I checked the text, and I believe we have a match. The main character is trying to infiltrate the underwater society. He is captured and agrees to the adaptation process after first asking:

'Bert, could you go back up above now if you changed your mind about staying? Or is what they did to let you breathe water impossible to reverse?'
  He smiled and used the stylus again.
  'We're not breathing water; that analysis misses on two counts. They did make an irreversible change, but it's not a very serious one. I could still live at the surface, though the shift back to air breathing would be somewhat lengthy and complicated.'

And then afterwards:

I  tried forcing myself to breathe.
  I found I could squeeze liquid slowly out of my lungs and get it back equally slowly, but it hurt and made me feel even dizzier than being right side up and upside down
  simultaneously. The liquid went into my windpipe; I could feel it, but there was no tendency to cough. I still think that must have been one of the trickiest parts of the conversion procedure, considering the nerve and muscle activity which coughing involves.
Comment: That was it exactly, thank you very much.  I repurchased the book via eBay for $6 and I am re-reading it 40 years later.  How did you find the text? Is it available as epub?  I did another search last week and came across my old post and your answer.  I quit looking after a year or so.
Comment: @Pioneer4x4, I don't recall now where I got the text. I may have typed it in from the paperback. It is available at archive.org in the IF magazine collection: Oct, Nov, & Dec 1967.
Comment: @Pioneer4x4: If this was the right answer, you should "accept" it by clicking the green check-mark to the left of it.
Comment: Thanks, I had trouble seeing it, it only turned green after I checked it.  Also the book was better than I remembered, I probably read this in the early 70s, the science premise was quite interesting.


XCode 4.5 Fails to Archive Phonegap Project

Question: Im trying to Archive a Phogap project using XCode 4.5 but whenever I do I'm getting the error: Command /usr/sbin/chown failed with exit code 1  Has anyone ever experienced this error before? If so, how can I fix it? Thanks
Answer: If the error mentions something about '3 Slices' none containing armv7s .
Goto Your Project > Build Settings > Remove 'armv7s' from 'Valid Architectures'
in both your project and the phonegap project as well.
Answer: The following worked for Xcode 4.5:
The fix is by removing the armv6 architecture from both your project and the CordovaLib project, clean
both projects, and rebuild.
Source


Which cybersecurity job is most likely to be remote

Question: When looking to get into cybersecurity what job/certification is most likely to be available as a remote position?
Comment: Probably a bit too broad - selling cybersecurity services might be entirely remote based, but probably isn't what you're after. Also, specific types of testing may have different requirements depending on the client - government work might require on-site presence, whereas a very similar test for a small business might be performed from home.
Comment: "Most likely"? That's an odd constraint. All can be. Managed services are becoming the norm. I could say that a cybersecurity reporter is most likely to be remote. As is being an author, or a penetration tester for hire. I think you will need to explain or narrow your scope.
Comment: Now that it's 2020 and a pandemic has hit, forcing everyone to reevaluate their WFH policies, it's possible that any job could be remote.
Answer: I see a few vCISO roles now and then that are virtual, they would be at the top of the career path. Further down the path you could find pen testing work that is purely virtual, as well as log analyst roles. Truly, you could make most infosec work virtual with the exception of forensics, in-person social engineering, and some incident response. I think the key will be using the right search terms, and using them correctly. Look for "virtual" as a location (to keep from returning "virtual machines" or such from polluting your results. Similarly, "remote" is only good as a location. Listings with many locations listed are more likely to be remote jobs.
PM me for a few good recruiters if you are interested in going that route.


ubuntu-16.10 unable to locate package

Question: I have recently setup Ubuntu 16.10 and am trying to install PuTTY but keep getting an 'E: Unable to locate package putty' error. I have run both 'sudo apt-get update' and 'sudo apt-get upgrade'. Both complete with success. I have verified that 'main', 'universe', 'restricted', and 'multiverse' are all enabled. But I continue to get the error when trying to install PuTTY. This is the command I'm using 'sudo apt install putty'.  Any thoughts on what might be going on?
Answer: I was able to finally determine that 16.10 was just too old to update properly. I have moved to 18.04 and successfully installed PuTTY.
Answer: This is a common problem when you follow the instructions on the PuTTY website.
Here is how to solve it for ubuntu:
instead of:
sudo apt-get install PuTTY

use:
sudo apt install putty

As expected the package is case sensitive but somehow even PuTTY haven't figured it out.

ubuntu-16.10 unable to locate package


About Electric potential

Question: when we bring a unit positive charge from infinity to a point in the electric field EF does work on the charge and external work is also done on the charge in same amount but in opposite sign. then how energy is gained by the charge at the point. I mean if both works done on charge are equal and opposite how energy is gained by the charge?
Answer: The definition of potential requires you to work out either the work done by an external force on the charge or the work done by the field but not both together.
If you consider both forces together you can use the work-energy theorem and say that as the net force on the charge is zero then the net work done on the charge is zero and hence the change in kinetic energy of the charge is zero.

About Electric potential


How to add link to translated content in text field

Question: I need some help on a multilingual D9 site, trying to add links to translated node in text field. In German version should be a link to the English and French version of this node, as part of the node content.
And I need them absolute, because the nodes are used as newsletter content: https://example.de/en/node/123
I created link with language part in url, but when I view the node, the links are rendered without translation. The en/ and fr/ part in the url are stripped.
I'm using CKEditor for content creation.
Any help appreciated, thanks.
Comment: The problem seems to be CKEditor. If I use a text format without CKEditor, the links are not altered. So my solution could be to create an extra text field for the translation links and render it above or underneath my content field.
Comment: There's a language switcher block available on the blocks page that will automatically generate links to the same page in the other enabled languages.  Does that do what you need?
Comment: I know about the language switcher block, but I need the links inside a text field with user generated content.
Answer: I solved this problem by using an extra field without CKEditor to add translation links. The links are working and no language part is stripped from URL.


Li-po - li-ion chargers

Question: I have bought a Li-po charger the other day and I looked up the IC and it stated Li-ion. Can you charge a Li-po with a Li-ion IC? And the other way around? The IC I am talking about is the TP4056.  I thought Li-po is quite difficult to charge because if your voltage is to low it does not charge anymore and if it is to high you can blow it up.
Comment: Why the down-vote? Why the votes to close?  Entirely reasonable question.
Answer: For charging purposes, the two types (Lithium Ion and Lithium polymer) can be considered equivalent.
Both types have the same issues you  noted; If the initial voltage is too low, it will very likely never recharge, and the terminal voltage must not exceed 4.2V or pyrotechnically interesting things can happen.
Comment: Thank you, this is exactly what I was wondering about. I gave you both an upvote, but since you were faster you get the answer ;)
Answer: I agree with the answer given by Peter Smith in that the Lion and Lipoly can be considered the same for charging purposes.  They certainly use the same algorithm.
However, I own 3 different battery chargers that charge all types of rechargable batteries.  They all make a small distinction between the Lion and Lipoly in terms of their voltage.  For LiOn, they claim a nominal voltage of 3.6v and a termination of charge at 4.1v.  For LiPoly, the nominal voltage is 3.7 and terminal charge voltage of 4.2v.
The datasheet of your battery will tell you which voltage you should charge to, 4.1 or 4.2.  But it seems like all lithium batteries in the last 5 years have been 4.2v for max voltage and LiOn and LiPoly are used interchangeably.

Li-po - li-ion chargers


How to get Facebook tagged photos in Groups to appear in profile?

Question: In a public group, photos can be tagged and the tags are shown within the group. Though, when viewed from a profile these tags do not appear. Is this the proper behaviour that photos within group are not connected to the profile? Or am I missing something?
Comment: Did you ever find a solution to this? I have similar problem.
Answer: It seems Facebook has this set as the default design.
An exhaustive search across the internet has led me in circles. It is not possible to get group tagged photos to appear in a Facebook profile/timeline.
Answer: I guess there's some issue with your album/picture sharing security settings. Just confirm whether you've set them to share with everyone/friends.
Comment: I don't believe it has to do with security settings. I've had the same problem and I've looked over all the security settings I could find and haven't seen anything to indicate the connections are being blocked or filtered.


how to start a process before windows login screen appears

Question: I am developing a windows login app (Credential Provider) which need SDK licensing service running for face detection and matching. Now I want to know how i can start that license process before login screen comes up.
Answer: create a service and have the provider createinstance call wait until the service is running before returning.  This will delay the logon screen from appearing since the UI will not appear until the provider initializes (if I am reading this correctly http://msdn.microsoft.com/en-us/magazine/cc163489.aspx).  This is probably best answered on Stackoverflow.
Answer: This is probably a question better asked on StackOverflow (if not already answered), but generally speaking, you want to create a program that runs as a windows service.


How does facebook manage image uploads?

Question: How does facebook manage the storage of images uploaded by their members?  I would imagine the files are divided across multiple storage devices, but how should the decision be made to determine which file system to add the file to, if there is a table that holds all of the paths to the respective storage devices?
Answer: Facebook's photo storage system is called Haystack.
Here's a technical introduction: http://www.facebook.com/note.php?note_id=76191543919
Note that the link is from 2009, so some of the numbers are probably wildly out of date by now.

How does facebook manage image uploads?


What type of aircraft would fly with an orange light at night?

Question: I know planes usually fly with red lights which seem small in the sky, but last night i saw a bright orange blinking light traveling through the sky; it was bigger then a star but was orange. I thought it might be a helicopter, although I read it could have been a comet or a meteor.
Comment: I'm sorry but it seems inconceivable that you would not be able to distinguish a comet, from a meteor, from something else. This not a well-researched question. Maybe start by asking a question of Astronomy Stack Exchange.  Anyway, was the blinking regular or irregular?
Comment: How fast was the blinking? Continuous?
Comment: If you can include your latitude & longitude, and the approximate time you saw it then it might be possible to pinpoint what you saw (eg, might have been the ISS - which is trackable)
Answer: While normally red in colour, anti collision lights can often appear more or less orange, depending on the background and atmospheric conditions. It is impossible to determine the aircraft type just based on the fact that it has a orange light on it, since almost all aircraft have reddish orange anti collision lights.
Comment: Needs the one frame when the light is on and reflecting off the bottom of the plane and looking very orange...
Answer: Others have mentioned red anti-collision beacons that might look orange in certain environmental conditions, but there's only one kind of aircraft that shows an intermittent orange light: balloons. The flame used to heat the balloon is orange, and most balloons are translucent enough that the whole balloon can appear orange when the flame is lit.
There wouldn't be a blinking orange light, but the flame is often used in short bursts to keep the balloon inflated, so it would be intermittent. Balloons don't move very fast, so if it was really "travelling across the sky" as you describe, perhaps this isn't the answer.
A meteor (which you've already guessed at) moves a lot faster, but tends to glow orange or white for a few seconds and then disappear: I don't think I've ever seen a blinking one.
Comment: Hot air balloons don't really fly at night, though.
Comment: @J.Hougaard It's rare but permitted. Dawn and dusk flights are a lot more common and the glow is still very visible then.
Comment: Not saying it doesn't happen, but I imagine most people try to avoid ending up in a situation where they have to land a hot air balloon in the dark (since you can't really control where you're going...)
Answer: Orange isn't a standard color for aircraft lighting. Best guess would be reflected sunlight on a metallic aircraft, although any number of other explanations are at least possible.
Comment: @user1871 Also depends on your latitude (It can be daylight at midnight), and the altitude of the object supposedly reflecting the sun.
Comment: Would the sun still reflect on an aircraft at midnight?
Comment: Don't believe the OP mentioned a time. Near sunset, such a reflection is possible; after the End of Evening Nautical Twilight, not so much.
Answer: In this new era of widespread droning, anything is possible.  People can and do put all kinds of non-authorized lighting on drones.  Maybe you saw a drone, possibly flown by an amateur hobbyist in a manner non-compliant with regulations.


Anova on discrete data

Question: I am currently checking for independence on the characteristics of the finger, based on gender, finger and hand. My lecturer recommended using anova test, but how do you implement it on outputs that are alphanumeric? Are there more appropriate tests? I have used the chi squared test for testing independence of genders and hands seperately but was told it was not appropriate. Could someone explain why? Any resources for any recommendations would be greatly appreciated.
Answer: Log-linear models may be useful.  They are used like multi-dimensional chi-square tests of association.  Also, it sounds like you have a three-dimensional table of counts;  the Cochran–Mantel–Haenszel test may be applicable.
Answer: ANOVA is not recommended, because like you said, it is a discrete data anda one of ANOVA's assumption is data normality. I dont understand your experiment at all, bu i think you shoul use multinomial logistic regression.

Anova on discrete data


How do I make sure that the user enters their input fields correctly?

Question: I have two input fields which need to be formatted a certain way. Ipv4 addresses need to have the "." to separate the numbers and ipv6 addresses need to have ":". I'm not sure what the best text should be in order to ensure the user enters their data correctly. Should I show them an example of the format (like the image below)? I wanted to use something like mm/dd/yyyy, but there is no equivalent for IP addresses.
Answer: There isn't a straightforward solution to this. Using masking is an option, but determining the number of digits in the IP is a challenge.
I would suggest adopting a post-interaction strategy: start by validating if the user is including dots in IPv4 and colons in IPv6. This initial step will assist in conducting a basic check to ensure that the format is correct. Although it doesn't confirm the validity of the address, it helps in maintaining the proper format.
Realistically, in most instances, users are unlikely to input information manually. They would rather copy and paste, which implies that your main task might be to sanitize blank spaces at most. Thus, I believe any possible issues will come with user testing, right now you can't do much than what you already did and the additional steps I mention. If it works or not is something you'll find out at a later time


How can I create my own software license?

Question: I have seen numerous licenses for any kind of software such as the GNU GPL, BSD License, MIT License, and LGPL. What is the process for creating a new software license, such as "John Doe's generic license"?
Comment: @Nick. You can write your own. But the probability that you will get what you actually wanted specified in the license is slim to non existent (unless you are a lawyer). Legal language is non trivial and complex and lawyers also have experience in the loop-holes (sorry specific conditions) that have held up/failed in court previously. All the licenses you mentioned have been carefully crafted by teams of experienced lawyers (even if the first versions were not).
Comment: Only advise I can give is don't take legal advise from programmers.....
Comment: This is a great question. Shame it's considered off topic.
Comment: @MartinYork Legal language is not some sort of special language. In English, they use English for contracts. Are there words not commonly used when you go to starbucks that appear in contracts? Sure, but that's true for gas station "jargon". Still, you can go to starbucks and ask them to fill a large handheld reservoir with simple black energy fuel up to 3.5$ (assuming that's the price for a large coffee), and you'll be perfectly understood, without using "coffee jargon"
Comment: @Andrei I would disagree. Legal language is a subset of the English Language. English can be very imprecise. So Legal language in contracts tend to use words with very specific meaning to make sure there is no ambiguity. As such it is a good idea to get an expert in writing in the correct style.
Comment: Thank you. So creating a license does not necessarily include getting a lawyer, I could also just write it myself and publish it on the net? Would that count as a license of would I have to submit it somewhere and pay?
Comment: If you are willing to use this license for open source, you should consider that your project may not be used/adopted by a larger project/community (or used by any company/individual) just because of your license not being compatible with that project's license.
Answer:
Open an editor.
Write terms of license.
Save file.
Include in project.

Generally speaking, that's all there is to it. Seriously.
Don't believe me? Check out Phil Sturgeon's Don't Be A Dick license (as well as the WTFPL that inspired it).
That said, it's hit or miss whether software licenses are legally enforceable, even if a lawyer wrote them (at least in the US, and it depends almost entirely on the whim of the judge or ruling copyright authority). More generally, though, as the software author, you are already covered by your country's copyright laws (assuming you've taken the steps for your work to be copyrighted; in the US, that's simply actually making the work).
It boils down to what you want to accomplish with your license, above and beyond (or in lieu of) copyright laws. In the case of such licenses like Phil's DBaD license, it's largely a matter of the honor principle and mutual respect of one another.
If you want it to be legally enforceable (or at least have a chance of doing so), such as releasing a closed-sourced product, then you'll very likely want to talk to a lawyer. That way, you can make sure there aren't any glaring loopholes and can have it written in such a way that a judge/ruling party is more likely to rule in your favor in the event that someone does violate it.
EllisLab (the company behind CodeIgniter) has a great explanation of software licensing (and entire week of blogs on the matter, in fact), both from a legal standpoint, and from the standpoint of a FLOSS developer and the honor principle I mentioned above.
(Disclaimer: I'm not a lawyer, this isn't legally binding advice, blah, blah, blah. I'm just a developer who's had to tip-toe around the legal minefield that is software development and licensing. When in doubt, talk to someone who actually is a lawyer, ideally with a copyright, patent, and/or licensing focus.)
Comment: +1 for answering the actual question then providing the correct course of action
Comment: I like the "Don't be a Dick" license.  It's even better than the WTFPL.
Comment: It might worth noting that ["Don't be a Dick License"](http://www.dbad-license.org/) is by no means a free / libre / 'open source' one.
Answer: Answer the following questions for the user of your software:

Is your product open-source, i.e. can I get a copy of the source code?

How can I use your source code?  Can I freely copy it into my own applications, or do I have to link to your binary?  If I use your code in my own application, is there a limit to how much of your code I can use?  Do I have to include your license or attribution header at the top of any source files I redistribute that contain your code, or in the Help/About box of my binary?

What can I do with your product?  Can I use it one one computer?  Can I use it on more than one computer?  Can I redistribute it or resell it?
If I use your product as part of another application I write, do I have to be open-source also?
How do you warranty your product?  Are you legally liable if I have problems?
What are your redistribution terms?  Can I redistribute your product as part of a product I write?  If you provide source, can I modify your library and redistribute the modifications, and if so, what conditions do I have to meet?

Study other licenses, and see how they have answered these questions.
Answer: It is actually surprisingly similar to what developers do:
You give a lawyer (or the legal department in a large corp) a list of requirements for what the license permits and what it does not permit (e.g. the license is for one user only, it's temporary, you cannot use the software for some purposes, etc.) and they "translate" this into lawyer-speak, pretty much like we translate requirements into code.
For example, "the license is for one user only" is usually translated into the something similar to this: "FooCorp Inc. grants you a single, non-exclusive, non-transferable license to  install and use FooSoftware for ...".
You might think this doesn't apply to F/OSS licenses, but it's more or less the same, except the requirements are different: you can find the simplest version of these requirements at http://opensource.org/docs/OSD.
But the lawyer part is essential and there's no point in trying to write a license by yourself (unless you're a lawyer with a lot of experience in IP-related and international law). Someone will just find a convenient loophole, which will allow him to use your product in a way he pleases (I'd go as far to compare this to exploits).
Of course I do not claim this explanation to be complete or even correct, but it's a rough sketch of the process, as far as software developers are involved (therefore I compare it to a programmer's work).
Comment: @Nick Certainly, programmers shouldn't generally tread there.
Comment: So it's basically a lawyer-orientated job?
Answer: To answer your question, anyone can write a license - and it shows!
Those of us who use software to build things and actually try to respect each license are ready to gouge our own eyeballs out with a hot poker every time someone talks about making new one.  There are already an uncountable number of software licenses in the world.  99% of them are a near duplicates of existing, more popular licenses, except not as well thought out.
One site I contribute to has a home-grown creative-commons non-commercial attribution type license, except it says nothing about derivative works.  Are they allowed?  Can they be distributed under the same license?  Beats me!
If other people contribute to a project and you ever want or need to change the license, you need to get them all to agree to change the license, or you are more-or-less stuck with it forever.  Some of them might become unreachable or even die.  I am not a lawyer, but if you don't have a signed contributor agreement from everyone that lets you make decisions about your license, I think you are SOL.
It's really hard to write a good license.  Even the best software licenses are full of holes.  There are new versions of the Apache and GPL licenses periodically.  Please, see if you can find a license which meets 90% of your needs and use it.  The GPL Linking Exception or "Classpath Exception" is a great example of this.  I'm a little worried about the statement the Hibernate people had on their site a few years, basically publicly mis-interpreting the GPLv2 to make it sound like it included the Classpath Exception.  Is that the classpath exception?  We won't know unless someone sues them.
It seems to me that getting sued is the most painful way to debug your license.  If you use a license that people have sued-over already, there is a certain assurance that it works, and also a sense of how it works.  Better yet, pick a license that has been to court in different countries.  Did you think about how your license would be interpreted in other parts of the world?
If you base your license off an existing one, then people who care about fulfilling the terms of your license might not have to study it for years to figure out exactly what it means.
If at all possible, I would encourage you to devote your energies and talents to improving an existing license rather than writing a new one.
Sorry to be such a kill-joy.  If I'm bitter, it's because I really read and try to respect people's licenses before I use their works.
Answer: Write down the terms, publish it with your software.  That's it.
One question, however, is why do you want to?  It is not unlikely that whatever conditions you wish to add are already present in another commonly-used licence.
This adds a preliminary step - check whether you actually have to create a licence.
It is also likely that some of the conditions you wish to enforce are flawed in some way;  adding another preliminary step - read around about the "unique" terms you want to apply.  Someone else is likely to have tried such terms, and found them unenforceable, or counterproductive in some way.


Artificial Plasma Creation

Question: It is intuitive that at high temperature matter converts into plasma because of intense energy of electrons that they break away from atoms.
But how can plasma be created at low pressures?
Comment: I know, I was asking you why the pressure part is even considered.  Gas pressure really does not matter in the ionization of a gas other than at higher pressures, there may be more opportunities for recombination (i.e., neutralization).
Comment: Why would pressure matter?  Why not ask how can plasmas be created at high pressures?
Comment: I'm just curious as to how can Plasma be formed at low pressure
Answer: Is your question about temperatures or pressures?
If you have a very high temperature at low pressure, you can create a plasma.
This graph resumes this well: density here is equivalent to pressure (they have the same effect)
Answer: To answer your question, we should clarify what a plasma actually is: an ionized gas, i.e. a gas consisting of particles with positive and negative charges and some neutrals.
Ok, we need to ionize particles. We can do that by, for example, accelerating some free electrons (which we always have, due to background radiation, for example). These electrons need to gain enough energy such that they can ionize particles of the gas by colliding with them. If we just apply enough energy to the system, enough ionization processes will occur and we have a plasma which behaves very different from a neutral gas (due to the long-range Coulomb-interaction).
As @honeste_vivere said in the comments, it doesn't really matter what the pressure is. Plasmas do exist over quiet a large parameter space, as shown in the answer by @Kaelan Donatella.
You said that it is clear to you how plasma is created at very high pressure but not at low pressures. Lower pressure just means that the mean free path, the average distance between two collisions, is longer. When you think of my initial example, where free electrons are initially accelerated (for example by some electric fields), a lower pressure is actually helpful as it is easier to let them gain enough energy to allow them to make ionizing collisions (since they are accelerated over a longer distance).

Artificial Plasma Creation


Java memmove?

Question: I'm in the process of porting some old C code to Java and, due to my inexperience with Java, I have encountered a problem locating a memmove equivalent.  Is there such a method in Java?  I've done enough research on it to assume that I'm overlooking something rather obvious.  Thanks in advance.
Comment: Could you ellaborate on what you are trying to accomplish?
Comment: I'm porting a forward error correcting codec from C to Java.  The original programmer used memset and memmove to manipulate the data in vector calculations.
Answer: You can use System.arraycopy, if all you want to do is shuffle things around in an array.
Specifically, that function permits the source and target to be in the same array, and the ranges are allowed to overlap; so in that sense it's like memmove.
Comment: It's doubtful I would have searched the System class for an array manipulation method.  Thanks very much!
Answer: Actually, the System.arrayCopy() is the old school approach to this, and should only be used if you're a) stuck on Java 5 or earlier, or b) have to copy between pre-existing arrays.  If you want to create a copy of an array you can use Arrays.copyOf() or Arrays.copyOfRange()
Comment: memmove specifically allows you to between overlapping ranges. (say, copy the range a[0-5] to a[1-6]. Do the ones you mention allow that?
Comment: I think when people ask about a memmove equivalent (as opposed to a memcpy equivalent), usually case (b) applies. Nonetheless, good answer.
Comment: @jalf: As Jherico mentioned in the answer, copyOf and copyOfRange both create new arrays, and cannot be used for copying stuff to the same array.

Java memmove?


What does Depth mean in Stockfish Game Analysis?

Question: When I'm observing a game, the evaluation bar works and above the moves list, there is a numerical value for depth. But what is Stockfish depth and what does it measure or tell?
Answer: Depth means roughly nothing. It used to mean the depth in the search tree that an engine searched but with modern search algorithms, there is enough pruning that the depth becomes fairly non-informative.
Comment: This answer would benefit from more details or with some citations to be more valuable for future visitors. For example, "with modern search algorithms, there is enough pruning that the depth becomes fairly non-informative"  [**citation needed**]
Answer: See the Chess Programmming wiki on depth. It is nominally a measure of how far ahead an engine is searching, so e.g. an engine that is looking at depth = 10 is looking five moves ahead (10 ply = 5 moves). "Nominally" because these days there are all sorts of pruning techniques and quiescent searches and similar that makes the number itself relatively meaningless; however, in terms of playing strength, a higher number is still always better than a lower one. As you let Stockfish run, you should see that its depth continuously increases, as well.


What is the best way to replicate a version control repository?

Question: Here is the scenario that I have. I have a cvs repository in one location (A) and I want to replicate it and keep it in sync with a repository in another location(B). This would be a single directional sync from A to B. What is the best way to do this? If it is not really feasible in CVS then which source code control system would you recommend to accomplish this? Thanks
Answer: When using CVS, I don't know any tools to do that other than files syncing. You can achieve that using tools like rsync (Unix) or xcopy/robocopy (Windows).
If you plan on migrating to Subversion, it provides a tool called svnsync that allows to sync a repository from another one.
Answer: I would recommend you migrate from CVS to a proper distributed version control system such as git, which will provide this sort of functionality very naturally.
Subversion also provides svnsync which does the same sort of thing.
Answer: If you do take the rsync/filecopy approach with CVS, it is important to only sync the files at a time when there is not an active commit. Otherwise, the repository's lock file will get copied over and you will be unable to checkout/update on the target side until the next sync.
This reason alone may make CVS a bad choice. The migration path from CVS to Subversion is pretty smooth and there are tools to import a full CVS repo, with history, into Subversion.
Consider Git or Mercurial if you want to get into true distributed versioning, but it sounds like that would be overkill for your "read only" needs.
Answer: The Best (and perhaps costliest) way is Clearcase Multisite
But if you are looking for opensource, Git is becoming quickly replacing svn everywhere..


Finding the laws of logic logically

Question: Consider the statement ''The laws of classical logic compraised of Identity, Excluding middle and Non-contradiction'', In which type of knowledge the above statement comes under? Is it purely primitive or authority? I dont think the laws can be find out with an apeal to intution.
Comment: Not very clear... Are you assuming that the only two sources of knowledge are: either primitive (innate?) or authority? See e.g. [The Sources of Knowledge](https://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780195301700.001.0001/oxfordhb-9780195301700-e-3).
Comment: Those laws are analytic.
Comment: Did we find these laws out or build them for our uses? Priest, Beall, Weir, and Field to name a few argue different reasons for adopting different logical laws than classical. One goal is we want to modify what is expressible in language. This  language first perspective might be identified with Wittgenstein among others. Where knowledge has something to do with how humans in society follow rules. (this isn't definitive, just one way to look at it that I think is supported)
Comment: The justification of all inference rules is the fact that whenever you have true propositions P and Q in one statement followed by P in a separate statement the pattern is what makes the inference rule. If the premises are related & are true then the conclusion turns out 100 percent accurate with no exceptions. So if you want to say most knowledge is found inductively then this historically holds to be true. There is no authority that creates the famous three laws you refer to which are known as the three laws of logic. Objective knowledge is important in philosophy.
Comment: Analytic in the sense of logic, right? Circular reasoning?
Comment: Sir, How objective knowledge differ (may be) from innate knowledge?
Answer:
''The laws of classical logic compraised of Identity, Excluding middle and Non-contradiction'', In which type of knowledge the above statement comes under? Is it purely primitive or authority? I dont think the laws can be find out with an apeal to intution. [sic]

Well, you come to the right place for an answer. This statement, true, false, or somewhere in between, is a statement investigated and contemplated by logicians. In fact, logicians themselves make arguments; and since logicians are generally involved in philosophy (mathematics, logic, and philosophy are highly interdependent in respect to Frege's notions that introduce formal systems), it's also fair to say that philosophers deal with the epistemology of logic, epistemology being the study of knowledge.
Now, if one comes from a naturalized epistemology, then one might approach the question from the perspective of a cognitive scientist, and ask questions about the brain and how it produces and uses the language to express logic and make decisions. (See Thinking, Fast and Slow, cognitive biases, and the Somatic Marker Hypothesis.) One might say that formal systems of logic are essentially the core that drives mathematical language, and therefore embrace constructivist ideas about logic. But certainly and traditionally, the study of informal logic, which goes back to the Ancient Greeks, has a tremendous body of knowledge that predates the 21st century. Ideas like Aristotelian syllogism are still mainstays of investigating logic, and for thousands of years, people put forth ideas that still have currency in the debate.
So, when one is discussing the Laws of Thought, the usual methods for determining truth apply: brute fact, intuition, rhetoric, and informal logic. In short? Philosophy!
Answer: Logic is an innate mental faculty, so any scientific investigation of the laws of logic would have to rely on the available empirical data, which are essentially our own personal logical intuition, as well as what kind of reasoning is regarded as valid by human beings. The work of logicians since Aristotle also qualifies as empirical data.
Nobody has ever carried out any scientific investigation of the laws of logic and nobody seems interested doing it today (see note), but if we are ever to understand human logic, this is how we will have to do it.
It is unclear how useful that would be, but logic is crucial to our understanding of our environment and of the universe, so understanding logic seems a priori a potentially very useful idea.
Logic has mostly been investigated by philosophers, and they keep at it today. However, the method of investigation philosophers are using today and have been using, apparently after Kant, is essentially logical analysis, which obviously cannot possibly work. This explains to some extent why modern logic is a complete fiasco.
The method used by Aristotle and by most logicians in the Aristotelian tradition until the Scholastic seems to have been to rely on their own intuition and on what other logicians had said. This would have been a bit better than the analytical method and may explain why they at least made some progress.

which type of knowledge the above statement comes under? Is it purely primitive or authority?

Most likely, Aristotle's results were based on a mix of intuitions and on the consideration of arguments used by philosophers and politicians at the time.

I dont think the laws can be find out with an apeal to intution.

With an appeal to intuition, no, but by using one's intuition, yes.
Intuition is routinely used by all intellectual workers, including scientists. Most people don't even realise that they use their intuition. There is no fundamental differences between intuition about the possible laws of logic and intuition about the possibly laws of space-time, and Einstein himself put a premium on his own intuition.
Note - I am well aware that the cognitive sciences have long been working on the logical capacity of humans, this is not in dispute. What I say is missing is the science of the laws of logic.
Scientists naively believe that mathematicians have done the job and that mathematical logic is the expertise on the laws of human logic. This point is in evidence in their presentation of their results. Further, given the daunting complication of mathematical logic, most scientists probably also believe that they are incompetent to investigate the laws of logic. Accordingly, they conclude that there is nothing left for them to investigate in this respect. Instead, they work on whether humans have a logical capacity, with the assumption that the laws of logic are as described by mathematical logic.
Comment: I've upvoted because your contribution has many good claims. It is utterly and demonstrably false that science doesn't investigate logic, however. This WP serves as an introduction to how modern day cognitive scientists often explore the biological and psychological origins of reason: https://en.wikipedia.org/wiki/Psychology_of_reasoning In fact, the [somatic marker hypothesis](https://en.wikipedia.org/wiki/Somatic_marker_hypothesis) and the [work in behavioral economics on cognitive biases is nobel-prize grade/winning research](https://en.wikipedia.org/wiki/Thinking,_Fast_and_Slow).
Comment: @JD Thanks for the upvote. You misconstrue the bit about the scientific investigation of logic. See my edit.

Finding the laws of logic logically


How can I list items on an FTP server in a list view? (Android)

Question: Ok, so I'm learning Java and I can't figure out how to list and display the contents of an FTP server in a list view, and I want to be able to download the files too.
Please show me code snippets if you can, Thanks.
Answer: these are basically two different questions. 1. how to get data from FTP and 2. how to use a ListView.
the answer to nr. 1 can be found here on stackoverflow: Android FTP Library
for nr. 2, please refere to the android documentation here, check the sample projects in the SDK or simply google for something like "android listview example".


Hot water in shower acting weird

Question: I just remodeled my bathroom and had all new plumbing installed about two months ago.  All faucets and fixtures are pretty much still brand new.  I have a gas tankless water heater also that is about 7 feet from my shower (it's a couple of years old).  Just during the last couple of weeks, when I'm showering, if I turn the Delta handle more towards the cold side, the hot water goes away completely.  I can turn it on all the way hot and it never gets even warm again.  But, if I turn the water completely off and turn it back on, it goes back to normal like nothing ever happened.  Anyone have any ideas?
Answer: This might be a problem with the water heater or it might be the shower valve. Or it might be due to an interaction between the water heater and the Delta shower valve which surely must have an antiscald feature.
The first thing I would try to resolve this is to turn the output temperature of the water heater to a lower setting. If you have the water heater output temperature set at 120 F, then lower it to 115 F. If that doesn't resolve the problem, lower the output temp by 10 deg F from the original setting. The purpose of this is to cause a higher flow rate through the water heater so that it does not go below the minimum and so shut off when you lower the temperature at the Delta valve. When the season changes to colder input water you could raise the temp of the output of the water heater.
What is happening is that when you turn the Delta valve to a lower temp the valve lowers the water flow rate from the water heater to such a low value that the heater shuts off the gas flame entirely. That is standard behavior for any tankless water heater. In deep summer the cold water going into the water heater is pretty warm so the water heater reacts by restricting the flow rate of water compared to the winter when the input water is much colder. So when you ask for lukewarm water the heater reduces the flow rate so low that the heater stops heating entirely. Water keeps flowing through the hot water pipes but it is not heated.
This does not explain why the heater does not resume heating when you turn the Delta valve back to the original higher temp. This could be a fault or a wrong setting on the heater or on the Delta thermostatic antiscald valve.
Also if your new Delta valve had a new shower head with it, then the shower head might be a lower flow rate "water saving" head. Code is requiring this in a lot of places, but what can happen is the the flow is so low that the water heater shuts off when you lower the temperature.
We have an old, outmoded Bosch tankless WH and find that we have to set the output temp much lower in the summer or the heater will shut off in mid shower when one reduces the temperature. But our heater will come back on if we turn the valve back to hot. However, our shower valve is an old two valve Price-Pfister that does not have an antiscald function. This type of valve is no longer allowed by code and would get flagged if we would put the house up for sale. We are not "in violation" beause the valve was installed before the code required anti-scald shower valve. Our valve is said to be "grandfathered in", but if we would replace it we would have to use the a valve with antiscald.
Comment: Don't rush to attribute the fault to the Delta valve. If you want to try to separate the function of the Delta valve and the tankless WH, do the following. With the WH output temp set at the value which causes the bad behavior, before you start the shower, open the hot valve on the bathroom lavatory to give a low moderate flow of water and leave it running during the shower. Does the bad behavior not happen when you shower (does the WH keep heating when the temp of the Delta is reduced to lukewarm)? If you do this, please report the result.
Comment: Thanks so much.  I'm inclined to think it might be an issue with the Delta shower valve itself.  I never had this particular issue before with the old stuff and same water heater.  I'm not sure the older one had an antiscald feature either though.  I will try the temp on the water heater first as you suggested.  I appreciate it.  I spent enough on the plumber this year, so I prefer not to have to call him back. :)
Comment: What is the current setting of the output water temp of the tankless WH?
Comment: Where is this, country, state?
Answer: Most tankless heaters have a minimum flow rate of 0.5-2 gallons per minute in order to activate the burners and avoid damaging the system. This explains why you get no hot water when your delta handle is requesting a very low amount of water.
However, it does not explain why turning it back to hot does not activate the heater. You should recreate the problem, walk over to your tank, and check if you can hear hot water flowing on the outlet side of your heater.
It's possible that your shower cartridge has a clog which only activates under the scenario you describe but unclogs when you spin it to the off position.

Hot water in shower acting weird


Showing Field extension contains elements

Question: We just started field extensions in my algebra course, so this is all relatively new to me, and I'm not sure where to even begin with this proof. For example, I'm trying to show that given a subfield $F$ of $\mathbb{C}$, if we define $E$ as $E=F(\sqrt{a}+\sqrt{b})$, $\sqrt{a}$ and $\sqrt{b}$ also belong to $E$. It seems obvious, but I'm not sure where to start. Thanks for any help
Comment: Well what do the elements of $F(\sqrt{a}+\sqrt{b})$ look like?
Comment: Hint: Hard to know what square root means. We will assume that we cannot have $\sqrt{b}=-\sqrt{a}$. Fields are closed under reciprocal, so $\frac{a-b}{\sqrt{a}+\sqrt{b}}$ is in $E$.
Answer: We will assume $a \neq b$.
It suffices to show that $\sqrt{a} \in E$, since then $\sqrt{b} = (\sqrt{a} + \sqrt{b}) - \sqrt{a}$ surely is, as well.
Note that: $(\sqrt{a} + \sqrt{b})^3 = a\sqrt{a} + 3a\sqrt{b} + 3b\sqrt{a} + b\sqrt{b} = (a+3b)\sqrt{a} + (3a + b)\sqrt{b}$.
Thus $(\sqrt{a} + \sqrt{b})^3 - (3a + b)(\sqrt{a} + \sqrt{b}) = 2(b-a)\sqrt{a}$.
Since $a \neq b$, $\dfrac{1}{2(b-a)} \in F$, thus:
$\sqrt{a} = \dfrac{1}{2(b  -a)}[(\sqrt{a} + \sqrt{b})^3 - (3a+b)(\sqrt{a} + \sqrt{b})] \in F(\sqrt{a} + \sqrt{b})$.


Non-Greek myths set in a wedding

Question: There are a many Greek myths that are set in a wedding, like Thetis wedding as the origin of the Trojan War (see also Harmonia's necklace and Orion's earthborn myth). Are there other tales in major mythologies that are set in a wedding? I am looking for myths that are set in a wedding and not origin myths of wedding rituals per se.
Comment: The Kalevala describes a wedding, and how the traditional family roles are connected to it, in chapters 20 to 24. This is, IMO, the most enjoyable part of the Kalevala. I hesitate in making this answer though, because the wedding proper probably doesn't count as mythology. The identity of the husband, and the impossible tasks that he has to perform as dowry, as described in the chapters before, those do count as mythology, but that is not what you're asking for.
Answer: There is a pretty famous Norse myth in which Thor has his hammer stolen by a giant, Thrym. The giant says he will only let Thor have the hammer back if Freyja, the goddess of beauty and fertility, will marry him. Freyja says no way (understandable) and Loki convinces Thor to dress up as a bride, and then go and pretend to be Freyja at the wedding. The two of them go to meet Thrym, and they have a banquet. As soon as Thor gets a sight of his hammer, which is brought out at the end of the feast as a wedding present, Thor grabs it and kills all the giants. The end.
Here's a link where you can read the myth for yourself
Answer: In Christian mythology, in John 2:1–11, at the Wedding at Cana, Jesus turns water into wine.

Non-Greek myths set in a wedding


Short Introduction to Planar Algebras

Question: Are there any good short expositions of planar algebras out there? I am interested primarily in seeing the main definition and some explicit examples.
Answer: My paper with Emily and Noah about the D_{2n} planar algebra includes our attempt at a friendly explanation. It's all about arguably the simplest non-trivial example of a subfactor planar algebra.
Answer: Emily and Noah wrote some really nice expository posts on planar algebras at the Secret Blogging Seminar. I also like Chris's posts on TQFTs and Planar Algebras to explain why I might care.
Answer: Vaughan Jones' paper "The planar algebra of a bipartite graph" is short and example-focused. It's got the "right" definition (phrased with operads) and the mechanics of the bipartite graph planar algebra.  While the bipartite graph planar algebra isn't a subfactor planar algebra, it's pretty darn close.
A subfactor planar algebra, by the way, is one which meets restrictions on the sizes of the spaces involved and has an inner product.  Unsurprisingly, they give rise to subfactors (through machinery of Popa).
Answer: You should first read the new Wikipedia page: Planar algebra.
Next, a neat course by Vijay Kodiyalam is available on YouTube, see here (HD).

Short Introduction to Planar Algebras


Why can't we use calculus in finding the M.L.E. of Uniform(-theta, theta)?

Question: I've understood that we use maximum/minimum of x's as MLE of theta. But no one so far has explained the reason why differentiation won't work. Please explain.
Comment: For the same reason we cannot use calculus for $U(0,\theta)$ and in general, when support of the distribution depends on the parameter of interest. See also https://math.stackexchange.com/questions/649678/how-do-you-differentiate-the-likelihood-function-for-the-uniform-distribution-in.
Comment: MLE derived [here](https://math.stackexchange.com/questions/2795320/maximum-likelihood-estimator-for-x-1-dots-x-n-sim-u-theta-theta?rq=1).
Comment: We can use calculus. In elementary math one learns to look at critical points which include the zeros of the first derivative, if they exist, but not restricted to them.
Answer: Differentiation won't work because the likelihood function, as a function of the parameter $\theta$, doesn't achieve its maximum where its derivative is zero.
The density function of the Uniform$[-\theta,\theta]$ distribution is
$$
f_\theta(x) =\frac1{2\theta}I(|x|\le\theta)
$$
so the likelihood function for a sample $x_1,x_2,\ldots,x_n$ is
$$
L(\theta)=\prod_{i=1}^nf_\theta(x_i)=\frac1{(2\theta)^n}\prod_{i=1}^n I(|x_i|\le\theta)
=\frac1{(2\theta)^n}I(\theta \ge\max_i |x_i|).
$$
The likelihood is a function of $\theta$ which is nonzero on the interval $I:=[\max_i|x_i|,\infty)$. Over this interval the function is strictly decreasing, so its derivative is never zero there. Outside of $I$ the function is zero so its maximum doesn't occur there. Thus calculus is no help in finding the maximum of $L$. On the other hand, since $L$ is nonzero and decreasing over $I$, it achieves its maximum at the left endpoint of $I$: $\hat\theta:=\max_i |x_i|$ and this is the MLE of $\theta$.


Do MySQL limitations apply to MariaDB as well on AWS RDS

Question: I am thinking of migrating MariaDB 10.3.13 to RDS, I want to do an analysis of what limitation can be caused by migrating to RDS, I could find no documentation by AWS for MariaDB for issues and imitations, however there is a MySQL doc for issues and limitations, I am wondering do the same limitations apply to MariaDB?
Comment: *do the same limitations apply to MariaDB?* Not all MariaDB limitations are inherited from MySQL, some of them were altered during the fork. And from this forking point the limitations history is independent. Taking into account that this was far ago (relatively, of course) the generic answer seems to be "unknown/unpredictable". And AWS may add its own limitations, both common and specific to MariaDB.
Comment: In addition to what Akina said, because AWS *likely* has additional limitations as well, your best bet is to talk to an **AWS Solutions Architect** or your **AWS Rep** to clarify all limitations.
Comment: I recommend to test every feature yourself if you're in doubt. I've gotten bad advice sometimes from AWS Solution Architects. Most are correct, but you can't know which ones. They are especially reluctant to tell you about limitations of their product. It's like they are all salespeople, and they try to hide any limitations or weaknesses. You have to double-check everything they seem to tell you, or else you'll be unpleasantly surprised.
Answer: Short answer:  Probably yes.
Long answer:
I reviewed that link.  All three (MySQL, MariaDB, RDS) are derived from the same original code.  As far as I know, all three have, or at least had all of those bugs/issues/etc.  When one of the 3 fixes a bug, the others may eventually pick up the fix.
For innodb_buffer_pool_instances, MariaDB seems to be leading the change:  MariaDB-10.5.1 changelog says that innodb_buffer_pool_instances deprecated.
WHERE indexed_col1 = 'value1' AND indexed_col2 = 'value2' -- This is almost certainly fixed by using a composite index rather than depending on index merge.
The 10x for blobs was a workaround that was mentioned in 5.6(?).  It may be relaxed in some later version.
lower_case_table_names is a kludge to porting to both Windows and Unix.  It got worse with 8.0's Data Dictionary.  So, beware of switching OS's
As for the 16TB limit for an InnoDB table, I thought it was 32TB or 64TB.  It won't be long before someone finds out for sure.
Lower limits such as 2 TB, and long ago, 2 GB are probably dictated by the underlying Filesystem on the Operating System.
long_query_time allowed microseconds long ago, in 5.1.21, before MariaDB forked off.  I don't understand why AWS mentions it.
My list of limits:
http://mysql.rjweb.org/doc.php/limits


Is transfer learning effective when the new task has more classes than the original?

Question: All examples of transfer learning I have seen for classification use initial weights of a network trained on a larger number of classes (say 1000 in the case of networks trained on ImageNet data) to address a new task that has a smaller number of classes. Can transfer learning be effectively used when the new task has more classes than the original? For example, can I effectively build on ResNet50 or parts of it for a new task that has 1500 classes? Thanks.
Answer: Yes, transfer learning can usually also be utilized when the number of classes differs from the original. Your model will however be more 'transferable' if it has been trained on a wide variety of data or data that is somewhat similar to your new data.
What one would usually do is freeze the weights of the lower layers of the network and only retrain the upper part of the network (e.g. the fully connected part). The most important question is, thus, not how many classes each network was trained on but instead: How high is the representational distance between the old and the new dataset?
Recently self-supervised pretraining has increased in popularity, this shows that general features are useful somewhat independent of how they were acquired.
Comment: Also, how many layers in the pre-trained model you decode to freeze could depend on how well you believe the convolutional layers are able to extract the feature of your new images.  If your images are a subset of the pre-training images, for example, then you may not need to train any of the convolutional layers.   If you are adding novel images, you may also have to train some of the deeper convolutional layers.  This can be a "hyperparameter" in your training.
Comment: The emphasis on representational distance concept is apropos.  For example, it has been shown that ImageNet trained models do not perform well on medical images.


Why do people call Linux a kernel rather than an OS?

Question: Why do people refer to Linux as a kernel rather than an operating system? To the best of my knowledge, the kernel is one part of the operating system.
Comment: Searching this site for "gnu/linux" brings among the first several hits questions like [What exactly do we mean when we say we are using Linux?](http://unix.stackexchange.com/q/87735/2465) (I'm voting to close as a duplicate of that question), [Can the GNU Project be seen as a distribution of Linux?](http://unix.stackexchange.com/q/81314/2465) and [Is GNU/Linux still relevant?](http://unix.stackexchange.com/q/1658/2465).
Comment: [The Confession of the Faith](http://stallman.org/saint.html) states that "There is no system but GNU, and Linux is one of its kernels". I think this is a pretty decent explanation. Also see http://en.wikipedia.org/wiki/GNU/Linux_naming_controversy.
Answer: A linux system is built out of many different parts.
The central part is the linux kernel. (You can get it from kernel.org, it is originally written by Linus Torvalds who named it "Linux".)
The kernel by itself is pretty much useless. It manages all kind of hardware and provides an interface for applications to use. To make any use of it you therefore need applications using this kernel.
On the other hand there is the GNU project, initiated by Richard Stallman. Its mission is to create a complete free operating system with all the standard tools around it.
After several years the GNU project went well, they wrote all the standard tools, but they still were missing a working kernel.
So it happened that at the same time there was a project for a kernel without tools (Linux), and a project with all the tools but without a kernel (GNU).
As both were written with the same UNIX mindset it was possible to combine them into a full operating system which people aptly called "GNU/Linux".
But even this kind of bare "GNU/Linux" system is not very useful without the software you want to run on it. (Like a browser, a mail server, or anything like this.)
Therefore some distributions (like Debian, RedHat, SuSE, Arch, etc.) went to package a Linux kernel, the GNU tools, and all kind of applications together for easy installation and maintenance. (There are also distributions which use other kernels. For example you can have a Debian system with FreeBSD or GNU-Hurd as a kernel.)
Now for "normal" people, (who for example just want to have a running web browser) this background is much too detailed and they want just a single name for it. So most of the time someone claims he installed "Linux" he usually really installed some distribution which came with a Linux kernel as one part of many.
To cut a long story short, people often just use the name of the central kernel to refer to the whole system.
It is for you to decide what you call an "operating system". Is it just the thing that manages hardware (like Linux) or is it the thing you interact with (like your favorite desktop environment), or maybe something in between like a basic (command line based) GNU/Linux.
Answer: A kernel is a constituent part of a larger operating system -- usually, in Linux distributions, the larger operating system contains a base of GNU tools, which is why many people refer to the kernel as Linux, and the overall operating system as "GNU/Linux" (although many people don't make this distinction).
People call Linux an operating system because they have to choose a name to refer to the overall operating system that is provided by the sum of parts. Linux is the lowest level component that is readily visible to the user, so it is natural that it would take on this name and role. People don't usually refer to GNU grep as "GNU grep", they usually just call it "grep" -- to many users, GNU tools are not as visibly tied together as the Linux kernel.
Richard Stallman is very insistent about having people call Linux "GNU/Linux", since it contains many GNU tools (the C compiler perhaps being the most critical). I'm not so fussed, and not many other people are, either.
If you want clarity, GNU/Linux unambiguously refers to the operating system (rather than the kernel). "Linux" is significantly more ambiguous out of context.
Comment: Solaris is *not* a Linux distro - it is an OS in its own right, with its own kernel.
Comment: I would not say it is called Gnu/Linux because of tools like gcc but because most of the POSIX APIs, i.e. what makes an OS a Unix is provided in the Linux case by Gnu software. These pieces are mainly libraries like the glibc and core utilities (shell and basic commands). On the other hand, the C compiler is not at all required or even AFAIK specified by the standard.
Comment: The OS in case of Linux would be its various flavors available, like Fedora, Red Hat, Solaris etc. All of these flavors are based on the same kernel i.e. Linux.
Comment: @debal they are also based on the same GNU tools so GNU/Linux would also work.
Comment: @D_Bye You should probably mention (with `@`) debal in that comment so that he can respond.
Comment: I'll give you a hand D_Bye and Chris @debal :)
Comment: @MichaelKjörling I was going to do it, but you can't mention more than two people in a comment (it refuses to let you post) :-(
Comment: @ChrisDown I know, and that's really a shame sometimes. Every once in a while it'd be really useful.
Comment: @D_Bye sorry i've mentioned that by mistake. Now that you mentioned it, I kind of feel stupid.. :) Thanks for pointing that out, and also the part where they use the same GNU tools.
Comment: Well, it's trivial enough to not use coreutils, and not use GNU utilities. I run Arch Linux and have almost entirely replaced GNU coreutils with 9base. Relieving gcc has historically been more difficult (although nowadays, many compilers can do it).
Answer:
To the best of my knowledge, the kernel is one part of the operating system.

The kernel is the "core" of the operating system. It provides most of the critical services. Starting, terminating and scheduling of processes, allocation of memory and swap space to processes, inter-process communication, files, the lower levels of the network stack, some level of IO abstraction.
On top of the kernel, we build a stack of other components. On the one hand, the kernel is not much use without at least some of those other components. On the other hand few of these components are truly mandatory, and most can be replaced with something else.
Exactly which of those components should be counted as part of the "operating system", is thus not clear-cut.

Why do people refer to Linux as a kernel rather than an operating system?

Two reasons.
The first is that strictly speaking it's true. The project called "Linux" only maintains a kernel. It is up to third party distributors to put together a collection of other software to actually turn it into a useful system.
The second is that the Free software foundation are somewhat annoyed that Linux gets all the glory. Especially as Linus is a Free software pragmatist not an idealist.
The GNU project was set up by the FSF to create a free software replacement for Unix. They created replacements for many important components, like the compiler, binutils (assembler, linker etc) libc (provides both the C standard library and the wrappers that turn the kernel's system call interface into posix), textutils, shellutils and fileutils (now merged as coreutils).
However they struggled to make a replacement for the Unix kernel. In 1987 Stallman decided to go with a system based on the Mach microkernel and a collection of Daemons known as Hurd. However the project was delayed starting by 3 years due to licensing concerns and has had a moribund development history ever since.
Meanwhile a student called Linus wrote his own kernel inspired by MINIX. A kernel alone doesn't make an OS, so GNU components were ported to the new kernel. Within a few months the kernel was sufficiently advanced and sufficient software had been ported to make the system "self-hosting", that is it was possible to build Linux on Linux. Soon after that Linux adapted the GPL and rapidly started to gain momentum.
For better or worse the term Linux came to be used to describe not just the kernel, but the system as a whole. This annoyed the FSF who saw the kernel as just one component among many. From their perspective they saw their project essentially co-opted.
Answer: Linux is itself is a kernel. It can be configured to a network workstation, a DNS server, a DHCP server, a web server, a mail server, a file and print server, database server, a firewall, a gateway router and many more.
Kernel makes memory management, device management and system calls efficient
Linux torvald created kernel. Richard stallman in return decide to build a free os. They were lacking kernel. Then, GNU/Linux was born.
Linux is not about just torvald.
Answer: Linux is a kernel. That is the main reason to call it a kernel. Ubuntu, Android and Chrome OS are fundamentally different operating systems. Applications for Ubuntu do not necessarily run on Android just like they do not necessarily run on Windows. For one simple reason: The operating system is different. Linux based operating systems also strongly differ from a security point of view. Chrome OS is known to be very secure. Android is not that secure: http://www.forbes.com/sites/gordonkelly/2014/03/24/report-97-of-mobile-malware-is-on-android-this-is-the-easy-way-you-stay-safe/


Is it acceptable to use mainly websites in a literature review?

Question: I'm writing a literature review for a project and due to the nature of the project (website and database) I haven't used any books etc. I have used a number of websites to get information on subjects such as hosting a website and a database using Azure, is it acceptable to use websites in a literature review?
Comment: Why do you think a website would NOT be acceptable in a literature review?
Comment: Because of the word literature. Look at the answers below
Comment: I think the better question would be "How can I identify reasonable websites to use in my literature review?" At which point Penguin_Knight's answer is perfect.
Answer: Website or not website is not the point. Printed journal articles are favored over website because i) they are peer-reviewed, carrying some degree of authority, ii) they are archived and retrievable, with payment or free of charge, and iii) once published, the contents do not change until they are formally revised, rebuked, or retracted.
So, it would mean that when citing websites, a few more steps need to be done to increase their worthiness. First, you'd need to assess the credibility of the website and be able to critically evaluate the contents. In journal articles this step is done to some degree but in websites you're on your own. Second, you may need to do most of the archiving (including downloading, dating, and archiving the data sets you may download, or printing and dating website into PDFs, etc.) or use website archiving to save a cross-section of the websites. There are free services like http://perma.cc and http://www.webcitation.org available.
Once they are archived, in your paper, it is a good practice to cite both original link and the archived link as well as the date you accessed the original link. For details, consult the project leader or editor. Some journals specialized in publishing Internet-related research such as Journal of Medical Internet Research may also provide useful format-related examples in their guide to the authors.
Answer: You can definitely put into references the official user documentation of your web server and database engine, as this software have not been just invented by you from scratch.
While websites can also be used in references, it seems to me that you may need more in depth coverage. Try to Google and find some real publications. If the topic seems too broad, it should be some reviews. If there is a Wikipedia article, check which references it uses.