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When is $2^n \pm 1$ a perfect power Is there an easy way of showing that $2^n \pm 1$ is never a perfect power, except for $2^3 + 1 = 3^2 $?  I know that   gives the result directly, but I'm hopefully for a more elementary method.    I can show that it is never a square, except for $2^3 + 1$.  Proof: Cases $n=1, 2, 3$ are easily dealt with. Henceforth, let $n\geq4$.  $2^n -1 \equiv 3 \pmod{4}$ hence is never a square.     If $2^n +1 =x^2$, then $2^n = (x-1)(x+1)$ and both of these are powers of 2. Thus we must have $(x-1) = 2, (x+1) = 4$. This gives the solution of $2^3 + 1 = 3^2$.    Let's now do an odd prime.  Say $2^n +1 = x^p$. Then $2^n = x^p - 1 = (x-1)(x^{p-1}+x^{p-2} + \ldots +1)$, so both terms are powers of 2. We have $ x = 2^m+1$ is odd. But the other term is the sum of $p$ odd numbers, hence is odd. Since this is clearly greater than 1, we get a contradiction.  Say $2^n -1 = x^p$. Then $2^n = x^p + 1 = (x+1)(x^{p-1} - x^{p-2} + \ldots -x +1 )$, so both terms are powers of 2. We have $x = 2^m -1$ is odd. But the other term is the sum of $p$ odd numbers, hence is odd. Since $x^p + 1 \neq x+1$ except for $p=1$, this means that the term is greater than 1. Hence we get a contradiction. What's the main difference between Intel processor generations? What are the main differences between each processor generation of Intel processors/chipsets? Is there a performance difference?  I've already seen processors from first to sixth generation, however I don't know/understand the exact differences. How to find sum of power series $\sum_{n=0}^\infty\frac{1}{n!(n+2)}$ by differentiation and integration? Let $$ S = \sum_{n=0}^\infty \frac{1}{n!(n+2)} $$  Integrate the Taylor Series of $xe^x$ to show that S = 1  Also, Differentiate the Taylor series of $\frac{e^x - 1}{x}$ to show that S = 1  For the integration one, I got $\int xe^x dx$ = $x^2\sum_{n=0}^\infty \frac{1}{n!(n+2)} x^n$ and pretty much stuck. I can see the S term but i have no idea how to move on from there Is it valid to have more than one question mark in a URL? Possible Duplicate:             I faced this issue today when I was replying to a comment posted which used @Transaction annotation.     @Trevor: @Transaction and ... @Transaction   When typing this comment, I got a warning. I have an expression for a covariance matrix $C$ in terms of the indices $i$ and $j$.  In this way I can analytically calculate the elements of my covariance matrix, however when I try to invert $C$ matlab gives a warning about the matrix being close to singular.  The inversion therefore doesn't work, by which I mean the multiplying $C$ by the inverse given i do not get the identity.  I have tried calculating the pseudo inverse but this also does not work.  I have also tried adding a small constant along the diagonal but again the results do not work.  In general I'm working with matrices of dimension 1200 but I will give low dimensional example matrix that has the same properties, i.e. the matrices are symmetric about the diagonal and the anti-diagonal     19.9939  19.9954  19.9958  19.9951  19.9933  19.9905  19.9865    19.9954  19.9973  19.9981  19.9978  19.9965  19.9940  19.9905    19.9958  19.9981  19.9993  19.9995  19.9985  19.9965  19.9933    19.9951  19.9978  19.9995  20.0000  19.9995  19.9978  19.9951    19.9933  19.9965  19.9985  19.9995  19.9993  19.9981  19.9958    19.9905  19.9940  19.9965  19.9978  19.9981  19.9973  19.9954    19.9865  19.9905  19.9933  19.9951  19.9958  19.9954  19.9939   As mentioned in the title the matrix isn't positive, however the the negative eigenvalues are very small suggesting that the matrix is not positive only due to machine precision.  The negative eigenvalues are $-1.4048e-14$ and $-2.4571e-15$.  How can I go about inverting these matrices? At 12 o'clock, the hour hand and minute hand of the clock can be swapped, and the clock still gives the same time, but at 6 o'clock, it can not be swapped. So in what cases when we swap the hour and the minute hand position does a clock still give a valid time? Bijection between an infinite set and its union of a countably infinite set I have $A$ as an infinite set and $S$ as a countably infinite set, (so that means there exists a one-to-one correspondence between $S$ and $\mathbb{N}$).  How do I show that there always exists a bijection between $A$ and $A\cup S$? Can it be done by showing that there is a bijection from $A$ to $S$ or from $A$ to itself? I am lost on this one.   Oh, and can it be possible that there is no bijection if it is between $S$ and $A\cup S$? What about a map that maps $\mathbb{N}\to \mathbb{N}\cup\mathbb{R}$? Why can't I suggest edits on SE meta sites? When you click &quot;edit&quot; on a meta site as a non-edit-privileged user, the error message appears:  Suggested edits are not allowed on non-tag-wiki posts on meta sites.  Right as I was going to post this as a possible bug, I finally found  which says that per-site Metas don't have suggested edits by-design because it's &quot;not needed&quot;. However, I've been participating a lot over at  where there is a lot still to be figured out in terms of site guidelines.  The Meta is one of the most active I've seen and a lot of the questions are really trying to gather community-built suggestions.  So maybe more of those need to be Community Wiki, but it still brings me to ask why don't per-site-metas &quot;need&quot; suggested edits? Adding bounty: The more betas I participate in, the more I don't get this. Google street view URL Hi I have the address of a property and my application can launch a browser to go to . If a good match is found it will take it directly there. Is there any thing I can append to the url to make it switch to streetview without having the exact coordinates? I dont't want to code any javascript or flash. In my profile my hat is on the correct position, but not in the top-bar. This is new. It worked until now.    Works neither in Chrome nor in Firefox. Which preposition in front of "line" — "on", "in", "at"? Using X11 in OSX 10.8.3 : "ld: library not found for -lX11" What's a (big) word or phrase to describe a book that is so good that I cannot put down when I start to read it? What is the fastest car in Saints Row 4, and where can I find it? I found an Attrazione in Salander, which has traditionally been fastest in previous games, but I don't like sinking upgrades money into a car unless it's for keeps. What car is fastest in Saints Row IV (before and after upgrades), and how can I get it? Is there any way to get stats on the car in-game? LibreOffice toolbar icons went blank after application hanged QGIS 2.0 - can't select items in Print Composer How to Expand , Resize "extended partition"? How should I add users non-interactively? I have a set of users I need to add to a new system as part of a BASH script that preps my server. How can I non-interactively add the user, create their home directory, set the group, and enter a default password?  For example, if I needed to add the following users, how would I go about that?  user: node password: spoon group: ultraMegaPermissions  user:serve password: spoot group: sortaNotMega  user:watcher password: spoogle group: notMega Is it possible to produce such design hints (on elements padding, spacing, width, height) automatically in photoshop? Is there any plugin that can potentially do this?