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 5h comment Show that a differentiable function $f:\mathbb{R} \to \mathbb{R}$ has a global max in $a$ if $a$ is its local max what does $A(a,f(a))$ mean? 1d comment How to find shortest distance between two skew lines in 3D? Incidentally, the claim that shortest distance is "only interesting" in the skew case is an odd one. Of course you can define shortest distance between parallel lines, and this is still very useful in applications like e.g. collision detection. Apr 30 comment Can we find prime numbers with any sum of digits (except those divisible by three) This seems like the kind of question that is incredibly hard to answer one way or the other, unless a small counterexample is found by direct search... Apr 30 answered Study the absolute minima and maxima of $f(x,y)=(x^2-y^2)(x-2)$ Apr 22 asked Deconvolution by disks Apr 11 answered Robustly map rotation matrix to axis-angle Mar 31 answered Mean curvature of a level set Mar 29 asked Critical points of a harmonic function Mar 22 awarded Revival Mar 17 comment Could we “invent” a number $h$ such that $h = {{1}\over{0}}$, similarly to the way we “invented” $i=\sqrt{-1}$? I think you can do it if you also introduce an indeterminate $k$, and define equality appropriately. Mar 17 accepted closed-form expression for roots of a polynomial Mar 16 awarded Popular Question Mar 14 awarded Notable Question Mar 14 comment Derivative of a linear transformation. +1 I really do hate that derivatives are taught as "slopes of tangent lines." If the differential were explained properly from the start it would demystify much of higher-dimensional calculus... Mar 2 answered Degrees of Polynomials Feb 26 awarded Yearling Feb 21 comment True or False: $f(z)=Ln(z)$ is periodic You will need to expand on what you mean for a complex function to be periodic, Under the definition you quoted, no, $Ln$ clearly is not periodic. Feb 19 comment Can $e^{ax}$ be said to be the eigenfunction of the operator $\frac{d^{(n)}}{dx}$? Note that the counter examples I'm aware of for the shape of a drum question are either high-dimensional or have boundary. As far as I know it's still open if there exists a pair of non-isometric, isospectral compact surfaces (without boundary) in $\mathbb{R}^3$. Feb 10 answered How to find the limit of a matrix $P^n = UD^nU^{-1}$ where $D$ is a diagonal matrix of eigenvalues and $U$ a matrix of eigenvectors? Feb 10 comment How can I show that for matrix $A$ , $A^t A$ is not equal to $A A^t$ in general? Have you tried arbitrary small (e.g. $2\times 2$) matrices?