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Python idiom to return first item or None Can we decrypt in this order when the message is encrypted twice? Is there really no way to power a DSLR by USB? How to replace a string on the 5th line of multiple text files? I want to replace the 5th line of multiple text files (file1.txt,file2.txt,file3.txt,file4.txt) with the string " Good Morning " using a single terminal command.  All the text files are located on my ~/Desktop.  Note: My desktop consists of 6 .txt files.I want to apply the change to above mentioned 4 text files only. Overriding "+=" in Python? (__iadd__() method) I really tried hard to do this on my own, but I keep failing ... Using multi-column and multi-row commands confused me. I know some of you might frown upon such a request but I really need help getting this table implemented .. So I'm sorry if I upset anyone.  My preamble has :   \documentclass[12pt,a4paper]{report} \usepackage[lmargin=3.81cm,tmargin=2.54cm,rmargin=2.54cm,bmargin=2.52cm]{geometry} \linespread{1.5}     Is there a way I can convert this from excel to LaTeX form ? I tried an add-in before but it didn't work for me. Also online generators cannot combine cells the way I want them (unless I didn't find a better table generator ? ) What are good out of the box alternative geocoding solutions to Esri's services? Esri's  will retire at the end of the year, what are good GIS services that can replace the North American and US Streets geocoders?  I have totally excluded Esri's replacement, the  as it is a subscription service. Attributes from an hstore-column as separate columns in a view? Let's assume we have a table where it has two columns, one an ID and one an hstore column, which stores key/value pairs.  You could query that column as such:  SELECT name, attributes-&gt;'device' as device FROM products WHERE attributes-&gt;'edition'= 'ebook'   Can you create a view for this table where the keys/values appear as columns in the view instead of all in the hstore column?  Example: I have an hstore key where it might have the key of amenity.  I want to create a view showing the the ID and the amenity values in a new view. Replicate data from one list to another list sharepoint I have a list A with columns Emp ID, First name &amp; last name and list B wih column name Emp ID, First name, last name , phone number and address.  I want functionality such that when i enter item in List A and save same data should get copied in 3 columns of list B. Can someone help me with the approach. Thnks Concentric circles with CSS checking if a number is a prime I was reading Wikipedia, and it was given that "all primes are of the form 6k ± 1"  (other than 2 and 3), where k = 1,2,3,4,...  Is this statement correct? If yes, can we use this to test if a given number is a prime number? For instance, we can say that 41 is a prime number, since there exists an integer K (K >= 1), such that 6k - 1 = 41 ==> k = 7.  I am confused why this test cannot be used to test if a number is prime?  Thanks, Sekhar Using principal component analysis (PCA) for feature selection How to get time difference in minutes in PHP How to calculate minute difference between two date-times in PHP? Find all the values of $w$ ∈ C that satisfy the equation. I always use lm() in R to perform linear regression of $y$ on $x$. That function returns a coefficient $\beta$ such that $$y = \beta x.$$  Today I learned about total least squares and that princomp() function (principal component analysis, PCA) can be used to perform it. It should be good for me (more accurate). I have done some tests using princomp(), like:  r &lt;- princomp( ~ x + y)   My problem is: how to interpret its results? How can I get the regression coefficient? By "coefficient" I mean the number $\beta$ that I have to use to multiply the $x$ value to give a number close to $y$. Is it true that the light from some galaxies will never reach us?   The explanation for that is that the Universe expanding faster than the speed of light. But, if the speed of light is constant in all reference frames, then in the reference frame of a galaxy, light must move at the speed of light and thus it must reach us (?).   Could you also explain  how is it possible that the Universe is expanding faster than the speed of light if an object can never be observed to move faster than light? (without Doppler effects). Primes dividing a polynomial Let $g(x)\in \mathbb{Z}[x]$, a nonconstant polynomial. Show that the set of primes $p$ such that $p\mid g(n)$ for some $n\in \mathbb{Z}$ is infinite.  I don't know how to start. I have tried asuming that the set is finite but i haven't reached any contradiction. This may have been asked before, however I was unable to find any duplicate. This comes from  of "Mathematical Analysis: An Introduction" by Browder. Problem 14:     If $(a_n)$ is a sequence in $\mathbb R$ and $a_n &gt; 0$ for every $n$. Then show:   $$ \liminf(a_{n+1}/a_n) \le \liminf((a_n)^{(1/n)}) \le \limsup((a_n)^{(1/n)}) \le \limsup(a_{n+1}/a_n)$$   The middle inequality is clear. However I am having a hard time showing the ones on the left and right. (It seems like the approach should be similar for each). This is homework, so it'd be great if someone could give me a hint to get started on at least one of the inequalities.  Thanks. Let $M$ be the space of all $m\times n$ matrices. And $C=\{X\in M|rank(X)\leq k\}$ where $k\leq \min\{m,n\}$. How to proof that C is a closed subset of M? Giving a single user write permissions I want to grant a single user write permissions to a directory owned by root over command line. With chmod, it seems that I can only change the permissions for owner, group and other, not for a specific user.