Paolo Leonetti
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 2d comment Find the interval of convergence $\displaystyle \sum_{n=1}^\infty \dfrac{(3n)!(x^n)}{(n!)^3}$ Well, weak approximations like $(n/4)^n\le n!\le (n/2)^n$ do not work for sure.. 2d revised Find the interval of convergence $\displaystyle \sum_{n=1}^\infty \dfrac{(3n)!(x^n)}{(n!)^3}$ deleted 9 characters in body 2d answered Find the interval of convergence $\displaystyle \sum_{n=1}^\infty \dfrac{(3n)!(x^n)}{(n!)^3}$ Apr 30 revised Smallest distance of a point to a surface edited title Apr 30 comment Can we find prime numbers with any sum of digits (except those divisible by three) I couldn't even consider it a heuristic.. Apr 30 revised Smallest distance of a point to a surface added 10 characters in body; edited title Apr 30 comment Smallest distance of a point to a surface You're right; anyway, apart from relabelings, the question should be clear enough Apr 30 comment Smallest distance of a point to a surface Indeed no, it is as if everything has been defined on $\mathbf{R}^{n-1}$ Apr 30 asked Smallest distance of a point to a surface Mar 29 comment Minimum of $\sum \left|7x-1\right|$ Well, I did know this pretty manual solution; but isn't any better option? Mar 29 comment Prove the following Theorem. For all positive integers n, some element of the set {n, n + 1} is divisible by 2. Come on, such a question shouldn't have been posted Mar 29 revised Prove the following Theorem. For all positive integers n, some element of the set {n, n + 1} is divisible by 2. added 104 characters in body Mar 29 revised Transformation of positive semi-definite matrices deleted 130 characters in body Mar 29 answered Prove the following Theorem. For all positive integers n, some element of the set {n, n + 1} is divisible by 2. Mar 28 revised Transformation of positive semi-definite matrices added 134 characters in body Mar 28 comment Transformation of positive semi-definite matrices Which limit are you talking about? Consider the matrix X=[1,2,2; 2,1,1; 2,1,1]. Then the determinants of $X_{123}$, $X_{23}$ and $X_3$ are nonnegative. But $X$ is not PSD. Mar 28 comment Transformation of positive semi-definite matrices A matrix $M$ is positive semi-definite if $x^TMx$ is nonnegative for each vector $x$. On the other hand, positive definiteness require strictly positiveness. The version of Sylvester criterion which you are talking about is about the latter case. Of course, there are variants of such criterion also for PSD matrices, but probably your condition is not sufficient. [I am just aware of a version requiring to check all principal minors, which is not quoted in Wikipedia] Mar 28 comment Transformation of positive semi-definite matrices Indeed, here we are not talking about positive definite matrices. Mar 28 comment Transformation of positive semi-definite matrices Where did you take such version of Sylvester criterion about positive semi-definite matrices? I believe too the answer is affirmative, but shouldn't we check all principal minors of $A$ and $B$? .. Mar 28 asked Transformation of positive semi-definite matrices