ziyuang
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 Feb 23 comment Can I get the upper bound of the $L_2$ norm of a symmetric matrix? 2-norm is the largest singular value. Feb 23 comment Proving that an $m \times n$ matrix has rank 1 Please post your exercise. Feb 10 comment If $(a_n)$ is a decreasing sequence of strictly positive numbers and if $\sum{a_n}$ is convergent, show that $\lim{na_n}=0$ How did you show $\lim_{n\to\infty}na_n$ exist? Feb 8 revised Derivative of trace of matrix tr -> \mathrm{tr}, use align environment Feb 8 suggested approved edit on Derivative of trace of matrix Feb 8 revised Derivative of trace of matrix <> -> \langle\rangle Feb 8 suggested approved edit on Derivative of trace of matrix Jan 29 comment Why is a vector function not smooth if $r'=0$ It's like you walk, stop, and (probably turn and) walk again. There will a singularity in the route. Jan 17 comment Laplace, Legendre, Fourier, Hankel, Mellin, Hilbert, Borel, Z…: unified treatment of transforms? @Startwearingpurple Can separable Hilbert space have uncountable orthonormal basis? Jan 15 revised Different versions of Mercer's theorem deleted 1 character in body Jan 15 asked Different versions of Mercer's theorem Jan 12 revised Continuity of a Linear function For grammar mistakes Jan 12 comment When do matrices have positive eigenvectors? What's a positive eigenvector? Jan 12 suggested approved edit on Continuity of a Linear function Jan 12 revised When do matrices have positive eigenvectors? For grammar mistakes Jan 12 suggested approved edit on When do matrices have positive eigenvectors? Jan 11 revised About sequential weak closure in $\ell^2$ Texified the title; <> t0 langle/rangle Jan 11 comment Prove that $(a-b)^n\mid (a^n-b^n) \iff n=1$ under given conditions Why is it the hardest? Jan 11 suggested approved edit on About sequential weak closure in $\ell^2$ Jan 11 comment Find the sum $\sum_{k=2}^n \frac{n!}{(n-k)!(k-2)!}.$ $LHS=\frac{n!}{(n-2)!}\sum_{k=0}^{n-2}\frac{(n-2)!}{(n-2-k)!k!}=n(n-1)2^{n-2}$