Brian B
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 Dec3 revised A function that is not a derivative simple should have been elementary Dec3 comment A function that is not a derivative Good catch, Matthew, and so edited. Dec2 awarded Yearling Dec2 answered A function that is not a derivative Nov20 comment Preparing for Spivak I'll never forget all the time I spent figuring out one of Spivak's problems: finding the antiderivative of $\sqrt{\tan(x)}$. Nowadays, Mathematica gets that one automatically. Mar18 comment Correlation matrix from Covariance matrix Thanks, I fixed that now. Mar18 revised Correlation matrix from Covariance matrix fixed to include square root Feb17 awarded Critic Feb17 comment How to show that for $|x|<1$, $1+2x+3x^2+\cdots=\frac1{(1-x)^2}$? Definitely untrue for certain values of $n$ and $x$ Feb12 answered Correlation matrix from Covariance matrix Mar22 awarded Editor Mar22 revised Dirac Delta function deleted 3 characters in body Mar22 comment Dirac Delta function @Carl: Agreed. The original dirac delta was a limit of gaussians, defined in such a way. It is possible to define a point mass functional as a limit of 1-sided kernels as well. I think of these things in functional analytic terms: the $\delta$ defines a functional taking $f$ to its value at the location of the point mass. A limit of even functions is an odder beast, taking $f$ to $f(0)/2$ if the point is on the boundary and $f(0)$ or $0$ otherwise. Note that defining $\delta(x)$ as a limit of even functions has the annoying property that $\int_{(-\infty,0)} f(x) \delta(x) dx = f(0)/2$. Mar22 answered Dirac Delta function Feb27 answered Is this function convex? Feb27 answered How to solve a multiple variable linear equation Feb24 comment Direct proof that $\pi$ is not constructible If there were such a direct proof wouldn't we have seen far fewer cranks trying to square the circle in the last century or so? Feb19 awarded Teacher Feb17 answered Line integration in complex analysis Feb15 comment Are sines of primes dense in $[-1,1]?$ On distributional principles it sure seems true. Very cool question.