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MathOverflow : Recent Questions and Answers (Page 34)

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Kernel of the map $mathbb{C}[G]^U to mathbb{C}[U^+]$

$DeclareMathOperator{SL}{operatorname{SL}}$Let $G=SL_k$ be the special linear group, $U$ the unipotent subgroup consisting of all lower unipotent triangular matrices, $U^+$ the unipotent subgroup consisting of all upper...

Asked on 01/13/2021 by Jianrong Li

0 answer

Finite fast tests for periodicity of certain matrices

Let $M=- U^{-1} U^T$ be an $n times n$-integer matrix, where $U$ is an upper triangular 0-1-matrix where all diagonal entries are equal to one. $M$...

Asked on 01/12/2021

1 answer

Are these two kernels isomorphic groups?

We have a finitely presented, infinite group $mathsf{B}$,coming from a geometric topology problem (it is the quotient of a braid group for a genus 2 surface). It is...

Asked on 01/12/2021 by Francesco Polizzi

0 answer

Definition of subcoalgebra over a commutative ring

Let $k$ be commutative ring and $(C, Delta)$ be a coalgebra over $k$. Let $D$ be a $k$-submodule of $C$. Notes I'm reading give...

Asked on 01/11/2021 by user839372

2 answer

Riesz Representation Theorem for $L^2(mathbb{R}) oplus L^2(mathbb{T})$?

The spaces $L^2(mathbb{R})$ (square-integrable functions) and $L^2(mathbb{T})$ (1-periodic square-integrable functions, considered over the real line $mathbb{R}$) are two subspaces of the space of tempered distributions $mathcal{S}'(mathbb{R})$...

Asked on 01/10/2021 by Goulifet

0 answer

Morphism of distinguished triangles where one of the arrows is a quasi-isomorphism

Let $R$ be any ring and let $Ato Bto Cto [1]$ and $A'to B'to C'to [1]$ be distinguished triangles of complexes of $R$-modules. Let $f:Ato...

Asked on 01/10/2021 by Stabilo

0 answer

Cut points and critical points of the exponential map

Suppose $ M $ is a complete Riemannian manifold with exponential map $ exp : TM rightarrow M $, and $ S subseteq M $ is an...

Asked on 01/09/2021 by Longyearbyen

0 answer

Eigendecomposition of $A=I+BDB^H$

Suppose that we have $$A = I_m + BDB^H$$ where matrix $A$ is $m times m$, matrix $B$ is $m times k$, $BB^H...

Asked on 01/09/2021 by user164237

0 answer

When is the log-permanent concave?

Let $operatorname{PSD}_n$ be the cone of $ntimes n$ semidefinite positive matrices. For any $Xin operatorname{PSD}_n$, define $$f(X)=log(det(X)).$$ Then $f$ is a concave function...

Asked on 01/09/2021 by Bill Bradley

1 answer

Laplace transform of the product of two gammas

Suppose that $X$ and $Y$ are both gamma distributed with shapes $a,b$ and unit scales / unit rates. To fix ideas, X has Laplace transform given by:...

Asked on 01/09/2021 by lrnv

1 answer

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