Fano varieties and polytopes

Transcription

1 Fano varieties and polytopes Olivier DEBARRE The Fano Conference Torino, September 29 October 5, 2002

2 Smooth Fano varieties A smooth Fano variety (over a fixed algebraically closed field k) is a smooth projective variety whose anticanonical divisor is ample Examples (1) A Fano curve is P 1 (2) Smooth Fano (del Pezzo) surfaces are P 1 P 1 and the blow-up of P 2 in at most 8 points, with no three colinear (3) Smooth Fano threefolds have been classified in characteristic zero (4) A finite product of Fano varieties is a Fano variety (5) A smooth complete intersection in P n+s defined by equations of degrees d 1 d s > 1 is a Fano variety if and only if d d s n + s In any given dimension n, there are only finitely many choices for the degrees (because s (d i 1) n)

3 (6) A cyclic covering X P n of degree d > 1 branched along a smooth hypersurface of degree de is a Fano variety if and only if (d 1)e n (again, there are only finitely many choices for d and e when n is fixed) (7) In characteristic zero, a projective variety acted on transitively by a connected linear algebraic group (such as a flag variety) is a smooth Fano variety (8) If X is a smooth (Fano) variety and D 1,, D r are nef divisors on X such that K X D 1 D r is ample, the (Grothendieck) projective bundle P( r i=1 O X (D i )) is a Fano variety

4 Toric varieties The algebraic torus of dimension n over k is T n = Spec(k[U 1, U1 1,, U n, Un 1 ]) A toric variety is an algebraic variety defined over k with an action of T n and a dense open orbit isomorphic to T n They can all be constructed from very explicit combinatorial data Affine toric varieties 1) Given a pair of dual lattices M Z n M R = M Z R N = Hom Z (M, Z) N R = N Z R 2) a cone σ in N R generated by a finite number of vectors in N and containing no lines, such as 0 τ σ

5 3) and the dual cone σ = {u M R u, v 0 for all v σ} 0 σ τ the semigroup σ M is finitely generated (by the dots) 4) Define an affine variety by X σ = Spec(k[σ M]) In our example, X σ = Spec(k[U, UV, UV 2 ]) Spec(k[X, Y, Z]/(XZ Y 2 ))

6 5) If τ is a face of σ, σ M τ M The corresponding map X τ X σ makes X τ into a principal open subset of X σ (If u M is such that τ = σ u, X τ = (X σ ) u ) In our example, X τ = Spec(k[U, U 1, V ]) = (X σ ) U X {0} = Spec(k[U, U 1, V, V 1 ]) = (X σ ) UV 6) The torus T n acts on X σ and X τ and the inclusion X τ X σ is equivariant General toric varieties A fan is a finite set of cones in N R as above such that: every face of a cone in is in ; the intersection of any two cones in is in Construct an irreducible n-dimensional k-scheme X with an action of T n by gluing the affine

7 varieties X σ and X σ along the open set X σ σ, for all σ and σ in The scheme X is irreducible, separated and normal Each X σ, hence also X, contains the open set X {0} = T n All toric varieties are obtained via this construction Example The toric surface associated with the fan that consists of σ 1, σ 2, σ 3 and their faces: σ 3 0 σ 2 σ 1 (a, 1) is the projective cone in P a+1 over the rational normal curve in P a For a = 1, this is P 2

8 Projectivity criterion is the set of cones spanned by the faces of a polytope Q with vertices in N and the origin in its interior X (or X Q ) projective Given a polytope Q, we denote by V (Q) the set of its vertices

9 Smooth toric Fano varieties Let Q be a polytope in N R Each facet of Q is the convex hull of a basis of N X Q smooth Fano In this case, the anticanonical divisor K XQ is very ample We call such a polytope a smooth Fano polytope Its only integral interior point is the origin Its dual P = {u M R u, v 1 for all v Q} is a polytope with vertices in M It is usually not a Fano polytope, although its only integral interior point is the origin We have Pic(X Q ) Z #V (Q) n h 0 (X, K X ) = #(M P ) ( K X ) n = n! vol(p )

10 Examples (1) There is only 1 smooth Fano polytope in R, the interval S 1 = [ 1, 1] (2) Up to the action of GL(2, Z), there are only 5 smooth Fano polytopes in R 2 : 0 0 S 2 P 1 P 1 P A 2 0 B 2 P 2 blown up P 2 blown up P 2 blown up at a point at 2 points at 3 points The dual polytopes are and the formulas above are easily checked in these cases

11 (3) The product of smooth Fano polytopes Q and Q is a smooth Fano polytope and X Q Q = X Q X Q (4) Let (e 1,, e n ) be a basis for N and set e 0 = e 1 e n The simplex S n = Conv({e 0, e 1,, e n }) is a smooth Fano polytope When n is even, A n = Conv({e 0, ±e 1,, ±e n }) B n = Conv({±e 0, ±e 1,, ±e n }) are smooth Fano polytopes When n = 2m + 1, the n + 1 vertices e 0, e 1, e 2,, e 2m+1 are on the same facet x 1 + x 2 x 3 + x 2m+1 = 1 One has X Sn P n, whereas X A2m and X B2m are obtained from P 2m by a series of blow ups (bu) and blow downs (bd), such as

12 P 1 P 2m 2 P 2 P 2m 3 P m 1 P m bu fixed points Y 1 bu inv P 1 bd exc div X 1 P 2m Y 2 P 2 bu inv bd exc div Y m 1 bu inv P m 1 bd exc div X 2 X 3 X m 1 X A2m = X m The following result, which improves on an earlier result of Ewald, is one of the few classification results that are valid in all dimensions Theorem (Casagrande) Any smooth Fano polytope in N R with a set of vertices symmetric about the origin that spans N R is isomorphic to a product of S 1, A 2r and B 2s

13 Kähler Einstein metrics Compact complex manifolds with a positive Kähler Einstein metric (ie, whose Ricci form is a positive (constant) multiple of the Kähler form) are Fano varieties, but it is a difficult problem to find conditions on a complex Fano manifold so that a Kähler Einstein metric exists However, there is a criterion for toric Fano manifolds (Tian, Batyrev & Selivanova): if Q N R is a smooth Fano polytope and the only point of N fixed by the finite group G Q = {g GL(N) g(q) = Q} is the origin, the Fano variety X Q has a Kähler Einstein metric This condition implies that the barycenter of Q (resp of Q ) is the origin We have the following implications: The only point of N fixed by G Q is the origin X Q has a KE metric Aut(X Q ) reductive and the Futaki character Lie(Aut(X Q )) C vanishes The set of points of M in the relative interiors of facets of Q is symmetric about the origin and the barycenter of Q is the origin

14 Examples (1) In dimension two, the answer to the question does X Q have a Kähler Einstein metric? is: Q S 2 A 2 B 2 Q YES YES NO NO YES The polytopes S n and B n all satisfy the Batyrev Selivanova criterion Using the criterion above, one checks that the automorphism group of X A2m is not reductive (2) Fix positive integers a 1,, a m, r 1,, r m with a i r i The projective bundle X = P(O(a 1, 0,, 0) O(0,, a m )) over P r 1 P r m is a smooth Fano variety (this is a particular case of Example (8)) whose automorphism group is reductive (by direct computation) It is the toric variety associated with the smooth Fano polytope Q with

15 vertices: g 1,, g m with g g m = 0 e i1,, e iri, a i g i (e i1 + + e iri ) for i = 1,, m If all the r i are equal and all the a i are equal, it satisfies the Batyrev Selivanova criterion, hence X has a Kähler-Einstein metric The barycenter of Q is the origin if and only if the integrals g 1,,g m 1 g 1,,g m =0 m g i j=1 (r j + 1 a j g j ) r j dµ all vanish for 1 i m I have checked that for r 1,, r m 10, they do only when all the r i are equal and all the a i are equal

16 Number of vertices Let Q N R be a smooth Fano polytope with dual P There is a oneto-one correspondence { vertices of P (resp Q) } { facets of Q (resp P ) u F u } For any u V (P ) and v V (Q), we have u, v Z, u, v 1 with equality if and only if v F u Theorem (Voskresenskĭı, Klyachko) A smooth Fano polytope of dimension n has most 2n 2 vertices Proof Let Q be a smooth Fano polytope, with dual P The origin is in the interior of P, hence in the interior of the convex hull of at most 2n vertices by Steinitz s theorem: we have 0 = λ 1 u λ d u d λ 1,, λ d > 0 where the vertices u 1,, u d of P generate M R and n < d 2n

17 Let v be a vertex of Q We have u i, v = 0 for at least one i, hence u i, v > 0 for at least one i This implies v F ui, hence the vertices of Q are all on F u1 F ud Since each facet has n vertices, Q has at most nd 2n 2 vertices Batyrev conjectures that a smooth Fano polytope of dimension n has at most 3n 3n 1 if n is even if n is odd vertices This holds for n 5 (Batyrev, Casagrande) There is equality for n even for A n/2 2 and for n odd for A (n 1)/2 2 S 1 The (asymptotically) best known bound is #V (Q) n n 3 (n 2 1)(2n 1) Corollary Up to the action of GL(n, Z), there are only finitely many smooth Fano polytopes in R n

18 Proof The number of vertices of such a polytope Q is at most 2n 2 Since each facet of Q has n vertices, their number is at most ( 2n 2 ), n ( 2n 2) n hence the volume of Q is at most n! 1 Fix the vertices e 1,, e n of a facet of Q and set S = Conv({0, e 1,, e n }) For any vertex v = v 1 e v n e n of Q, the volume of Conv(v, S) is at most n! 1 ( 2n 2) n, hence v i ( 2n 2 ) n The integral vector v may therefore take only finitely many values

19 Volume of the dual polytope Let Q be a smooth Fano polytope The integer ( K XQ ) n = n! vol(q ) is often called the degree of the smooth Fano variety X Q Theorem Let Q be a smooth Fano polytope of dimension n with ρ + n vertices, ρ > 1 We have ( (n 1) ρ+1 ) n 1 n ρn vol(q ) n 2 Equivalently, a smooth toric Fano variety X of dimension n and Picard number ρ 2 satisfies ( K X ) n n!n ρn (1) Remarks (1) When ρ = 1, we have X P n, hence ( K X ) n = (n + 1) n When ρ = 2, we have by classification ( K X ) n n 2n

20 For any integers ρ 2 and n 4 such that n log n 2ρ 2, there are examples of smooth toric Fano varieties X of dimension n and Picard number ρ with ( K X ) n ( n ρ 2 ρ2 1 (log n) ρ 1 so the bound (1) is not too far off (2) The pseudo-index ι X = ) n min ( K X C) C X rational curve of X can be computed in a purely combinatorial way from Q One can get the more precise bound (for ρ > 1) vol(q ) ( n(n + 1 ι X ) ρ 1) n (3) If X Q is a compact complex manifold of dimension n with a positive Kähler-Einstein metric, methods from differential geometry give vol(q ) (2n 1) n 2n+1 n! (2n)! 2(2e) n 1/2 (4) Taking a 1 = r 1 and a 2 = = a m = r 2 = = r m = 1 in Example (2) above, one gets

21 a smooth toric Fano variety X of dimension n = r 1 + 2m 2 and Picard number m + 1 with reductive automorphism group and Taking m = [ ( K X ) n m n r r 1 n 2 log n 1 ], we get ( KX ) n ( an 2 log n for some positive constant a In particular, X does not have a Kähler-Einstein metric (5) Ewald conjectures that given a smooth Fano polytope Q, there is a basis of N in which the vertices of Q have all their coordinates in {0, ±1} It holds for all the examples presented here Let (e 1,, e n ) be such a basis, let u be a vertex of Q, and let v 1,, v n be the vertices of the face F u of Q Write ) n v i = j α ij e j, α ij {0, ±1}, det(α ij ) = ±1 The coordinates u, e 1,, u, e n of u, solutions of the linear system 1 = u, v i = n j=1 α ij u, e j i = 1,, n

22 are n n determinants whose entries are in {0, ±1} By Hadamard s inequality, their absolute value is bounded by n n/2, hence vol(q ) 2 n n n2 /2

23 The theorem on the volume of the dual of a smooth Fano polytope is a consequence of the following two lemmas Lemma Let P be a polytope in R n whose only integral interior point is the origin Let b be such that for each vertex u of P and each vertex v of P, we have b u, v 1 Then vol(p ) (b + 1) n Proof The hypothesis implies 1 b P P Assume vol(p ) > (b + 1) n and set P = 1 ε b + 1 P We have vol(p ) > 1 for ε positive small enough hence, by a theorem of Blichfeldt, P contains distinct points p and p such that p p Z n Since P is convex and 1 b P P, the point p p b + 1 = p + b ( 1 b p ) b + 1 is in P, hence p p is in (b + 1)P, which is contained in the interior of P This contradicts the fact that the origin is the only integral interior point of P

24 Lemma Let Q be a smooth Fano polytope of dimension n with ρ + n vertices, ρ > 1 For each vertex u of Q and each vertex v of Q, we have n 1 n 2 ((n 1)ρ 1) u, v 1

25 Singular Fano varieties In many cases, such as in the Minimal Model Program, it is necessary to allow some kind of singularities for Fano varieties We assume the characteristic of k is 0 Let X be a normal variety such that K X is a Q-Cartier divisor For any desingularization f : Y X, write K Y f K X + i a i E i where the E i are f-exceptional divisors, a i Q, and denotes numerical equivalence The discrepancy of X is the minimum of the a i for all possible desingularizations f It can be computed from any desingularization f whose exceptional locus is a divisor with normal crossings by the formula discr(x) = min(1, a i ) if all a i are 1 otherwise It is therefore either or a rational number in [ 1, 1], and is 1 if X is smooth We

26 say that X has log terminal singularities if discr(x) > 1 Definition A Fano variety X is a normal projective variety with log terminal singularities such that K X is an ample Q-Cartier divisor Example Let Y be a projectively normal smooth subvariety of P n, with hyperplane divisor H, such that K Y qh for some rational number q (eg, Y a smooth curve) Set π : X = P(O Y O Y (H)) Y and let Y 0 be the section such that Y 0 Y0 H The cone X in P n+1 over Y is normal; it is the contraction f : X X of Y 0, and hence K X 2Y 0 + π (K Y H) 2Y 0 + π ((q 1)H) K X (q 1)H (These two Weil divisors coincide outside of the vertex of X) and K X ample q < 1

27 Writing K X f K X + ay 0, restricting to Y 0 yields a = 1 q, hence X has log terminal singularities q < 0 It follows that X is a Fano variety if and only if Y is a Fano variety

28 Toric Fano varieties We have Vertices of Q are primitive in N X Q Fano Moreover, if P = Q and if, for each vertex u of P, we let σ u be the cone in N R generated by the facet F u of Q, we have discr(x Q ) = 1 + min u V (P ) v σ u N V (Q) {0} u, v 0 F u u, =1 σ u The polytope P has rational vertices and Vertices of mp are in M mk X Cartier

29 In particular, X is a Gorenstein variety if and only if P has integral vertices (a polytope Q with this property is called reflexive; its dual has the same property)

30 Example The projective cone in P a+1 over the rational normal curve in P a is the toric surface associated with the polytope with dual 0 F v u σ u ( 1,a+1) 0 u=( 2 a, 1) (a, 1) It is a Fano surface (this is a particular case of the example above, with q = 2/a) We recover discr(x) = 1 + u, v = a

31 Also, the smallest positive integer m such that mk X is a Cartier divisor is a if a is odd, a/2 otherwise This shows that there are, in each dimension at least 2, infinitely many isomorphism classes of toric Fano varieties However, by bounding the discrepancy away from 1, we get finitely many isomorphism types: since the vertices of a Fano polytope are primitive, we have for any r > 0 discr(x Q ) r u, v 1 r for all v σ u N This implies Int(Q) rn = {0} Corollary Given positive integers n and r, there are only finitely many isomorphism types of toric Fano varieties of dimension n and discrepancy r defined over k

32 This is due to Borisov & Borisov (1993), who appear to have been unaware of an earlier article of Hensley (1983), which gives the finiteness of the number of isomorphism classes of polytopes Q R n such that Int(Q) rz n = {0} under the action of GL(Z n ) It also provides explicit (large) bounds in terms of n and r for the smallest integer m such that mk X is Cartier and for the degree ( K X ) n It is conjectured that for each positive integer n and r, there are only finitely many deformation types of Fano varieties of dimension n with discrepancy 1 + 1/r The point is to bound ( K X ) n and an integer m such that mk X is a Cartier divisor M c Kernan claims in the preprint Boundedness of log terminal Fano pairs of bounded index (mathag/ ) that given integers n and m, there are only finitely many deformation types of complex Fano varieties X of dimension n such that mk X is a Cartier divisor