User Wilberd van der Kallen

53 votes

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

answered Mar 11, 2022 at 8:32

35 votes

Accepted

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

answered Sep 18, 2015 at 7:38

21 votes

Accepted

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

answered May 1, 2011 at 10:42

14 votes

Accepted

Is Lusztig's conjecture solved?

answered Mar 28, 2010 at 10:24

14 votes

Accepted

Tensor product of rings of Witt vectors

answered Mar 4, 2011 at 12:36

14 votes

Accepted

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

answered Sep 8, 2021 at 8:59

11 votes

Non-finite version of Nakayama's lemma?

answered Jul 27, 2010 at 14:32

10 votes

Accepted

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

answered Mar 21, 2011 at 15:49

9 votes

Proper subgroup of GL(n,Z) isomorphic to GL(n,Z)?

answered Sep 16, 2010 at 9:56

9 votes

How to think about parabolic induction.

answered Dec 10, 2012 at 13:23

9 votes

Normalizer of SL_2(Z) in GL_2(R)

answered Feb 21, 2014 at 10:03

8 votes

Accepted

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

answered Sep 18, 2012 at 18:29

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

8 votes

Accepted

Connected components of the orthogonal group O(2n) in characteristic 2.

answered Mar 25, 2010 at 16:28

7 votes

Accepted

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

answered Jul 27, 2012 at 14:19

7 votes

Isomorphism between varieties of char 0

answered Aug 21, 2011 at 12:23

7 votes

GIT over integers

answered Jul 19, 2014 at 18:14

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

6 votes

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

answered Sep 21, 2016 at 8:08

6 votes

Accepted

Global homological dimension of reductive groups

answered Dec 19, 2019 at 12:15

5 votes

Accepted

Behavior of invariants under reduction mod p

answered Jul 2, 2020 at 8:11

5 votes

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

answered Oct 10, 2011 at 7:22

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

Accepted

Intersections of conjugates of the icosahedral group in SO(3)

answered Mar 21, 2011 at 15:24

5 votes

Applications of finite continued fractions

answered Feb 20, 2011 at 9:52

4 votes

Accepted

Tilting modules in positive characteristic

answered Jun 6, 2016 at 10:00

4 votes

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

answered Jun 5, 2018 at 10:04

4 votes

Are the Weyl modules projectives?

answered Jun 20, 2012 at 12:15

3 votes

Accepted

finite generation of a certain type of subring

answered May 28, 2015 at 8:13

3 votes

Quotient space of algebraic group

answered Jun 17, 2012 at 7:50

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37 Answers

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

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

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

Is Lusztig's conjecture solved?

Tensor product of rings of Witt vectors

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

Non-finite version of Nakayama's lemma?

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

Proper subgroup of GL(n,Z) isomorphic to GL(n,Z)?

How to think about parabolic induction.

Normalizer of SL_2(Z) in GL_2(R)

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

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

Connected components of the orthogonal group O(2n) in characteristic 2.

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

Isomorphism between varieties of char 0

GIT over integers

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

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

Global homological dimension of reductive groups

Behavior of invariants under reduction mod p

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

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

Intersections of conjugates of the icosahedral group in SO(3)

Applications of finite continued fractions

Tilting modules in positive characteristic

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

Are the Weyl modules projectives?

finite generation of a certain type of subring

Quotient space of algebraic group