# Compatibly split subvarieties of Hilb n (A 2 k)

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1 Compatibly split subvarieties of Hilb n (A 2 k) Jenna Rajchgot MSRI Combinatorial Commutative Algebra December 3-7, 2012

2 Throughout this talk, let k be an algebraically closed field of characteristic p > 2.

3 Definition: Hilb n (A 2 k ) is the scheme parametrizing dimension-0, degree-n subschemes of A 2 k. So, set theoretically, Hilb n (A 2 k) := {I k[x, y] : dim(k[x, y]/i ) = n as a vector space over k}. Properties: [Hartshorne] Hilb n (A 2 k ) is connected. [Fogarty] Hilb n (A 2 k ) is non-singular. This 2n-dimensional scheme has a T 2 = (k ) 2 action induced by the standard action of T 2 on A 2 k (i.e. (t 1, t 2 ) (x, y) = (t 1 x, t 2 y)). The T 2 -fixed points of Hilb n (A 2 k ) are the colength-n monomial ideals. Goal: To understand a particular stratification of Hilb n (A 2 k ). This stratification will consist of finitely many (locally closed) strata which are automatically reduced, regular in codimension 1, and stable under the T 2 -action.

4 We begin by stratifying Hilb 2 (A 2 k ) by reduced, T 2 -invariant subvarieties such that the open strata are regular in codimension 1. Consider Hilb 2 (A 2 k ) and the reduced, T 2 -invariant divisor D where D = at least one point is on a coordinate axis. The two components of D will be the codimension 1 subvarieties in our stratification. We can intersect the irreducible components of this divisor and decompose the intersection to obtain some new subvarieties. These subvarieties are reduced!

5 Neither irreducible component of D is regular in codimension 1. So, we include the codimension 1 component of the singular loci in the union of codimension 2 subvarieties to appear in the stratification of Hilb 2 (A 2 k ). Intersecting each one of these subvarieties with the union of the others and then decomposing each intersection yields the following codimension 3 subvarieties: Repeating this procedure once more produces the T 2 -fixed points.

6 This sequence of intersecting, decomposing, and including non-r1 loci produces the following stratification by reduced subvarieties: This is precisely the collection of compatibly Frobenius split subvarieties of Hilb 2 (A 2 k ).

7 Definition: Let R be a (commutative) k-algebra and let X = Spec(R). Say that R (or X ) is Frobenius split by φ : R R if: φ(a + b) = φ(a) + φ(b), φ(a p b) = aφ(b), φ(1) = 1 for any a, b R. (Notice that φ is an R-module map which splits the Frobenius endomorphism F : R R, r r p. That is, φ F = Id.) It immediately follows from the definition that if R is Frobenius split then R has no nilpotents. So, X = Spec(R) is reduced. Definition: Let I R be an ideal. We say that I (or V (I )) is compatibly Frobenius split if φ(i ) I. In this case, there is an induced splitting, φ : R/I R/I and we get that I is a radical ideal. The following are some consequences which we have already used: 1. Intersections, unions and components of compatibly split subschemes are compatibly split. 2. The non-r1 locus of any compatibly split subvariety is compatibly split. Note: The above definitions and results generalize to schemes (X, O X ).

8 Theorem: 1. For X regular, Hom OX (F O X, O X ) = H 0 (X, F (ω 1 p X )), where ω X is the canonical bundle on X. Thus, certain anticanonical sections determine Frobenius splittings. 2. [Kumar-Mehta] Let X be an irreducible, normal variety which is Frobenius split by σ H 0 (X, F (ω 1 p X )). If Y is compatibly split then Y V (σ) or Y sing(x ). Example: The divisor {x 1 x 2 x n = 0} determines a Frobenius splitting of A n. This splitting of A n is called the standard splitting. By intersecting the components of the divisor, decomposing the intersections, intersecting the new components, etc., we obtain the collection of coordinate subspaces. This is precisely the set of compatibly split subvarieties of A n with the standard splitting. Theorem: [Lakshmibai-Mehta-Parameswaran] Let f k[x 1,..., x n ]. If there is a term order on k[x 1,..., x n ] such that init(f ) = x 1 x 2 x n then {f = 0} determines a splitting of A n that compatibly splits {f = 0}. Theorem: [Kumar-Thomsen] The anticanonical divisor described by at least one point is on an axis determines a Frobenius splitting of Hilb n (A 2 k ).

9 Algorithm: [Knutson-Lam-Speyer] Input: (X, X ) where X is Frobenius split and X is the anticanonical divisor which induces the splitting. Output: Suppose that X = D 1 D r. Let E i = D 1 ˆD i D n. There are two cases. 1. If X is normal, then return (D 1, D 1 E 1 ),...,(D n, D n E n ). 2. If X is not normal, return ( X, ν 1 ( X X non-r1 )) where ν : X X is the normalization of X. Repeat until neither 1. nor 2. can be applied. When finished, map all subvarieties back to the original Frobenius split variety to obtain a list of many (for large p) compatibly split subvarieties. At each stage of the algorithm, check if a component of the singular locus that is both compatibly split and of codimension 2. (Hard!) If so, add it (and its compatibly split subvarieties) to the list. In certain cases, the final list consists of all compatibly split subvarieties of (X, X ).

10 As an example of the algorithm, we consider (again) the case of Hilb 2 (A 2 k ). Start with (Hilb 2 (A 2 k ), D) where D is as before. Apply 1. Due to the symmetry, continue with just the first of the two pairs. Next, recall that the components of D are not regular in codimension 1. Apply 2.

11 From the previous slide, we have: Apply 1. Applying 1. once more obtains the preimage of the T 2 -fixed points under map π : X n Hilb 2 (A 2 k ) where X n denotes the isospectral Hilbert scheme (i.e. the scheme of labelled points in the affine plane).

12 The algorithm produces the following stratification of Hilb2 (A2k ):

13 In contrast to previously studied cases (eg. the flag variety), not every compatibly split subvariety of Hilb n (A 2 k) is normal. As a result, additional non-split subvarieties naturally arise. Notice that ν Y is generically 2:1 but is ramified along the locus where the two points collide. Letting s = y 1 + y 2 and m = y 1y 2 be the two coordinates on ν(y ) = A 2 k/s 2, we can check that the splitting of ν(y ) is given by the section (s 2 4m) (p 1)/2 m p 1. From this we see that {m = 0} is compatibly split. It would be nice to be able to say that {s 2 = 4m} (which agrees with the ramification locus of ν Y ) is half split.

14 The stratification of Hilb 3 (A 2 k ) by all compatibly split subvarieties:

15 The compatibly split subvarieties of Hilb4 (A2k ):

16 Proposition: Y Hilb n (A 2 k ) is a compatibly split subvariety if and only if Y is the closure of the image of the morphism i : Hilb a ( punctured y-axis ) Hilb b ( punctured x-axis ) Hilb c (A 2 k \{xy = 0}) Z Hilbn (A 2 k ) (I 1, I 2, I 3, I 4 ) I 1 I 2 I 3 I 4 for some a, b, c 0 with a + b + c n and for some compatibly split Z Hilb n a b c (A 2 k ), where Z is contained inside of the punctual Hilbert scheme of n a b c points all supported at the origin. Thus, the problem of finding all compatibly split subvarieties of Hilb n (A 2 k ) is equivalent to the problem of finding all compatibly split subvarieties of Hilb m (A 2 k ), m n, where all points are at the origin.

17 Notice that for n = 1, 2, 3, 5, all torus fixed points are indeed 0-dimensional compatibly split subvarieties of Hilb n (A 2 k ). However, when n = 4, { x 2, y 2 } is not compatibly split. For n 8, the colength-n monomial ideals which do not correspond to any of the following standard sets (or their transposes ) are compatibly split: Conjecturally, a torus fixed point is a 0-dimensional compatibly split subvariety of Hilb n (A 2 k ) if and only if the associated monomial ideal is integrally closed.

18 For the remainder of the talk, we will restrict to a specific open patch of Hilb n (A 2 k ) (for arbitrary n).

19 Definition: Let λ be a colength-n monomial ideal. U λ is the set of all I Hilb n (A 2 k ) such that the monomials outside λ form a vector space basis of k[x, y]/i. Example: Let λ = x, y 2 Hilb 2 (A 2 k ). Then I = y 2 + y, x + 2 U λ as {1, y} spans the vector space k[x, y]/i. Unless otherwise indicated, we consider U x,y n from now on. We ll study the simpler stratification of U x,y n by all of its compatibly split subvarieties. All colength-n ideals in U x,y n have a Gröbner basis of the form: y n b 1 y n 1 b 2 y n 2 b n 1 y b n xy n 1 a 1 y n 1 c 12 y n 2 c 1(n 1) y c 1n xy n 2 a 2 y n 1 c 22 y n 2 c 2(n 1) y c 2n. xy a n 1 y n 1 c (n 1)2 y n 2 c (n 1)(n 1) y c (n 1)n x a ny n 1 c n2 y n 2 c n(n 1) y c nn where each c ij is a polynomial in a 1,..., a n, b 1,..., b n. We see that U x,y n = A 2n = Spec k[a 1, b 1,..., a n, b n ].

20 As before, we begin with the n = 2 case. The compatibly split subvarieties of U x,y 2 are the non-empty Y U x,y 2 such that Y Hilb 2 (A 2 k ) is compatibly split. The subvarieties to the left of the red curve have non-trivial intersection with U x,y 2 Hilb 2 (A 2 k ).

21 Recall that U x,y 2 = Spec(k[a 1, b 1, a 2, b 2]). By imposing the condition at least one point is on an axis, we obtain the divisor {f 2 = 0} where f 2 = a 1b 1a 2b 2 a 2 1b 2 + a 2 2b 2 2. Under the term order Revlex b2, Lex a2, Revlex b1, Lex a1, init(f 2) = a 1b 1a 2b 2. In fact, for any compatibly split ideal I, init(i ) (under the same term order) is a squarefree monomial ideal. We may therefore associate a simplicial complex to each init(i ).

22 The n = 2 case generalizes. Proposition: 1. With respect to the term order Revlex bn, Lex an,..., Revlex b1, Lex a1, init(f n) = a 1b 1... a nb n and (by a theorem of Knutson) all compatibly split subvarieties of U x,y n degenerate to Stanley-Reisner schemes. 2. More precisely, if Y U x,y n is compatibly split, then Lex an Revlex bn Y is a compatibly split subvariety of U x,y n 1 A 2 k. 3. Y U x,y n is compatibly split if and only if it is of one of the following four types: 4. Each compatibly split subvariety of U x,y n degenerates to the Stanley-Reisner scheme of a shellable simplicial complex. Thus, each compatibly split subvariety of U x,y n is Cohen-Macaulay. 5. Suppose that Y is compatibly split and that a stratum representative of Y either has no points in A 2 k \ {xy = 0} or has at most one point on the punctured y-axis, then init(y ) is the Stanley-Reisner scheme of a simplicial ball.

23 We now present the ideas in the proof of the proposition. To begin, consider the following theorem. Theorem: Let f k[x 1,..., x n] be a degree n polynomial such that, under some term order, init(f ) = i x i. 1. [LMP] {f = 0} determines a Frobenius splitting of A n. 2. [Knutson] If I is compatibly split with respect to this splitting, then init(i ) is compatibly split with respect to the splitting determined by {init(f ) = 0}. This theorem applies to our situation: All elements of U x,y n are ideals generated by polynomials of the form: y n b 1 y n 1 b 2 y n 2 b n 1 y b n xy n 1 a 1 y n 1 c 12 y n 2 c 1(n 1) y c 1n xy n 2 a 2 y n 1 c 22 y n 2 c 2(n 1) y c 2n. xy a n 1 y n 1 c (n 1)2 y n 2 c (n 1)(n 1) y c (n 1)n x a ny n 1 c n2 y n 2 c n(n 1) y c nn where each c ij is a polynomial in a 1,..., a n, b 1,..., b n. Let M n be the matrix of coefficients ( c ij ) 1 i,j n where c i1 = a i. The divisor that determines the splitting on U x,y n is given by {f n = 0} where f n = b n(detm n). Under the term order Revlex bn, Lex an,..., Revlex b1, Lex a1, init(f n) is a 1b 1 a nb n.

24 For example, we have: M 4 = Ä ä a1 a M 2 = 2 b 2, M a 2 (a 1 b 1 a 2 ) 3 = Å a1 ã (a 2 b 2 + a 3 b 3 ) a 2 b 3 a 2 (a 1 b 1 a 2 ) a 3 b 3 a 3 (a 2 b 1 a 3 ) (a 1 b 1 a 2 b 2 a 3 ) Ç a1 å (a 2 b 2 + a 3 b 3 + a 4 b 4 ) (a 2 b 3 + a 3 b 4 ) a 2 b 4 a 2 (a 1 b 1 a 2 ) (a 3 b 3 + a 4 b 4 ) a 3 b 4 a 3 (a 2 b 1 a 3 ) (a 1 b 1 a 2 b 2 a 3 ) a 4 b 4 a 4 (a 3 b 1 a 4 ) (a 2 b 1 a 3 b 2 a 4 ) (a 1 b 1 a 2 b 2 a 3 b 3 a 4 ) Computing the determinant of M 4 using cofactors along the last column, we get: det M 4 = (M 4 ) 44 (det M 3 ) + b 4 ( ). Taking the terms with the smallest power of b 4 (i.e. computing Revlex b4 (det M 4 )) yields: (M 4 ) 44 (det M 3 ). Taking Lex a4 of this polynomial yields: a 4 b 3 (det M 3 ).

25 Proposition: 1. With respect to the term order Revlex bn, Lex an,..., Revlex b1, Lex a1, init(f n) = a 1b 1... a nb n and (by a theorem of Knutson) all compatibly split subvarieties of U x,y n degenerate to Stanley-Reisner schemes. 2. More precisely, if Y U x,y n is compatibly split, then Lex an Revlex bn Y is a compatibly split subvariety of U x,y n 1 A 2 k. 3. Y U x,y n is compatibly split if and only if it is of one of the following four types: 4. Each compatibly split subvariety of U x,y n degenerates to the Stanley-Reisner scheme of a shellable simplicial complex. Thus, each compatibly split subvariety of U x,y n is Cohen-Macaulay. 5. Suppose that Y is compatibly split and that a stratum representative of Y either has no points in A 2 k \ {xy = 0} or has at most one point on the punctured y-axis, then init(y ) is the Stanley-Reisner scheme of a simplicial ball.

26 We do not sketch the proof of 3. here. For 4., we first determine init(y ) for each compatibly split subvariety Y by understanding the degenerations given by Revlex bn and Lex an. Lemma: Let Y be a subvariety of Spec k[x 1,..., x n]. Let H be the hyperplane {x 1 = 0}. If Y H then Revlex x1 (Y ) = Y O x1 H Spec k[x 1]. If Y H then Revlex x1 (Y ) = (Y H) Spec k[x 1] H Spec k[x 1]. In our case, H = {b n = 0}, which is the subvariety one point is on the x-axis. The Lex an degenerations can be described by the following pictures:

27 Using the step by step degenerations, we can compute the initial schemes of a compatibly split ideal Y. We can therefore determine the simplicial complexes associated to init(y ). Example: Thus, init(i (Y )) is a 2 b 1 a 1 and the associated simplicial complex is

28 To each compatibly split subvariety of U x,y n, the step by step degenerations allow us to associate words in the following letters : (1) a, (2) â, (3) aa, (4) aa, (5) â Let Y be a compatibly split subvariety of U x,y n Hilb n (A 2 k ). Suppose that a general element of Y has L points in A 2 k \ {xy = 0}, K points on the punctured y-axis, and R points vertically stacked at the origin. For example: Proposition: The facets of the simplicial complex associated to init(y ) are in one-to-one correspondence with words of the following form: (word in (1), (2), (3)) (word in (4), (5)) (a iff R + 1 at origin) such that #(1) + #(3) + #(4) = L, #(2) + #(3) = K, #(4) + #(5) = R.

29 What about other patches U λ which are isomorphic to A 2n? Conjecture: There are specific coordinates a 1,..., a n, b 1,..., b n, chosen in an analogous manner to the λ = x, y n case, such that {f λ = 0} is a residual normal crossings divisor. (This has been checked in Macaulay 2 for n 8.) The term order such that initf λ = a 1 b 1 a n b n depends on the shape of the partition associated to λ and is a generalization (in a precise way) of the term order in the U x,y n case. Question: What about on patches that are not isomorphic to affine space? Can you do this in a formal neighborhood of a torus fixed point?

30 Finally, we consider a connection to Poisson geometry. Consider A 2 with the Poisson tensor xy d dx d dy. This induces a Poisson tensor on the Hilbert scheme. (See a paper of Bottacin for the general situation.) On the open patch U x,y n the Poisson bracket is given by : {a i, b j } = j 1 {b i, b j } = 0, {a i, a j } = a k a j (k i), i < j n a k (j i) b k, i j, k=j k=1 j 1 {a i, b j } = a i j a i j+k b k, i > j The Pfaffian of the (2n 2n)-matrix gives the anticanonical divisor that determines the induced splitting of U x,y n. With respect to the same weighting of the variables which gives init(f n ) = a 1 b 1... a n b n, we can degenerate the Poisson tensor to a log canonical tensor. Furthermore, all compatibly split subvarieties of U x,y n are Poisson subvarieties. k=1 Conjectural for large n.

31 Thank You.

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