Johannes Kloos
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 Mar 5 awarded Yearling Feb 22 comment When is matrix multiplication commutative? Here's an example why this condition is not neccesary: Take any non-diagonalizable matrix $A$. It will always commute with the unit matrix $1$, but $A$ and $1$ are clearly not simultaneously diagonalizable. Feb 12 comment When is matrix multiplication commutative? It's not neccesary, AFAICT. I can't think of a counter-example right know, though. Jan 31 awarded Revival Jan 22 comment $f(x) \in R[X]$ irreducible $\Rightarrow (f(x))$ ideal? Just to make sure: Do you mean by $(f)$ the ideal generated by $f$? Jan 11 comment Powers of $2\times 2$ matrices, such that $A^n = I$ Do you know about rotation matrices? Dec 19 awarded Caucus Sep 30 awarded Explainer Sep 25 awarded Nice Answer Sep 13 comment What is a polynomially bounded function? Your argument would be correct if, say, $2^x-1$ was actually a polynomial. But by definition, a polynomial is of the form $a_n x^n + a^{n-1} x^{n-1} + \cdots + a_0$ for some parameters $a_0, \ldots, a_n$, and $2^x-1$ is not of this form. Sep 13 revised What is a polynomially bounded function? Not particularly computer science-related, texified Sep 2 reviewed Approve spectral-graph-theory tag wiki excerpt Aug 2 reviewed Approve Finding the volume of $(x-4)^2+y^2 \leqslant 4$ Aug 2 reviewed Reject Finitely generated ideal in Boolean ring; how do we motivate the generator? Jul 6 comment Help with function proof Indeed, my bad. Ignore my previous comment. Jul 6 comment Help with function proof @CameronWilliams: If you prefer, write $\mathbb{N}_0$. My natural numbers start at 0. Jul 6 comment Help with function proof To everybody trying to show a proof: Please make sure that what you want to prove is actually correct. In particular, consider $A = B = \mathbb{N}$, $f(x) = 0$, $Y = 2\mathbb{N}$ (the set of even numbers). Jul 6 comment Who named “Quotient groups”? Skimming the paper, Hölder uses the term "Faktorgruppe" (factor group), and refers to an expression "factor of composition" that he attributes to Jordan. May 31 comment How to extend an existing orthogonal set of vectors? Would guessing a linearly independent vector and then taking the orthogonal part work? You can make a vector orthogonal to a given set of vectors using methods similar to Gram-Schmidt... Apr 26 reviewed Reject Uniform convergence of $\sum_{k=1}^\infty \frac{1}{k^{1+x}}$