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? 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 12 comment When do matrices have positive eigenvectors? What's a positive eigenvector? 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 comment Find the sum $\sum_{k=2}^n \frac{n!}{(n-k)!(k-2)!}.$ $LHS=\frac{n!}{(n-2)!}\sum_{k=0}^{n-2}\frac{(n-2)!}{(n-2-k)!k!}=n(n-1)2^{n-2}$ Jan 7 comment Show that $rank(A) \ngeq \frac{[tr(A)]^2 }{tr(A^2)}$ Does >/= mean