Why do some keys in NSUserDefaults have quote marks while others do not?
Description of NSDictionary - why are some key names with quotes?
How can I start automatically a command on Ubuntu 18.04? "Newtonian limit" property of special relativity Books say that special relativity is indistinguishable from Newtonian mechanics when the speed of the primed frame ($v$) is small compared to the speed of light ($c$). This is what I mean by the "Newtonian limit" property of special relativity. (I don't know the correct name for it). But the transformation of the time coordinate  $$t' = \gamma \left(t + \frac{vx}{c^2}\right)$$  involves the spatial quantity $x$ which, if large enough, could balance out the smallness of $v/c^2$. So why doesn't this observation mess up the "Newtonian limit" property? Statistical inference when the sample "is" the population Imagine you have to do reporting on the numbers of candidates who yearly take a given test. It seems rather difficult to infer the observed % of success, for instance, on a wider population due to the specifity of the target population. So you may consider that these data represent the whole population.   Are results of tests indicating that the proportions of males and females are different really correct? Does a test comparing observed and theoretical proportions appear to be a correct one, since you consider a whole population (and not a sample)? the cone is contractible Why I can't remove linux partitions on macOS Sierra I'm trying to remove linux partition from my iMac (21.5-inch, Mid 2011) on MacOS Sierra 10.12 Beta 2, So I tried existing solutions by using Disk Utility, I tried from Recovery as well, But it says      You can not remove this volume because the previous volume can not be resized         Some  use Terminal but I have no idea about that.  I need a simplest and easiest way to remove these linux partitions and grow my mac partition. How to determine the direction of a wave propagation? In the textbook, it said a wave in the form $y(x, t) = A\cos(\omega t + \beta x + \varphi)$ propagates along negative $x$ direction and $y(x, t) = A\cos(\omega t - \beta x + \varphi)$ propagates along positive $x$ direction. This statement looks really confusing because when it says the wave is propagating along $\pm$ x direction, to my understanding, we can drop the time term and ignore the initial phase $\varphi$ while analyzing the direction, i.e. $y(x, 0) = A\cos(\pm\beta x)$, however, because of the symmetry of the cosine function, $\cos(\beta x)\equiv \cos(-\beta x)$, so how can we determine the direction of propagation from that?  I know my reasoning must be incorrect but I don't know how to determine the direction. So if we don't go over the math, how to figure out the direction of propagation from the physical point of view? Why $-\beta x$ corresponding to the propagation on positive x direction but not the opposite? Why is saltatory conduction in myelinated axons faster than continuous conduction in unmyelinated axons? Transfer subtype Descriptions form long integer value into a string at new diffrent field What do "." and ".." mean when in a folder? when I used ls -a to view hidden files in folder there were 2 hidden files created by default what is the significance of this two files . and ..?  ls -a   Produces  .  .. Is there a way to show the console in a Windows application?  I want to do something like this:  static class Program {     [STAThread]     static void Main(string[] args) {         bool consoleMode = Boolean.Parse(args[0]);          if (consoleMode) {             Console.WriteLine("consolemode started");             // ...         } else {             Application.EnableVisualStyles();             Application.SetCompatibleTextRenderingDefault(false);             Application.Run(new Form1());         }     } } Is the set of partial order relations over $ \mathbb{N}$ countable? I think it's uncountable but don't know how to prove this. I would be very helpful if someone could also give some kind of a strategy for proving uncountability, in particular how to find injective or surjective functions that help proving this. Why is a conservative force defined as the negative gradient of a potential? I'm learning about work in my dynamics class right now. We have defined the work on a particle due to the force field from point A to point B as the curve Integral over the force field from point A to B. From math I know that if a vector field has a potential, we only need to evaluate the potential at point B minus the potential at point A to get the result of the curve Integral. In the text that I'm reading, it's explained that if the integral over a force-field is path-independent, then the force field $F = -{\rm grad}(V)$, where $V$ is the potential. Why is it defined as the negative gradient? Doesn't one determine the potential from $F$ mathematically. Why do we impose the sign on the potential? The most prominent place I've noticed this is when SSH-ing at work, but I feel like I've observed this behaviour elsewhere too.  When I try to log into Linux servers from my Windows desktop at work, I've noticed that if I mis-type my password, it takes a good 5 seconds or so before I get "Access Denied" back. Then when I type my password correctly, the login (along with welcome messages etc) is virtually instant.  Is there any logical reason for this, or would it be down to some odd configuration that's specific to the machines here at work? Pandas Merging 101 The tutorial I read is based on 2.79. So I'm not sure if it's still the same name in 2.8. But I can't find anything providing similar functionalities. Any open subset of $\Bbb R$ is a countable union of disjoint open intervals Central limit theorem and the law of large numbers I have a very beginner's question regarding the Central Limit Theorem (CLT):  I am aware that the CLT states that a mean of i.i.d. random variables is approximately normal distributed (for $n \to \infty$, where $n$ is the index of the summands) or the standardized random variable would have a standard normal distribution.  Now the Law of Large Number states roughly speaking that the mean of i.i.d random variables converges (in probability or almost surely) to their expected value.  What I don't understand is: If, as the CLT states, the mean is approximately normally distributed, how then can it also converge to the expected value at the same time?   Convergence would imply for me that with time the probability that the mean takes a value which is not the expected value is almost zero, hence the distribution would not really be a normal but almost zero everywhere except at the expected value.  Any explanation is welcome. Can the cometd library be used in Salesforce to listen to external buses? I'll confess up front that my knowledge of event architecture is limited:  Can the cometd library that we can use to listen for Streaming API and Platform Events also be used to listen to external (i.e. non-Salesforce) event buses?  We're hoping to make Salesforce a subscriber to replace polling integrations through the REST API. How can I install Ubuntu without removing Windows? I need my pre-installed version of Windows 7 (or any other version of Windows), how could I install Ubuntu without erasing it? Find the representing matrix with the respect to the standard basis Let $\Bbb R^2$ be a finite-dimensional vector space and $T:\mathbb{R}^2\to \mathbb{R}^2$ a linear operator. Let $[T]_B =  \begin {bmatrix}0 &amp; 0 \\-1 &amp; 1 \end{bmatrix} $ be a matrix representation of $T$ in the basis $B= (1,1),(-1,0)$.   How can I find the matrix $[T]_E$ where $E$ is the standard basis $(1,0),(0,1)$ if the transformation isn't given in the question?  Thanks,  Alan