User Anton Geraschenko

19 votes

4 answers

2k views

Example of a smooth morphism where you can't lift a map from a nilpotent thickening?

asked Apr 21, 2010 at 5:38

18 votes

2 answers

4k views

What does the nilpotent cone represent?

asked Jan 8, 2010 at 17:41

18 votes

3 answers

2k views

Can a coequalizer of schemes fail to be surjective?

asked Oct 2, 2009 at 5:33

15 votes

12 answers

6k views

Alternatives to pi day [closed]

asked Oct 19, 2009 at 19:40

15 votes

0 answers

633 views

Are "fpqc algebraic spaces" algebraic spaces?

asked Oct 9, 2011 at 0:22

14 votes

2 answers

1k views

If a map restricts to an isomorphism on a closed subscheme and its open complement, must it be an isomorphism?

asked Dec 10, 2010 at 2:02

14 votes

1 answer

845 views

Can a single DVR witness all specializations on a variety?

asked Oct 26, 2010 at 0:41

14 votes

1 answer

631 views

Does a degeneration always have a larger-dimensional automorphism group?

asked Mar 12, 2011 at 21:14

14 votes

6 answers

2k views

Does every morphism BG-->BH come from a homomorphism G-->H?

asked Oct 22, 2009 at 4:34

14 votes

2 answers

928 views

Do orbits and stable loci of group actions have natural scheme structures?

asked Oct 28, 2009 at 19:40

13 votes

1 answer

2k views

Are root stacks characterized by their divisor multiplicities?

asked Sep 7, 2010 at 18:36

13 votes

4 answers

2k views

Does smoothness descend along flat morphisms?

asked Feb 22, 2012 at 1:06

13 votes

1 answer

1k views

Do canonical stacks exist over Spec(Z)?

asked Jan 25, 2015 at 20:33

12 votes

2 answers

894 views

Are representations of a linearly reductive group discretely parameterized?

asked Feb 20, 2011 at 22:17

12 votes

2 answers

1k views

Is an affine "G-variety" with reductive stabilizers a toric variety?

asked Apr 4, 2011 at 15:38

12 votes

2 answers

2k views

Is the fixed locus of a group action always a scheme?

asked Oct 29, 2009 at 2:23

11 votes

2 answers

9k views

Is a subgroup of a free abelian group free abelian?

asked Oct 30, 2009 at 5:21

11 votes

5 answers

3k views

Is the long line paracompact?

asked Oct 9, 2009 at 17:12

11 votes

1 answer

1k views

Can ⨁_I A be isomorphic to ∏_I A for infinite I?

asked Dec 15, 2010 at 10:05

10 votes

1 answer

2k views

Can the valuative criteria for separatedness/properness be checked "formally"?

asked Oct 3, 2009 at 5:15

9 votes

3 answers

1k views

If $\Omega_{X/Y}$ is locally free of rank $\mathrm{dim}\left(X\right)-\mathrm{dim}\left(Y\right)$, is $X\rightarrow Y$ smooth?

asked Sep 30, 2009 at 20:14

9 votes

1 answer

1k views

Example where you need non-DVRs in the valuative criteria

asked Oct 18, 2009 at 3:56

9 votes

3 answers

2k views

Reference request: is the punctual Hilbert scheme irreducible?

asked Apr 4, 2010 at 7:08

9 votes

2 answers

469 views

Can the difference of non-conjugate pseudoreflections lie in the commutator subgroup?

asked May 5, 2014 at 22:40

8 votes

3 answers

864 views

Is $\mathbb{A}²$ the universal smooth scheme which is a finite cover of $\mathbb{A}²/μ₂$?

asked Aug 16, 2010 at 1:02

8 votes

2 answers

848 views

If a representation has enough reductive stabilizers, is it a direct sum of characters?

asked Mar 25, 2011 at 4:37

8 votes

2 answers

1k views

Is every subgroup of an algebraic group a stabilizer for some action?

asked Dec 13, 2009 at 23:16

6 votes

2 answers

1k views

Does projectiveness descend along field extensions?

asked Oct 20, 2009 at 21:58

6 votes

0 answers

367 views

Do simplicial toric varieties have "lots" of base point free linear systems?

asked Aug 23, 2011 at 18:05

5 votes

1 answer

464 views

If Spec(A) has a G-fixed point and a dense G-orbit, is Spec(A) a cone?

asked Mar 31, 2011 at 7:03

Stack Exchange Network

68 Questions

Example of a smooth morphism where you can't lift a map from a nilpotent thickening?

What does the nilpotent cone represent?

Can a coequalizer of schemes fail to be surjective?

Alternatives to pi day [closed]

Are "fpqc algebraic spaces" algebraic spaces?

If a map restricts to an isomorphism on a closed subscheme and its open complement, must it be an isomorphism?

Can a single DVR witness all specializations on a variety?

Does a degeneration always have a larger-dimensional automorphism group?

Does every morphism BG-->BH come from a homomorphism G-->H?

Do orbits and stable loci of group actions have natural scheme structures?

Are root stacks characterized by their divisor multiplicities?

Does smoothness descend along flat morphisms?

Do canonical stacks exist over Spec(Z)?

Are representations of a linearly reductive group discretely parameterized?

Is an affine "G-variety" with reductive stabilizers a toric variety?

Is the fixed locus of a group action always a scheme?

Is a subgroup of a free abelian group free abelian?

Is the long line paracompact?

Can ⨁_I A be isomorphic to ∏_I A for infinite I?

Can the valuative criteria for separatedness/properness be checked "formally"?

If $\Omega_{X/Y}$ is locally free of rank $\mathrm{dim}\left(X\right)-\mathrm{dim}\left(Y\right)$, is $X\rightarrow Y$ smooth?

Example where you need non-DVRs in the valuative criteria

Reference request: is the punctual Hilbert scheme irreducible?

Can the difference of non-conjugate pseudoreflections lie in the commutator subgroup?

Is $\mathbb{A}²$ the universal smooth scheme which is a finite cover of $\mathbb{A}²/μ₂$?

If a representation has enough reductive stabilizers, is it a direct sum of characters?

Is every subgroup of an algebraic group a stabilizer for some action?

Does projectiveness descend along field extensions?

Do simplicial toric varieties have "lots" of base point free linear systems?

If Spec(A) has a G-fixed point and a dense G-orbit, is Spec(A) a cone?