User David Zureick-Brown

43 votes

7 answers

21k views

How do you show that $S^{\infty}$ is contractible?

asked Oct 8, 2009 at 14:34

34 votes

3 answers

4k views

What is interesting/useful about big Witt Vectors?

asked Oct 14, 2009 at 21:46

26 votes

6 answers

4k views

Where can I find a comprehensive list of equations for small genus modular curves?

asked Feb 3, 2010 at 21:12

21 votes

6 answers

5k views

Is a quotient of a reductive group reductive?

asked Sep 28, 2009 at 17:42

20 votes

5 answers

2k views

Equivalence of ordered and unordered cech cohomology.

asked Dec 29, 2009 at 19:35

18 votes

2 answers

2k views

Are Jacobians principally polarized over non-algebraically closed fields?

asked Dec 1, 2009 at 21:00

18 votes

3 answers

2k views

Is there an example of a variety over the complex numbers with no embedding into a smooth variety?

asked Oct 8, 2009 at 15:42

17 votes

2 answers

4k views

What is interesting/useful about Castelnuovo-Mumford regularity?

asked Oct 3, 2009 at 9:52

17 votes

2 answers

3k views

Are curves with `fractional points' uniquely determined by their residual gerbes?

asked Oct 20, 2009 at 18:23

17 votes

2 answers

1k views

Can Hom_gp(G,H) fail to be representable for affine algebraic groups?

asked Nov 4, 2009 at 7:24

17 votes

2 answers

968 views

What is an explicit example of a variety X which is finite over Spec F_p but which does not lift to a scheme Y which is finite and flat over Spec Z_p?

asked May 5, 2011 at 2:59

16 votes

2 answers

3k views

Is there an example of a formally smooth morphism which is not smooth?

asked Oct 8, 2009 at 12:06

16 votes

2 answers

3k views

Is the Torelli map an immersion?

asked Dec 15, 2009 at 2:08

16 votes

3 answers

2k views

Cech to derived spectral sequence and sheafification

asked Dec 15, 2009 at 17:45

15 votes

1 answer

656 views

Can an algebraic space fail to have a universal map to a scheme?

asked Nov 8, 2009 at 3:58

15 votes

1 answer

2k views

Reference for the `standard' Tate curve argument.

asked Oct 16, 2009 at 2:59

15 votes

7 answers

2k views

Examples of rational families of abelian varieties.

asked Oct 16, 2009 at 20:14

15 votes

1 answer

1k views

Is there an example of a scheme X whose reduction X_red is affine but X is not affine?

asked Oct 4, 2009 at 21:33

15 votes

2 answers

2k views

What is an example of a smooth variety over a finite field F_p which does not embed into a smooth scheme over Z_p?

asked Oct 13, 2009 at 5:59

15 votes

2 answers

305 views

S integral points of an elliptic curve, with S of positive density

asked May 14, 2014 at 5:21

13 votes

2 answers

568 views

Existence of points on varieties which avoid a given number field.

asked Jun 11, 2012 at 19:51

13 votes

2 answers

2k views

Can a singular Deligne-Mumford stack have a smooth coarse space?

asked Oct 21, 2009 at 3:05

13 votes

3 answers

788 views

Constructing a degeneration (as a group scheme) of G_m to G_a

asked Nov 3, 2009 at 7:07

12 votes

3 answers

1k views

Motivation for uniform surjectivity of mod l representations associated to elliptic curves

asked Nov 23, 2009 at 5:48

12 votes

1 answer

1k views

When is an Albanese variety principally polarized?

asked Dec 1, 2009 at 0:48

12 votes

1 answer

704 views

Is there a non-quotient stack with affine stabilizers whose good moduli space is a geometric point?

asked Feb 28, 2011 at 0:05

12 votes

1 answer

330 views

Are coarse spaces of 1-dimensional smooth proper Artin stacks smooth?

asked Oct 15, 2015 at 15:51

11 votes

3 answers

1k views

Does Ribet's level lowering theorem hold for prime powers?

asked Oct 18, 2009 at 1:05

9 votes

1 answer

1k views

Kodaira-Spencer Theory and moduli of curves

asked Oct 22, 2009 at 0:20

9 votes

2 answers

2k views

What is an example of a smooth variety over a finite field F_p which does not lift to Z_p?

asked Oct 13, 2009 at 14:29

Stack Exchange Network

45 Questions

How do you show that $S^{\infty}$ is contractible?

What is interesting/useful about big Witt Vectors?

Where can I find a comprehensive list of equations for small genus modular curves?

Is a quotient of a reductive group reductive?

Equivalence of ordered and unordered cech cohomology.

Are Jacobians principally polarized over non-algebraically closed fields?

Is there an example of a variety over the complex numbers with no embedding into a smooth variety?

What is interesting/useful about Castelnuovo-Mumford regularity?

Are curves with `fractional points' uniquely determined by their residual gerbes?

Can Hom_gp(G,H) fail to be representable for affine algebraic groups?

What is an explicit example of a variety X which is finite over Spec F_p but which does not lift to a scheme Y which is finite and flat over Spec Z_p?

Is there an example of a formally smooth morphism which is not smooth?

Is the Torelli map an immersion?

Cech to derived spectral sequence and sheafification

Can an algebraic space fail to have a universal map to a scheme?

Reference for the `standard' Tate curve argument.

Examples of rational families of abelian varieties.

Is there an example of a scheme X whose reduction X_red is affine but X is not affine?

What is an example of a smooth variety over a finite field F_p which does not embed into a smooth scheme over Z_p?

S integral points of an elliptic curve, with S of positive density

Existence of points on varieties which avoid a given number field.

Can a singular Deligne-Mumford stack have a smooth coarse space?

Constructing a degeneration (as a group scheme) of G_m to G_a

Motivation for uniform surjectivity of mod l representations associated to elliptic curves

When is an Albanese variety principally polarized?

Is there a non-quotient stack with affine stabilizers whose good moduli space is a geometric point?

Are coarse spaces of 1-dimensional smooth proper Artin stacks smooth?

Does Ribet's level lowering theorem hold for prime powers?

Kodaira-Spencer Theory and moduli of curves

What is an example of a smooth variety over a finite field F_p which does not lift to Z_p?