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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?