Extract rows of a table, that include less than x NULLs
Count where any 3 columns have values (not null)
E: dpkg was interrupted... run 'sudo dpkg --configure -a' What characters do I need to escape when using sed in a sh script? Take the following script:  #!/bin/sh sed 's/(127\.0\.1\.1)\s/\1/' [some file]   If I try to run this in sh (dash here), it'll fail because of the parentheses, which need to be escaped. But I don't need to escape the backslashes themselves (between the octets, or in the \s or \1). What's the rule here? What about when I need to use {...} or [...]? Is there a list of what I do and don't need to escape? Is $\cot(z)-\frac1z$ bounded on a given circle? Consider the sequence defined as  $x_1 = 1$  $x_{n+1} = \sin x_n$  I think I was able to show that the sequence $\sqrt{n} x_{n}$ converges to $\sqrt{3}$ by a tedious elementary method which I wasn't too happy about.  (I think I did this by showing that $\sqrt{\frac{3}{n+1}} &lt; x_{n} &lt; \sqrt{\frac{3}{n}}$, don't remember exactly)  This looks like it should be a standard problem.   Does anyone know a simple (and preferably elementary) proof for the fact that the sequence $\sqrt{n}x_{n}$ converges to $\sqrt{3}$? Does $A$ a UFD imply that  $A[T]$ is also a UFD? I'm trying to prove that $A$ a UFD implies that $A[T]$ is a UFD.   The only thing I am sure I could try to use is Gauss's lemma.  Also, how can we deduce that the polynomial rings $\mathbb{Z}[x_1,\ldots,x_n]$ and $k[x_1,\ldots,x_n]$ are UFDs? Understanding Python super() with __init__() methods I'm trying to understand the use of super(). From the looks of it, both child classes can be created, just fine.   I'm curious to know about the actual difference between the following 2 child classes.  class Base(object):     def __init__(self):         print "Base created"  class ChildA(Base):     def __init__(self):         Base.__init__(self)  class ChildB(Base):     def __init__(self):         super(ChildB, self).__init__()  ChildA()  ChildB() How old should you be when you begin Jedi training? What is the difference between necessary and sufficient conditions? If $p\implies q$ ("$p$ implies $q$"), then $p$ is a sufficient condition for $q$. If $\lnot p\implies \lnot q$ ("not $p$ implies not $q$"), then $p$ is a necessary condition for $q$.   I don't understand what sufficient and necessary mean in this case. How do you know which one is necessary and which one is sufficient? scope resolution operator without a scope Prove a differentiable function between banach spaces is continuous I really don't know where to start with this problem. It intuitively makes sense from basic calculus knowledge but I have absolutely no clue how to show it. , How to enable top bar background opacity? There is a way to change top bar background opacity in ubuntu 20.04 lts.  I installed the hide top bar extension but is not a option to change it. I don't really remember the plot but I think the a couple of alien girls (look human) crash land on Earth and are helped by human boy (18-19 ish). They get back to their ship but the older girl, with white hair, decides to stay on Earth to be with the boy.  At the beginning of the TV movie the boy wakes up and uses a ham radio and then goes to work on a ranch/farm(?).  I think it might have been one of these old Disney Sunday night movies. How do I get my players to form a PC party without just forcing them to? I believe that it is generally a good thing that all the PCs travel together, as a "party". This way the GM has to manage one environment – not, say, five. The problem is how the GM can ensure such a relationship between the characters.  One simple and really great solution is to require the players to design their characters so that they know each other. This has the added advantage of cross-backstories – as the players try to adjust their stories to fit together, they get and create hooks for more narrative!  However, I am now experimenting with giving the players complete freedom at character creation and later meeting in-game. No luck so far.  It is not easy, as trusting someone with your life (what happens during combat) is not a trivial thing. Furthermore, people generally like others with the same behaviour/interests/skin colour. Fantasy is all about diversity – you do not character-generate a strong man with a sword, no, you generate a half-ogre with an axe never before seen!  How can the GM get players, with diverse characters who are strangers to each other, decide to have their PCs act as a coherent group. I absolutely want to avoid asking my players to just solve this through metagaming ("Hey Joe, think of a way to get together with those, I can't story-tell two separate groups at the same time!").  I'm not having the stereotypical problem where a bunch of PCs meet for the first time and are automatically so paranoid of strangers that they start trying to kill each other. My players have just created a bunch of characters who have their own legitimate interests, and are faithfully following those in completely different directions that don't result in a classic "party". I put the PCs in great peril together and everyone escaped by their own devices, but now I have to figure out get the guy who ran for the hills back together with the couple that entered the city. And, I want to do it by resorting only to reasonable in-game events, not out-of-game metagame suggestions. Write Python stdout to file immediately When trying to write the stdout from a Python script to a text file (python script.py &gt; log), the text file is created when the command is started, but the actual content isn't written until the Python script finishes. For example:  script.py:  import time for i in range(10):     print('bla')     time.sleep(5)   prints to stdout every 5 seconds when called with python script.py, but when I call python script.py &gt; log, the size of the log file stays zero until the script finishes. Is it possible to directly write to the log file, such that you can follow the progress of the script (e.g. using tail)?   EDIT It turns out that python -u script.py does the trick, I didn't know about the buffering of stdout. Why does this double arithmetic give two different answers on a single machine? Proof by induction of summation inequality: $1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\dots+\frac1{2^n}\ge 1+\frac{n}2$ How to understand degrees of freedom? From , there are three interpretations of the degrees of freedom of a statistic:     In statistics, the number of degrees of freedom is the number of   values in the final calculation of a statistic that are free to vary.      Estimates of statistical parameters can be based upon different   amounts of information or data. The number of independent pieces of   information that go into the estimate of a parameter is called the   degrees of freedom (df). In general, the degrees of freedom of an   estimate of a parameter is equal to the number of independent scores   that go into the estimate minus the number of parameters used as   intermediate steps in the estimation of the parameter itself (which,   in sample variance, is one, since the sample mean is the only   intermediate step).      Mathematically, degrees of freedom is the dimension of the domain of a   random vector, or essentially the number of 'free' components: how   many components need to be known before the vector is fully   determined.   The bold words are what I don't quite understand. If possible, some mathematical formulations will help clarify the concept.  Also do the three interpretations agree with each other? A real nxn matrix always has a real eigenvalue when n is odd what do it mean by "A real nxn matrix always has a real eigenvalue when n is odd?" Shouldnt it be that regardless of odd or even there will always have a real root?  hope to hear from you guys soon:) how to set Image and text at the center of the button Random contacts on my iPhone 5 I recently bought a new iPhone 5. When I was going through my contacts today, I noticed a lot of email adresses stored along with the other numbers. I didn't even know most of them! What has happened?