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 Feb4 revised New proof about normal matrix is diagonalizable. added 4 characters in body Feb4 answered New proof about normal matrix is diagonalizable. Jan29 awarded Yearling Sep30 awarded Explainer Jul2 awarded Curious May24 revised Does every function with $f_x,f_y>0,f_{xx},f_{yy}<0$ with particular condition have to satisfy $f_{xy}/f_{xx} = -x/y$? deleted 3 characters in body May24 comment Does every function with $f_x,f_y>0,f_{xx},f_{yy}<0$ with particular condition have to satisfy $f_{xy}/f_{xx} = -x/y$? When you say $\infty$, it is $+\infty$ or $-\infty$? May21 answered Inverse of a sum of positive definite matrices May20 answered Proving $A+2B+3C+4D < 2.5$ with given conditions May20 comment If $f'(x) = 0$ for all $x \in \mathbb{Q}$, is $f$ constant? You may want to consider the fundamental theorem of Lebesgue integral calculus, which requires $f'(x)=0$ almost everywhere to get to your result. However, $\mathbb{Q}$ is far away from almost everywhere.. Feb12 awarded Nice Answer Jan29 awarded Yearling Sep12 accepted Maximal abelian subgroups in a $p$-group are always normal? Sep12 comment Maximal abelian subgroups in a $p$-group are always normal? thanks, I didn't realise it was so simple. Sep11 comment Maximal abelian subgroups in a $p$-group are always normal? @DonAntonio, yeah, I am assuming the $p$-group is finite. But I am talking about maximal among abelian subgroups, not just maximal among any subgroups. Sep11 asked Maximal abelian subgroups in a $p$-group are always normal? Aug5 comment $\frac {dy}{dx} \sin y = (1-x\cos y)\cos y$ Hint: $\sin ydy=-d\cos y$. Jul22 comment Are these 2 graphs isomorphic? @MarkMcClure, do you know this kind of animation is available by tex coding or not? Jul22 answered Group Theory Normal Subgroups Jul21 comment Solving $x^2-7[x]+5=0.$ to find values of $x$. Hint: find the intersection of $y=x^2+5$ and $y=7[x]$.