Geometric interpretations for the algebra of conformal blocks

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1 Geometric interpretations for the algebra of conformal blocks Angela Gibney The University of Georgia 2015

2 This talk is about recent work with Prakash Belkale and Anna Kazanova.

3 Conformal blocks

4 Conformal blocks informally, these are vector spaces V (C, p) that one can associate to any stable n-pointed curve (C, p) of genus g.

5 Conformal blocks informally, these are vector spaces V (C, p) that one can associate to any stable n-pointed curve (C, p) of genus g. They were originally constructed using integrable representations of simple affine Lie algebras by Tsuchiya, Ueno, Yamada [1989].

6 Vector spaces V (C, p) = V (g, λ, l) (C, p)

7 Vector spaces V (C, p) = V (g, λ, l) (C, p) are determined by: (1) a simple Lie algebra g (2) a positive integer l; (3) an n-tuple λ = (λ 1,..., λ n ) of dominant integral weights for g at level l.

8 If C is a smooth curve:

9 If C is a smooth curve: There is a canonical isomorphism: V (C, p) = H 0 (X (C, p), L (C, p) ),

10 If C is a smooth curve: There is a canonical isomorphism: V (C, p) = H 0 (X (C, p), L (C, p) ), where X (C, p) is a projective variety and L (C, p) is a natural ample line bundle on it.

11 If C is a smooth curve: There is a canonical isomorphism: V (C, p) = H 0 (X (C, p), L (C, p) ), where X (C, p) is a projective variety and L (C, p) is a natural ample line bundle on it. Beauville Laszlo 1994; Faltings 1994; Kumar, Narasimhan, Ramanathan 1994;

12 If C is a smooth curve: There is a canonical isomorphism: V (C, p) = H 0 (X (C, p), L (C, p) ), where X (C, p) is a projective variety and L (C, p) is a natural ample line bundle on it. Beauville Laszlo 1994; Faltings 1994; Kumar, Narasimhan, Ramanathan 1994; Laszlo, Sorger 1997; Pauly 1996.

13 Crucially:

14 Crucially: m Z >0 V [m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) ), V [m] = V (g, m λ, ml).

15 Crucially: m Z >0 V [m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) ), V [m] = V (g, m λ, ml). for example, if V[m] = V(sl r+1, {m λ 1,..., m λ n }, m l), then m λ i = r m c i ω i. i=1

16 This is interesting because:

17 This is interesting because: On the one hand, it tells you the algebra of conformal blocks (on the left), is finitely generated;

18 This is interesting because: On the one hand, it tells you the algebra of conformal blocks (on the left), is finitely generated; On the other hand, one can compute the dimensions of the vector spaces V [m] using the Verlinde formula, and study X (C, p).

19 A natural question:

20 A natural question: Do such geometric interpretations hold for V [m] (C, p) m Z >0 at SINGULAR curves C?

21 Another way to say this:

22 Another way to say this: Given a point (C, p) M g,n, where C is SINGULAR, and a vector space of conformal blocks V (C, p), can one always find a projective polarized pair (X (C, p), L (C, p) ) such that

23 Another way to say this: Given a point (C, p) M g,n, where C is SINGULAR, and a vector space of conformal blocks V (C, p), can one always find a projective polarized pair (X (C, p), L (C, p) ) such that m Z >0 V [m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) )?

24 Another way to say this: Given a point (C, p) M g,n, where C is SINGULAR, and a vector space of conformal blocks V (C, p), can one always find a projective polarized pair (X (C, p), L (C, p) ) such that m Z >0 V [m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) )? The answer is NO.

25 Another way to say this: Given a point (C, p) M g,n, where C is SINGULAR, and a vector space of conformal blocks V (C, p), can one always find a projective polarized pair (X (C, p), L (C, p) ) such that m Z >0 V [m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) )? The answer is NO. Counter examples are given by vector bundles of conformal blocks.

26 Vector bundles of conformal blocks V = V(g, λ, l) on M g,n

27 Vector bundles of conformal blocks V = V(g, λ, l) on M g,n given by: (1) a simple Lie algebra g (2) a positive integer l; (3) an n-tuple λ = (λ1,..., λ n ) of dominant integral weights for g at level l.

28 Vector bundles of conformal blocks V = V(g, λ, l) on M g,n given by: (1) a simple Lie algebra g (2) a positive integer l; (3) an n-tuple λ = (λ1,..., λ n ) of dominant integral weights for g at level l. At (C, p) M g,n, V (C, p) = V (C, p).

29 Vector bundles of conformal blocks V = V(g, λ, l) on M g,n given by: (1) a simple Lie algebra g (2) a positive integer l; (3) an n-tuple λ = (λ1,..., λ n ) of dominant integral weights for g at level l. For g = 0 the bundles are globally generated and so c 1 (V) are base point free. At (C, p) M g,n, V (C, p) = V (C, p).

30 Vector bundles of conformal blocks V = V(g, λ, l) on M g,n given by: (1) a simple Lie algebra g (2) a positive integer l; (3) an n-tuple λ = (λ1,..., λ n ) of dominant integral weights for g at level l. At (C, p) M g,n, V (C, p) = V (C, p). For g = 0 the bundles are globally generated and so c 1 (V) are base point free. Fakhruddin (improved by Mukhopadhyay) (g=0), and Marian, Oprea, Pandharipande (g >0) have given formulas for c 1 (V).

31 Theorem (Belkale, G, Kazanova) There are vector bundles of conformal blocks V on M g,n, and points (C, p) M g,n, for which there are no polarized varieties (X (C, p), L (C, p) ) such that holds. V [m] (C, p) = m m H 0 (X (C, p), L m (C, p) )

32 For example:

33 For example: There is no geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2,, m) C, on M 2 where C is the curve with a separating node.

34 Similarly:

35 Similarly: There is no geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2, {m ω 1,..., m ω 1 }, m) (C, p), on M 2,n, n even where C is the curve with a separating node.

36 To prove this theorem:

37 To prove this theorem: We use intersection theory on M g,n to give obstructions to geometric interpretations.

38 To prove this theorem: We use intersection theory on M g,n to give obstructions to geometric interpretations. We did this in cases where if geometric interpretations were to hold, then we would have useful information about the polarized varieties (X (C, p), L (C, p) ), at boundary points (C, p) M g,n.

39 To prove this theorem: We use intersection theory on M g,n to give obstructions to geometric interpretations. We did this in cases where if geometric interpretations were to hold, then we would have useful information about the polarized varieties (X (C, p), L (C, p) ), at boundary points (C, p) M g,n. For example, we found obstructions in case (X (C, p), L (C, p) ) = 0.

40 Theorem (Belkale, G, Kazanova) Suppose that V has -invariant zero rank scaling, and that for all (C, p), there are polarized pairs (X (C, p), L (C, p) ) such that Then for m 1, m Z >0 V[m] (C, p) = m Z >0 H 0 (X (C, p), L m (C, p) ). c 1 (V[m]) = D α i (m)c 1 (V[i]), (1) i=1 the α i (m) are (explicitly given) polynomials in m, and D is the volume of V.

41 For example:

42 For example: If there were a geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2,, m) C, at all points C M 2

43 For example: If there were a geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2,, m) C, at all points C M 2 ( ) m + 3 rk V(sl 2,, m) = = V has (P 3, O(1)) -scaling, 3

44 For example: If there were a geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2,, m) C, at all points C M 2 ( ) m + 3 rk V(sl 2,, m) = 3 and = V has (P 3, O(1)) -scaling, ( ) m + 3 c 1 (V[m]) = c 1 (V). (2) 4

45 For example: If there were a geometric interpretation for the algebra of conformal blocks m Z >0 V(sl 2,, m) C, at all points C M 2 ( ) m + 3 rk V(sl 2,, m) = 3 and = V has (P 3, O(1)) -scaling, ( ) m + 3 c 1 (V[m]) = c 1 (V). (2) 4 we show Eq 2 fails by intersecting with F-curves.

46 The -invariant of a projective variety

47 The -invariant of a projective variety Definition Let X be an irreducible projective variety with ample line bundle L. The -invariant of the pair (X, L) is (X, L) = dim(x) + L dim(x) h 0 (X, L).

48 If (X, L) = 0

49 If (X, L) = 0 [Fujita] L is very ample, and embeds X into a projective space whose image has deg(x) = h 0 (X, L) dim(x) = 1 + codim(x).

50 If (X, L) = 0 [Fujita] L is very ample, and embeds X into a projective space whose image has deg(x) = h 0 (X, L) dim(x) = 1 + codim(x). For any nondegenerate variety X P N deg(x) 1 + codim(x).

51 If (X, L) = 0 [Fujita] L is very ample, and embeds X into a projective space whose image has deg(x) = h 0 (X, L) dim(x) = 1 + codim(x). For any nondegenerate variety X P N deg(x) 1 + codim(x). (X, L) = 0 = X a proj. variety of minimal degree.

52 This is great for three reasons:

53 This is great for three reasons: (1) Suppose that every point (C, p) M g,n, there exists (X (C, p), L (C, p) ), such that m Z >0 V (C, p) = m Z >0 H 0 (X (C, p), L (C, p) ),

54 This is great for three reasons: (1) Suppose that every point (C, p) M g,n, there exists (X (C, p), L (C, p) ), such that m Z >0 V (C, p) = m Z >0 H 0 (X (C, p), L (C, p) ), (X (C, p), L (C, p) ) = 0 some (C, p) M g,n,

55 This is great for three reasons: (1) Suppose that every point (C, p) M g,n, there exists (X (C, p), L (C, p) ), such that m Z >0 V (C, p) = m Z >0 H 0 (X (C, p), L (C, p) ), (X (C, p), L (C, p) ) = 0 some (C, p) M g,n, = (X (C, p), L (C, p) ) = 0 (C, p) M g,n.

57 (2) Projective varieties of minimal degree are known:

58 (2) Projective varieties of minimal degree are known: 0. rational normal curves; 1. projective spaces; 2. quadric hypersurfaces; 3. rational normal scrolls; 4. Veronese surfaces; and 5. Cones over the above.

59 (3) The function f (m) = rk V[m] determines the Delta invariant (X, L) = dim(x) + L dim(x) h 0 (X, L).

60 (3) The function f (m) = rk V[m] determines the Delta invariant (X, L) = dim(x) + L dim(x) h 0 (X, L). 1. f (1) = h 0 (X, L),

61 (3) The function f (m) = rk V[m] determines the Delta invariant (X, L) = dim(x) + L dim(x) h 0 (X, L). 1. f (1) = h 0 (X, L), 2. deg(f (m)) = dim(x);

62 (3) The function f (m) = rk V[m] determines the Delta invariant (X, L) = dim(x) + L dim(x) h 0 (X, L). 1. f (1) = h 0 (X, L), 2. deg(f (m)) = dim(x); and 3. c deg(f (m))! = L dim(x), where c is the leading coefficient of f (m).

63 (3) The function f (m) = rk V[m] determines the Delta invariant (X, L) = dim(x) + L dim(x) h 0 (X, L). 1. f (1) = h 0 (X, L), 2. deg(f (m)) = dim(x); and 3. c deg(f (m))! = L dim(x), where c is the leading coefficient of f (m). Definition Vol V = L dim(x).

64 For example: 0. rk(v[m]) = dm + 1, then V has (P 1, O(d))-scaling; 1. rk(v[m]) = ( ) d+m m, V has (P d, O(1))-scaling; 2. rk(v[m]) = 2 ( ) ( m+d 1 d + m+d 1 ) d 1, V has quadric hypersurface scaling; 3. rk(v[m]) = (m + 1)(1 + m(a+b) 2 ), V has (S(a, b), O(1)) scaling 1 ; 4. rk(v[m]) = (m + 1)(2m + 1), V has Veronese surface scaling; 1 there are more general scrolls S(a 1,..., a d ) = (P(E), O(1))

65 Definition We say that V has -invariant zero rank scaling if (X (C, p), L (C, p) ) = 0 for (C, p) M g,n such that V (C, p) = H 0 (X (C, p), L (C, p) ).

67 We checked many many examples, and found that the predicted scaling identities held except possibly on particular loci.

68 We have similar results for bundles that don t have -invariant zero rank scaling.

69 We have similar results for bundles that don t have -invariant zero rank scaling. For example, pairs (X, L) isomorphic to Coble s Quartic Hypersurface in P 7, and Coble s Cubic hypersurface in P 8 [BGK], and more recently in work with Prakash we look at other examples as well.

70 We also have some results which give conditions that guarantee geometric interpretations do exist.

72 Theorem (BGK) Let V = V(sl r+1, λ, l) on M g,n. If rk(v[m]) = ( ) m+d d m, then ( ) m + d c 1 (V) = c 1 (V[m]) + D m, d + 1 where D m is an effective Cartier divisor supported on the boundary of M g,n. Geometric extensions hold if and only if D m = 0.

74 Our experimentation has led to some conjectures.

75 Original Conjecture:

76 Original Conjecture: Conjecture (Belkale, G, Kazanova) Given a vector bundle of conformal blocks V of type A on M g,n, for every x = (C, p) Z = M rt g,n 0 irr, there is a polarized pair (X x, L x ) such that m Z >0 V x = m Z >0 H 0 (X x, L m x ).

77 Original Conjecture: Conjecture (Belkale, G, Kazanova) Given a vector bundle of conformal blocks V of type A on M g,n, for every x = (C, p) Z = M rt g,n 0 irr, there is a polarized pair (X x, L x ) such that m Z >0 V x = m Z >0 H 0 (X x, L m x ). M rt g,n is the set of points in M g,n with one irreducible component having genus g.

78 Original Conjecture: Conjecture (Belkale, G, Kazanova) Given a vector bundle of conformal blocks V of type A on M g,n, for every x = (C, p) Z = M rt g,n 0 irr, there is a polarized pair (X x, L x ) such that m Z >0 V x = m Z >0 H 0 (X x, L m x ). M rt g,n is the set of points in M g,n with one irreducible component having genus g. For g = 0, Z = M 0,n.

79 Modification

80 Modification Conjecture (Belkale, G) Let V = V(sl r, λ, l), and let x = [C] M g, g 2 be any (closed) point corresponding to a nodal curve C. Then there is a projective variety X x and a big (equivalent to an ample plus effective) line bundle L x on X x such that m 0 V[m] x = m 0 H 0 (X x, L m x ), where m = m if C is an irreducible curve, and m = m 2 otherwise. If r = 2, then L x is ample.

81 Work in progress

82 Work in progress Expected Theorem (Belkale, G)

83 Work in progress Expected Theorem (Belkale, G) Let V = V(g, λ, l) be any vector bundle of conformal blocks on M g,n, and (C, p) M g,n any point. There is an isomorphism of algebras V[m] (C; p) = H 0 (Parbun G (C, p), L G (C, p) m ), m Z 0 m Z 0

84 Work in progress Expected Theorem (Belkale, G) Let V = V(g, λ, l) be any vector bundle of conformal blocks on M g,n, and (C, p) M g,n any point. There is an isomorphism of algebras V[m] (C; p) = H 0 (Parbun G (C, p), L G (C, p) m ), m Z 0 m Z 0 Parbun G (C, p) quasi-parabolic G-bundles on C, L G (C, p) natural line bundle.

85 Thank you!

Nef line bundles on M 0,n from GIT

Nef line bundles on M 0,n from GIT David Swinarski Department of Mathematics University of Georgia November 13, 2009 Many of the results here are joint work with Valery Alexeev. We have a preprint: arxiv:0812.0778