User Wilberd van der Kallen

21 votes

Accepted

Universal cover of SL2(R) admits no central extensions?

answered May 1, 2011 at 10:42

14 votes

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Tensor product of rings of Witt vectors

answered Mar 4, 2011 at 12:36

51 votes

Conceptual reason why the sign of a permutation is well-defined?

answered Mar 11, 2022 at 8:32

14 votes

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Invariants of matrices (by simultaneous $\mathrm{GL}_n$ conjugation) over arbitrary rings

answered Sep 8, 2021 at 8:59

5 votes

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Behavior of invariants under reduction mod p

answered Jul 2, 2020 at 8:11

6 votes

Accepted

Lifting $G$-invariants from characteristic $p\gg 0$ to characteristic 0 for a reductive algebraic group $G$

answered Jan 20, 2020 at 12:07

8 votes

Accepted

For regular tetrahedron $ABCD$ with center $O$, and $\overrightarrow{NO}=-3\overrightarrow{MO}$, is $NA+NB+NC+ND\geq MA+MB+MC+MD$?

answered Jan 8, 2020 at 10:03

6 votes

Accepted

Global homological dimension of reductive groups

answered Dec 19, 2019 at 12:15

2 votes

Number of solutions and minimal clues in Sixy Sudoku

answered Oct 11, 2019 at 13:38

2 votes

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Openly available software to work with Demazure modules

answered Mar 13, 2019 at 10:02

4 votes

$R$ a DVR with fraction field $K.$ What are the $R$-submodules of $K^n?$

answered Jun 5, 2018 at 10:04

6 votes

Polynomial ring $S[X]$ over domain $S$

answered Sep 21, 2016 at 8:08

1 vote

Maps between symmetric powers of the natural module for $SL_2 (k)$ in prime characteristic

answered Jul 24, 2016 at 9:31

4 votes

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Tilting modules in positive characteristic

answered Jun 6, 2016 at 10:00

1 vote

Extension property for unipotent linear groups over rings

answered Apr 25, 2016 at 7:40

35 votes

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How to prove this polynomial always has integer values at all integers?

answered Sep 18, 2015 at 7:38

3 votes

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finite generation of a certain type of subring

answered May 28, 2015 at 8:13

7 votes

GIT over integers

answered Jul 19, 2014 at 18:14

9 votes

Normalizer of SL_2(Z) in GL_2(R)

answered Feb 21, 2014 at 10:03

8 votes

How to think about parabolic induction.

answered Dec 10, 2012 at 13:23

8 votes

Accepted

Are homogeneous components of f.g graded modules f.g ?

answered Sep 18, 2012 at 18:29

1 vote

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Comparing Spectral Sequences

answered Aug 15, 2012 at 20:04

7 votes

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Subgroups of $GL_n(\mathbb Z)$ with finite coinvariants

answered Jul 27, 2012 at 14:19

4 votes

Are the Weyl modules projectives?

answered Jun 20, 2012 at 12:15

3 votes

Quotient space of algebraic group

answered Jun 17, 2012 at 7:50

5 votes

Accepted

Can the 'linkages' between equivalent extensions of modules of an algebraic group be taken to have bounded length?

answered Nov 24, 2011 at 9:26

5 votes

A question about the additive group of a finitely generated integral domain

answered Oct 10, 2011 at 7:22

7 votes

Isomorphism between varieties of char 0

answered Aug 21, 2011 at 12:23

0 votes

Convergence of eigenvectors

answered Mar 25, 2011 at 10:58

10 votes

Accepted

Question about the representation theory of SL(n,Z)

answered Mar 21, 2011 at 15:49

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37 Answers

Universal cover of SL2(R) admits no central extensions?

Tensor product of rings of Witt vectors

Conceptual reason why the sign of a permutation is well-defined?

Invariants of matrices (by simultaneous $\mathrm{GL}_n$ conjugation) over arbitrary rings

Behavior of invariants under reduction mod p

Lifting $G$-invariants from characteristic $p\gg 0$ to characteristic 0 for a reductive algebraic group $G$

For regular tetrahedron $ABCD$ with center $O$, and $\overrightarrow{NO}=-3\overrightarrow{MO}$, is $NA+NB+NC+ND\geq MA+MB+MC+MD$?

Global homological dimension of reductive groups

Number of solutions and minimal clues in Sixy Sudoku

Openly available software to work with Demazure modules

$R$ a DVR with fraction field $K.$ What are the $R$-submodules of $K^n?$

Polynomial ring $S[X]$ over domain $S$

Maps between symmetric powers of the natural module for $SL_2 (k)$ in prime characteristic

Tilting modules in positive characteristic

Extension property for unipotent linear groups over rings

How to prove this polynomial always has integer values at all integers?

finite generation of a certain type of subring

GIT over integers

Normalizer of SL_2(Z) in GL_2(R)

How to think about parabolic induction.

Are homogeneous components of f.g graded modules f.g ?

Comparing Spectral Sequences

Subgroups of $GL_n(\mathbb Z)$ with finite coinvariants

Are the Weyl modules projectives?

Quotient space of algebraic group

Can the 'linkages' between equivalent extensions of modules of an algebraic group be taken to have bounded length?

A question about the additive group of a finitely generated integral domain

Isomorphism between varieties of char 0

Convergence of eigenvectors

Question about the representation theory of SL(n,Z)