A Numerical Criterion for Lower bounds on K-energy maps of Algebraic manifolds
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1 A Numerical Criterion for Lower bounds on K-energy maps of Algebraic manifolds Sean Timothy Paul University of Wisconsin, Madison
2 Outline Formulation of the problem: To bound the Mabuchi energy from below on the space of Kähler metrics in a given Kähler class [ω]. Tian s program 88-97: In algebraic case should restrict K-energy to Bergman metrics". Representation theory : Toric Morphisms and Equivariant embeddings. Discriminants and resultants of projective varieties: Hyperdiscriminants and Cayley- Chow forms. Output: A complete description of the extremal properties of the Mabuchi energy restricted to the space of Bergman metrics.
3 Formulating the problem Set up and notation: (X n, ω) closed Kähler manifold H ω := {ϕ C (X) ω ϕ > 0} (the space of Kähler metrics in the class [ω] ) ω ϕ := ω + 1 2π ϕ Scal(ω): = scalar curvature of ω µ = 1 V X Scal(ω)ωn (average of the scalar curvature) V =volume
4 Definition. (Mabuchi 1986 ) The K-energy map ν ω : H ω R is given by ν ω (ϕ) := 1 V 1 0 X ϕ t(scal(ω ϕt ) µ)ω n t dt ϕ t is a C 1 path in H ω satisfying ϕ 0 = 0, ϕ 1 = ϕ Observe : ϕ is a critical point for ν ω iff Scal(ω ϕ ) µ (a constant) Basic Theorem (Bando-Mabuchi, Donaldson,..., Chen-Tian) If there is a ψ H ω with constant scalar curvature then there exits C 0 such that ν ω (ϕ) C for all ϕ H ω.
5 Question ( ) : Given [ω] how to detect when ν ω is bounded below on H ω? N.B. : In general we do not know (!) if there is a constant scalar curvature metric in the class [ω]. Special Case: Assume that [ω] is an integral class, i.e. there is an ample divisor L on X such that [ω] = c 1 (L) We may assume that X P N (embedded) and ω = ω F S X
6 Observe that for σ G := SL(N + 1, C) there is a ϕ σ C (P N ) such that σ ω F S = ω F S + This gives a map 1 2π ϕ σ > 0 G σ ϕ σ H ω The space of Bergman Metrics is the image of this map B := {ϕ σ σ G} H ω. Tian s idea: RESTRICT THE K-ENERGY TO B
7 Question ( ) : Given X P N how to detect when ν ω is bounded below on B?
8 Definition. Let (G) be the space of algebraic one parameter subgroups λ of G. These are algebraic homomorphisms λ : C G λ ij C[ t, t 1 ]. Definition. (The space of degenerations in B) (B) := {C ϕ λ B ; λ (G)}.
9 Theorem. ( Paul 2012 ) Assume that for every degeneration λ in B there is a (finite) constant C(λ) such that lim α 0 ν ω(ϕ λ(α) ) C(λ). Then there is a uniform constant C such that for all ϕ σ B we have the lower bound ν ω (ϕ σ ) C.
10 Equivariant Embeddings of Algebraic Homogeneous Spaces G reductive complex linear algebraic group: G = GL(N + 1, C), SL(N + 1, C), (C ) N, SO(N, C), Sp 2n (C). H := Zariski closed subgroup. O := G/H associated homogeneous space.
11 Definition. An embedding of O is an irreducible G variety X together with a G-equivariant embedding i : O X such that i(o) is an open dense orbit of X.
12 Let (X 1, i 1 ) and (X 2, i 2 ) be two embeddings of O. Definition. A morphism ϕ from (X 1, i 1 ) to (X 2, i 2 ) is a G equivariant regular map ϕ : X 1 X 2 such that the diagram commutes. O i 1 i 2 X 1 ϕ X 2 One says that (X 1, i 1 ) dominates (X 2, i 2 ).
13 Assume these embeddings are both projective (hence complete) with very ample linearizations satisfying L 1 Pic(X 1 ) G, L 2 Pic(X 2 ) G ϕ (L 2 ) = L 1. Get injective map of G modules ϕ : H 0 (X 2, L 2 ) H 0 (X 1, L 1 )
14 The adjoint (ϕ ) t : H 0 (X 1, L 1 ) H 0 (X 2, L 2 ) is surjective and gives a rational map : O i 1 X 1 ϕ P(H 0 (X 1, L 1 ) ) (ϕ ) t i 2 X 2 P(H 0 (X 2, L 2 ) )
15 We abstract this situation : 1. V, W finite dimensional rational G-modules 2. v, w nonzero vectors in V, W respectively 3. Linear span of G v coincides with V (same for w) 4. [v] corresponding line through v = point in P(V) 5. O v := G [v] P(V) ( projective orbit ) 6. O v = Zariski closure in P(V).
16 Definition. (V; v) dominates (W; w) if and only if there exists π Hom(V, W) G such that π(v) = w and the rational map π : P(V) P(W) induces a regular finite morphism π : G [v] G [w] O i v O v π P(V) π i w O w P(W)
17 Observe that the map π extends to the boundary if and only if ( ) G [v] P(ker π) =. π(v) = W V = ker(π) W (G-module splitting) Identify π with projection onto W v = (v π, w) v π 0 ( ) is equivalent to ( ) G [(v π, w)] G [(v π, 0)] = (Zariski closure inside P(ker(π) W ) )
18 Given (v, w) V W set O vw := G [(v, w)] P(V W) O v := G [(v, 0)] P(V {0}) This motivates: Definition. (Paul 2010) The pair (v, w) is semistable if and only if O vw O v =
19 Example. Let V e and V d be irreducible SL(2, C) modules with highest weights e, d N = homogeneous polynomials in two variables. Let f and g in V e \ {0} and W d \ {0} respectively. Claim. (f, g) is semistable if and only if e d and for all p P 1 ord p (g) ord p (f) d e 2. When e = 0 and f = 1 conclude that (1, g) is semistable if and only if ord p (g) d 2 for all p P1.
20 Toric Morphisms If the pair (v, w) is semistable then we certainly have that T [(v, w)] T [(v, 0)] = for all maximal algebraic tori T G. Therefore there exists a morphism of projective toric varieties. T T [(v, w)] π P(V W) π T [(0, w)] P(W) We expect that the existence of such a morphism is completely dictated by the weight polyhedra : N (v) and N (w).
21 Theorem. (Paul 2012) The following statements are equivalent. 1. (v, w) is semistable. Recall that this means O vw O v = 2. N (v) N (w) for all maximal tori H G. We say that (v, w) is numerically semistable. 3. For every maximal algebraic torus H G and χ A H (v) there exists an integer d > 0 and a relative invariant f C d [ V W ] H dχ such that f(v, w) 0 and f V 0.
22 Corollary A. If O vw O v then there exists an alg. 1psg λ (G) such that lim α 0 λ(α) [(v, w)] O v.
23 Equip V and W with Hermitian norms. The energy of the pair (v, w) is the function on G defined by G σ p vw (σ) := log σ w 2 log σ v 2. Corollary B. inf p σ G vw (σ) = if and only if there is a degeneration λ (G) such that lim α 0 p vw (λ(α)) =.
24 Hilbert-Mumford Semistability Semistable Pairs For all H G d Z >0 and For all H G and χ A H (v) f C d [ W ] H such that d Z >0 and f C d [ V W ] H dχ f(w) 0 and f(0) = 0 such that f(v, w) 0 and f V 0 0 / G w O vw O v = w λ (w) 0 w λ (w) w λ (v) 0 for all 1psg s λ of G for all 1psg s λ of G 0 N (w) all H G N (v) N (w) all H G C 0 such that log σ w 2 C all σ G C 0 such that log σ w 2 log σ v 2 C all σ G
25 To summarize, the context for the study of SEMISTABLE PAIRS is 1. A reductive linear algebraic group G. 2. A pair V, W of finite dimensional rational G- modules. 3. A pair of (non-zero) vectors (v, w) V W.
26 Resultants and Discriminants Let X be a smooth linearly normal variety X P N Consider two polynomials: R X := X-resultant X P n 1 := X-hyperdiscriminant Let s normalize the degrees of these polynomials X R = R(X) := R deg( X P n 1 ) X X = (X) := deg(r X) X P n 1
27 It is known that R(X) E λ \ {0}, (n + 1)λ = ( n+1 N n {}}{{}}{ r, r,..., r, 0,..., 0 ). (X) E µ \ {0}, nµ = ( {}} n N+1 n {{}}{ r, r,..., r, 0,..., 0 ). r = deg(r(x)) = deg( (X)). E λ and E µ are irreducible G modules. The associations X R(X), X (X) are G equivariant: R(σ X) = σ R(X) (σ X) = σ (X).
28 K-Energy maps and Semistable Pairs Let P be a numerical polynomial P (T ) = c ( T ) ( T ) n + cn 1 + O(T n 2 ) c n Z >0. n n 1 Consider the Hilbert scheme H P P N := { all (smooth) X P N with Hilbert polynomial P }. Recall the G-equivariant morphisms R, : H P P N P(E λ ), P(E µ ).
29 Theorem (Paul 2012 ) There is a constant M depending only on c n, c n 1 and the Fubini Study metric such that for all [X] H P P N and all σ G we have ν ωf S X (ϕ σ ) p R(X) (X) (σ) M.
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