Statistical Mechanics & Enumerative Geometry:

Transcription

1 Statistical Mechanics & Enumerative Geometry: Christian Korff University Research Fellow of the Royal Society Department of Mathematics, University of Glasgow joint work with C Stroppel (U Bonn) Adv Math 225 (2010) arxiv: ; arxiv: ; arxiv: July 2010

2 Outline Part 1: Quantum Cohomology Motivation Vicious walkers on the cylinder Gromov-Witten invariants Fermions hopping on Dynkin diagrams nil-temperley-lieb algebra and nc Schur polynomials New recursion formulae for Gromov-Witten invariants Part 2: ŝl(n) k Verlinde algebra/wznw fusion ring Relating fusion coefficients and Gromov-Witten invariants Main result New algorithm for computing Gromov-Witten invariants

3 Motivation Quantum cohomology originated in the works of Gepner, Vafa, Intriligator and Witten (topological field & string theory) Witten s 1995 paper The Verlinde algebra and the cohomology of the Grassmannian: The fusion coefficients of ĝl(n) k WZNW theory can be defined in geometric terms using Gromov s pseudoholomorphic curves Kontsevich s formula ( big quantum cohomology) How many curves of degree d pass through 3d 1 points in the complex projective plane? For more on big and small quantum cohomology see eg notes by Fulton and Pandharipande (alg-geom/960811v2)

4 Vicious walkers on the cylinder: the 5-vertex model Define statistical model whose partition function generates Gromov-Witten invariants Consider an n N square lattice (0 n N) with quasi-periodic boundary conditions (twist parameter q) in the horizontal direction Allowed vertex configurations and their weights Here x i is the spectral parameter in the i th lattice row (Vicious walker model, cf [Fisher][Forrester][Guttmann et al])

5 x 1 w (μ) 1 w (μ) 2 w (μ) 3 x 2 x 3 x n w (ν) 1 w (ν) 2 w (ν) 3

6 Vicious walkers on the cylinder: transfer matrix Example of an i th lattice row configuration (n = 3 and N = 9): The variable x i counts the number of horizontal edges, while the boundary variable q counts the outer horizontal edges divided by 2 Definition of the transfer matrix Given a pair of 01-words w = , w = of length N, the transfer matrix Q(x i ) is defined as Q(x i ) w,w := # of outer edges # of horizontal edges q 2 x i allowed row configuration

7 Interlude: 01-words and Young diagrams Row configurations are described through 01-words w in the set { W n = w = w 1 w 2 w N w = } w i = n, w i {0, 1} i Let l 1 < < l n with 1 l i N be the positions of 1-letters in a word w Then w = 0 01 l ln 0 0 λ = (λ 1,, λ n ), λ i = l n+1 i i defines a bijection from W n into the set P n,k := {λ = (λ 1 λ 2 λ n ) λ 1 k and λ n 0} which are the partitions whose Young diagram fits into a n k bounding box with k = N n

8 Vicious walkers on the cylinder: partition function Given µ, ν P n,k, let w(µ), w(ν) be the corresponding 01-words Boundary conditions: fix the values of the edges on the top and bottom to be w(µ) and w(ν), respectively The partition function is the weighted sum over all allowed lattice configurations and is given by Z µ ν (x 1,, x n ; q) = (Q(x n ) Q(x n 1 ) Q(x 1 )) w(ν),w(µ) Theorem (Generating function for Gromov-Witten invariants) The partition function has the following expansion in terms of Schur functions s λ, Z µ ν (x 1,, x n ; q) = λ (n,k) q d C ν,d λµ s λ(x 1,, x n ), where C ν,d λµ are 3-point Gromov-Witten invariants (d = λ + µ ν N )

9 Reminder: Schur functions The ring of symmetric function Λ plays an important role in representation theory, combinatorics and enumerative geometry Λ = lim Λ n, Λ n = Z[x 1,, x n ] Sn An important bases are Schur functions which we define as s λ (x) = x T, x T x α 1 1 x α n n, T =λ where α(t ) = (α 1,, α n ) is the weight of a (semi-standard) tableau T of shape λ Example Let n = 3 and λ = (2, 1) Then T = 1 1 2, 1 2 2, 1 3 2, 1 2 3, 1 3 3, s (2,1) = x 2 1 x 2 + x 1 x x 1 x 2 x 3 + x 1 x x 2 2 x 3 + x 2 x 2 3, 2 3 3

10 Special cases of Schur functions are the elementary symmetric functions λ = (1 r ) = (1,, 1) }{{} r e r (x) = x p 1 1 x p n n = x i1 x ir i 1 < <i r p r, p i =0,1 complete symmetric functions, λ = (r, 0, 0, ) h r (x) = x p 1 1 x p n n = x i1 x ir p r i 1 i r Product of Schur functions via Littlewood-Richardson coefficients: s λ s µ = ν c ν λµ s ν

11 Schubert varieties and rational maps Let Gr n,n be the Grassmannian of n-dimn l subspaces V in C N Given a flag F 1 F 2 F n = C N the Schubert variety Ω λ (F ) is defined as Ω λ = {V Gr n,n dim(v F k+i λi ) i, i = 1, 2, n} Definition of 3-point Gromov-Witten invariants C ν,d λ,µ = # of rational maps f : P1 Gr n,n of degree d which meet the varieties Ω λ (F ), Ω µ (F ), Ω ν (F ) for general flags F, F, F (up to automorphisms in P 1 ) If there is an number of such maps, set C ν,d λ,µ = 0 ν = (k ν n,, k ν 1 ) (complement Poincaré dual)

12 Small Quantum (= q-deformed) Cohomology Define qh (Gr n,k ) := Z[q] Z H (Gr n,k ) Small quantum cohomology ring (Gepner, Witten, Agnihotri, ) The product σ λ σ µ := d,ν qd C ν,d λµ σ ν, with σ λ := 1 [Ω λ ], turns qh (Gr n,k ) into a commutative ring Theorem (Siebert-Tian 1997) Set Λ n = Z[e 1,, e n ] then σ λ s λ is a ring isomorphism qh (Gr n,n ) = (Z[q] Z Λ n )/ h k+1,, h n+k 1, h n+k + ( 1) n q Specialisations q = 0: cup product in cohomology ring H (Gr n,n+k ) q = 1: fusion product of the gauged ĝl(n) k WZNW model (TFT)

13 Fermions on a circle Consider a circular lattice with N sites, 0 n N particles (called fermions ) and k = N n holes Example Pauli s exclusion principle: at each site at most one particle is allowed (vicious walker constraint)!

14 Fermion creation and annihilation Consider the finite-dimensional vector space F = N F n, F n = CW n = (C 2 ) N, n=0 where F 0 = C{0 0} = C and w = 0 0 is the vacuum Define n i (w) = w w i, the number of 1-letters in [1, i] For 1 i N define the (linear) maps ψ i, ψ i : F n F n±1, { ψ i (w) := 0, { ( 1) ni 1(w) w, w i = 0 and w j = w j + δ i,j w i = 1 ψ i (w) := ( 1) ni 1(w) w, w i = 1 and w j = w j δ i,j 0, w i = 0

15 Example Take n = k = 4 and µ = (4, 3, 3, 1) The boundary ribbon (shaded boxes) starts in the (3 n) = 1 diagonal Below the diagram the respective 01-words w(µ) and w(ψ 3µ) are displayed

16 Clifford algebra Proposition The maps ψ i, ψ i : F n F n 1 yield a Clifford algebra on F = 0 n N F n, ie one has the relations ψ i ψ j + ψ j ψ i = ψ i ψ j + ψ j ψ i = 0, ψ i ψ j + ψ j ψ i = δ ij Introducing the scalar product w, w = i δ wi,w i one has ψ i w, w = w, ψ i w for any pair w, w F Remark The Clifford algebra turns out to be the fundamental object in the description of quantum cohomology

17 Nil affine Temperley-Lieb algebra Proposition (hopping operators) The map u i ψ i+1 ψ i, i = 1,, N 1, u N ( 1) n 1 qψ 1ψ N yields a faithful rep of the nil affine TL algebra in End(F n ), ui 2 = u i u i+1 u i = u i+1 u i u i+1 = 0, i Z N u i u j = u j u i, i j mod N > 1 Proposition (nc complete symmetric polynomials) The transfer matrix of the vicious walker model is given by Q(x i ) = 0 r<n x r i h r (u) with h r (u) = (q( 1) n 1 ψ N ) p 0 u p N 1 N 1 up 1 1 (ψ 1) p 0 p r Q possesses a complete eigenbasis independent of x i, hence [Q(x i ), Q(x j )] = 0 for any pair x i, x j

18 Noncommutative Schur functions Define s λ (u) := det(h λi i+j(u)) 1 i,j N, note [h r (u), h r (u)] = 0 Theorem (Postnikov 2005) Combinatorial product formula for the quantum cohomology ring: λ µ = q d C ν,d λµ ν = s λ(u)µ d,ν Proposition (CK, Stroppel: noncommutative Cauchy identity) Let Q be the transfer matrix of the vicious walker model, then Q(x 1 ) Q(x 2 ) Q(x n ) = s λ (u)s λ (x 1,, x n ) λ Partition function of the vicious walker model Taking the scalar product ν, µ in the nc Cauchy identity now proves the earlier stated expansion of the partition function

19 Explicit construction of eigenbasis Construct common eigenbasis {b(y)} of s λ (u 1,, u N ) s using Clifford algebra (Jordan-Wigner transformation): s λ (u)b(y) = s λ (y)b(y), b(y) := ˆψ (y 1 ) ˆψ (y n ), where ˆψ (y i ) = a y a i ψ a and y N 1 = = y N n = ( 1) n 1 q Siebert-Tian presentation of qh (Gr n,n ) h k+1 = = h n+k 1 = 0, h n+k = ( 1) n 1 q Bertram-Vafa-Intrilligator formula for C ν,d λµ C ν,d λµ = y ν, s λ (u)b(y) b(y), µ b(y), b(y)

20 Fermion fields and nc Schur functions Proposition The following commutation relation holds true, s λ (u, q)ψ i = ψ i s λ (u, q) + l(λ) ψ i+r r=1 λ/µ=(r) s µ (u, q) where we set ψ j+n = ( 1)n +1 qψ j, n = particle number operator

21 Fermion creation of quantum cohomology rings ψ i, ψ i induce maps qh (Gr n,n ) qh (Gr n±1,n ) Corollary (Recursive product formula) One has the following relation for the product in qh (Gr n,n ), λ ψ i (µ) = s λ (u, q)ψ i (µ) = λ 1 r=0 λ/ν=(r) where denotes the product in qh (Gr n 1,N ) with q q Corollary (Recursion formula for Gromov-Witten invariants) Suppose ψ j µ 0 for some 1 j N Then ψ i+r (ν µ) λ1 C ν,d λµ (n, N) = r=0 ( 1) d j (µ,ν) λ/ρ=(r) C ψ j+r ν,dr ρ,ψ j µ (n 1, N)

22 Example Consider the ring qh (Gr 2,5 ) Via ψ i : qh (Gr 1,5 ) qh (Gr 2,5 ) one can compute the product in qh (Gr 2,5 ) through the product in qh (Gr 1,5 ): = ψ ( = = qψ 2+2 ( ψ = q + q Inductive algorithm One can successively generate the entire ring hierarchy {qh (Gr n,n )} N n=0 starting from n = 0 ( ) + ψ 2+3 ( ) ) ( qψ ) )

23 Corollary (Fermionic product formula) One has the following alternative product formula in qh (Gr n,n ), λ µ = T =λ ψ l 1 (µ)+α n ψ l 2 (µ)+α n 1 ψ l 3 (µ)+α n 2 ψ l 4 (µ)+α n 3, where l i are the particle positions in µ, α is the weight of T, ψ i = ψ i for i = 1,, N and ψ i+n = ( 1) n q ψ i Corollary (Quantum Racah-Speiser Algorithm) For π S n set α i (π) = (l i (ν) l π(i) (µ)) mod N 0 and d(π) = #{i l i (ν) l π(i) (µ) < 0} Then C ν,d λµ = π S n, d(π)=d where K λ,α are the Kostka numbers ( 1) l(π)+(n 1)d K λ,α(π),

24 Example Set N = 7, n = N k = 4 and λ = (2, 2, 1, 0), µ = (3, 3, 2, 1) Step 1 Positions of 1-letters: l(µ) = (l 1,, l 4 ) = (2, 4, 6, 7) Step 2 Write down all tableaux of shape λ such that l = (l 1 + α n,, l n + α 1 ) mod N with l i l j for i j , 3 (3,5,7,9) , 4 (3,5,7,9) , 4 (3,4,8,9) , 4 (4,5,6,9) , 3 (3,6,7,8) , 4 (3,6,7,8) , 4 (3,6,8,7) , 4 (4,5,7,8) Step 3 Impose quasi-periodic boundary conditions, ie for each l i > N make the replacement ψ l 1 ψ l n ( 1)n+1 qψ l 1 ψ l i N ψ l n , 4 (4,5,7,8) Step 4 Let l be the reduced positions in [1, N] Choose permutation π S n st l 1 < < l n and multiply with ( 1) l(π) Done (4,5,8,7)

25 The three tableaux , 4 (4,5,7,8) , 4 (4,5,7,8) (4,5,8,7) yield the same 01-word w = , λ(w) = (3, 2, 2, 0) but with changing sign, ψ l 1 +2 ψ l 2 +1ψ l 3 +1 ψ l 4 +1 = ψ l 1 +2 ψ l 2 +1ψ l 3 +1 ψ l 4 +1 = We obtain the product expansion ψ l 1 +2 ψ l 2 +1ψ l 3 +2 ψ l 4 = q ψ 1ψ 4ψ 5ψ 7 = q +2q +q +q +q 2