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Generalized Gelfand-Zeitlin systems
Benjamin Hoffman
Department of MathematicsCornell University
Groupes de Lie et espaces des modules 28 April 2020
Joint with Jeremy Lane (McMaster/Fields)
Benjamin Hoffman (Cornell University) Generalized Gelfand-Zeitlin systemsGroupes de Lie et espaces des modules 28 April 2020 Joint with Jeremy Lane (McMaster/Fields) 1
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Outline
Gelfand-Zeitlin systemsCanonical bases and their parametrizationsGeneralized Gelfand-Zeitlin systemsProof of the main result
Benjamin Hoffman (Cornell University) Generalized Gelfand-Zeitlin systemsGroupes de Lie et espaces des modules 28 April 2020 Joint with Jeremy Lane (McMaster/Fields) 2
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Linear Poisson manifolds
For a Lie group K , let (k, [·, ·]) be the Lie algebra of K , and k∗ its lineardual.
Define a Poisson bracket on C∞(k∗):
{X ,Y }(ξ) = 〈ξ, [X ,Y ]〉
for X ,Y ∈ k ∼= (k∗)∗ ⊂ C∞(k∗) and ξ ∈ k∗. Then (k∗, {·, ·}) is a linearPoisson manifold.
Symplectic leaves of k∗ are the coadjoint orbits.
If (M, ω, µ) is a Hamiltonian K -manifold, then the moment mapµ : M → k∗ is a Poisson map.
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Gelfand-Zeitlin functions on u(n)∗
Consider the unitary group U(n).Identify u(n)∗ with H(n), the set of n × n Hermitian matrices.
For 1 ≤ i ≤ j ≤ n, let
µij : H(n)→ R
where µij(M) is the ith largest eigenvalue of the j × j submatrix M[1,j],[1,j].
The functions µin are Casimirs of the Poisson structure on H(n).
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Some properties of the Gelfand-Zeitlin system
Collectively:µ : H(n)→ Rn(n+1)/2.
It is smooth on an open dense subset.
The vector fields {µij , ·} and {µi′j ′ , ·} commute, for all i , i ′, j , j ′. The
flows of these vector fields descend to an effective torus action(S1)n(n−1)/2 on the smooth locus of µ.
? Completely integrable, on a dense subset. ?
The image of µ is the polyhedral cone in Rn(n+1)/2 cut out by theinequalities µij+1 ≥ µij ≥ µ
i+1j+1
This is the Gelfand-Zeitlin system
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Generalizations of the Gelfand-Zeitlin system
Guillemin-Sternberg (83): Original construction for u(n) and o(n).The construction uses a chain of subalgebras, eg:
o(1) ⊂ o(2) ⊂ · · · ⊂ o(n − 1) ⊂ o(n).
Harada (04): Completely integrable system on sp(2n)∗.
Harada-Kaveh (15): Completely integrable systems on integralcoadjoint orbits, for arbitrary compact k.
Goal: give a generalization of the GZ system on k∗ for all compact K .
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Outline
Gelfand-Zeitlin systemsCanonical bases and their parametrizationsGeneralized Gelfand-Zeitlin systemsProof of the main result
Benjamin Hoffman (Cornell University) Generalized Gelfand-Zeitlin systemsGroupes de Lie et espaces des modules 28 April 2020 Joint with Jeremy Lane (McMaster/Fields) 7
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Lie Theory Notation
Throughout what follows:
K is a compact Lie group, and G is its complex form.
B,B− ⊂ G is choice of opposite Borels. N ⊂ B,N− ⊂ B− aremaximal unipotent subgroups.
H = B ∩ B− is Cartan subgroup, and T = H ∩ K is maximal torus.Fraktur letters denote Lie algebras.
P = Hom(H,C×) ↪→ h∗ is the set of weights of G .P+ is the set of dominant weights of g, and t
∗+ is the positive Weyl
chamber.
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Canonical bases
Theorem (Lusztig): There is a canonical basis B of U(n−).
If λ ∈ P+ is a dominant integral weight of G , letV (λ) be the irreducible G module with high weight λ
vλ ∈ V (λ) be a fixed highest weight vectorThen Bλ = B · vλ is a weight basis of V (λ). The dual basis B∗λ is a weightbasis of V (λ)∗.
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Parametrizations of canonical bases
Let λ ∈ P+. A polyhedral parametrization of Bλ (or B∗λ) is a bijection
Bλ ↔4λ
where 4λ ⊂ Zm is the set of lattice points of a rational convex polytope inRm.
A polyhedral parametrization of B∗ :=∐λ∈P+ B
∗λ is
C =∐λ∈P+
{λ} ×4λ ⊂ P × Zm.
We require that C is the set of lattice points of a rational convexpolyhedral cone in Rm+r .
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Gelfand-Zeitlin cone
Let G = GLnC. Then P ∼= Zn and λ ∈ P+ can be written:
λ =(λ1, λ2, . . . , λn) ∈ Zn
λ1 ≥ λ2 ≥ · · · ≥ λn
The Gelfand-Zeitlin pattern 4GZλ associated with λ is the set of λij ∈ Z,1 ≤ i ≤ j ≤ n, so that
λij+1 ≥ λij ≥ λi+1j+1There exists a bijection Bλ ↔4GZλ .
Moreover,CGZ =
∐λ∈P+
{λ} ×4GZλ ⊂ Zn × Zn(n−1)/2
is the set of integral points of a convex polyhedral cone.
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String parametrizations
Let G be a reductive complex Lie group. Let i = (i1, i2, . . . , im) be areduced expression for the longest element w0 of the Weyl group W of G .
There is a polyhedral parametrization 4iλ of B∗λ which depends only on thechoice of i. Each 4iλ is a string polytope (Berenstein-Zelevinsky).
Moreover,Ci =
∐λ∈P+
{λ} ×4iλ ⊂ P × Zm
is the set of integral points of a convex polyhedral cone (Littelmann).
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String parametrizations, in detail
Fix λ ∈ P+, and consider b∗ ∈ B∗λ ⊂ V (λ)∗. Define
vi(b∗) = (v1, . . . , vN) ∈ 4iλ ⊂ ZN
by:
v1 = max{v | F vi1b∗ 6= 0}
v2 = max{v | F vi2Fv1i1b∗ 6= 0}
...
vN = max{v | F viNFvN−1iN−1
· · ·F v1i1 b∗ 6= 0}
Then vi is a bijection B∗λ ↔4iλ.
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Example
Let G = SL3C. Let i = (1, 2, 1). Generators of Ci:
F : N6 → Ci ⊂ P+ × N3
(a1, a2, a3, a12, a13, a23) 7→
∑i
ai ,∑ij
aij , a2 + a23, a3 + a13 + a23, a3
Then:
F (0, 1, 0, 0, 1, 0) = F (1, 0, 0, 0, 0, 1).
C[Ci] =C[z1, z2, z3, z12, z13, z23]
(z2z13 − z1z23)
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Outline
Gelfand-Zeitlin systemsCanonical bases and their parametrizationsGeneralized Gelfand-Zeitlin systemsProof of the main result
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Problem statement
The moment map image µ(u(n)∗) of the Gelfand-Zeitlin system isR≥0 · CGZ .
The integral points CGZ parametrize∐λ∈P+ B
∗λ, the dual canonical bases
of GLn(C) modules.
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Problem statement
Let k be the Lie algebra of a compact Lie group K , and let C ⊂ ZN be apolyhedral parametrization of of
∐λ∈P+ B
∗λ. Find a proper continuous map
µ : k∗ → RN which satisfies the following:1 For each open face σ of t∗+, the map µ is smooth on an open dense
subset of K · σ.2 On its smooth locus, µ : K · σ → RN generates a completely
integrable torus action.
3 µ(k∗) = R≥0 · C.
Theorem (H.-Lane)
For all compact Lie groups K and all string parametrizations Ci, such amap µ : k∗ → RN exists.
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Problem statement
Let k be the Lie algebra of a compact Lie group K , and let C ⊂ ZN be apolyhedral parametrization of of
∐λ∈P+ B
∗λ. Find a proper continuous map
µ : k∗ → RN which satisfies the following:1 For each open face σ of t∗+, the map µ is smooth on an open dense
subset of K · σ.2 On its smooth locus, µ : K · σ → RN generates a completely
integrable torus action.
3 µ(k∗) = R≥0 · C.
Theorem (H.-Lane)
For all compact Lie groups K and all string parametrizations Ci, such amap µ : k∗ → RN exists.
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Outline
Gelfand-Zeitlin systemsCanonical bases and their parametrizationsGeneralized Gelfand-Zeitlin systemsProof of the main result
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Proof outline
1 Pass to a stratified space ET ∗K which is symplectic realization of k∗.2 Identify ET ∗K with the affine variety G � N.3 Take a toric degeneration of G � N to a toric variety XCi .4 Use gradient-Hamiltonian flow to build a map G � N → XCi .5 Compose with a moment map XCi → RN , and divide by a small torus
action, to getk∗ → RN
satisfying the desired properties.
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Symplectic implosion
Step 1: Pass to a space ET ∗K which is a symplectic realization of k∗.
The symplectic implosion ET ∗K of T ∗K is a stratified space constructedby Guillemin-Jeffrey-Sjamaar.
As a set: ET ∗K =∐σ≤t∗+
(K/[Kσ,Kσ])× σ.
It is a topological space. Each piece (K/[Kσ,Kσ])× σ is a smoothsymplectic manifold.
There is a proper map ET ∗K → k∗. It is piecewise Poisson.
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Ambient affine space
Step 2: Identify ET ∗K with the affine variety G � N.
Fix a string parametrization Ci of∐λ∈P+ B
∗λ. It is a semigroup.
Let Π ⊂ P+ be a finite subset so that∐λ∈Π{λ} ×4iλ generates Ci as a
semigroup.
Let E = ⊕λ∈ΠV (λ). It is a G -module.
Example: G = SL3(C), Π = {ω1, ω2}, E = V (ω1)⊕ V (ω2).
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Base affine space
C[G ] ∼= ⊕λ∈P+V (λ)∗ ⊗ V (λ) as a G × G module.
C[G ]1×N ∼= ⊕λ∈P+V (λ)∗ ⊗ Cvλ ∼= ⊕λ∈P+V (λ)∗.Then B∗ =
∐λ∈P+ B
∗λ is a basis for C[G ]N .
Let G � N = SpecC[G ]N .
⊕λ∈ΠV (λ)∗ generates C[G ]N , so
G � N ↪→ ⊕λ∈ΠV (λ) = E .
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Example
Let G = SL3C, Π = {ω1, ω2}. Then
C[G ]N ∼=C[z1, z2, z3, z12, z13, z23](z2z13 − z1z23 − z3z12)
Since
C[V (ω1)] ∼= C[z1, z2, z3]C[V (ω2)] ∼= C[z12, z13, z23]
this embedsG � N ↪→ V (ω1)⊕ V (ω2).
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Example
C[G ]N ∼= C[z1,z2,z3,z12,z13,z23](z2z13−z1z23−z3z12)
The dual canonical basis B∗ of C[G ]N is the set of monomials in
z1, z2, z3, z12, z13, z23
which are not divisible by z2z13.
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Kähler structure
Give E a K -invariant Hermitian form 〈·, ·〉.Then E is Kähler, with Kähler form ωE = −=〈·, ·〉.
Theorem (Guillemin-Jeffrey-Sjamaar, Hilgert-Manon-Martens): ET ∗K isisomorphic to G � N, equipped with the restriction of ωE .
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High weight map
E = ⊕λ∈ΠV (λ) is a Hamiltonian K × T space.
T moment map:
hw : E → t∗+ ⊂ t∗∑λ∈Π
uλ 7→ π∑λ∈Π||uλ||2λ, uλ ∈ V (λ)
Key fact: hw is proper.
Recall ET ∗K =∐σ≤t∗+
K[Kσ ,Kσ]
× σ. Stratification:
K
[Kσ,Kσ]× σ ∼= G � N ∩ hw−1(σ)
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Toric degenerations
Step 3: Take a toric degeneration of G � N to a toric variety XCi .
Let XCi = SpecC[Ci].
Theorem (Caldero): There is a flat family π : X → C whereπ−1(C×) ∼= G � N × C× and π−1(0) ∼= XCi
Reformulation, due to Kaveh: The string parametrization can be extendedto a valuation
vi : C[G � N]→ P × ZN
with one dimensional leaves.
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Toric degenerations
Embed it into E × C:X E × C
C
←↩ →←
→π
←→ pr2
Technical point: Let T be the compact torus of XCi . Then the embeddingX ↪→ E × C can be chosen so that T acts on E via unitarytransformations (and hence is a Hamiltonian action).
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Example
Let G = SL3C, i = (1, 2, 1) as before. Then
C[Ci] =C[z1, z2, z3, z12, z13, z23]
(z2z13 − z1z23)
Take the algebra:
R = C[z1, z2, z3, z12, z13, z23, t](z2z13 − z1z23 − tz3z12)
R is a C[t] module. X = SpecR → SpecC[t] = C.R[t−1] ∼= C[G ]N ⊗ C[t±1]R/(t) ∼= C[Ci].
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Stratified gradient-Hamiltonian flow
Step 4: Use gradient-Hamiltonian flow to build a map G � N → XCi .
For each σ ≤ t∗+, restrict attention to X ∩ (hw−1(σ)× C). Smooth awayfrom π−1(0). Let
V σ = − ∇
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Stratified gradient-Hamiltonian flow
LetV =
∐σ≤t∗+
V σ : X\π−1(0)→ T (E × C).
Technical fact: V is continuous.(Note this doesn’t always happen for stratified gradient flows).
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Stratified gradient-Hamiltonian flow
Fact: π∗V = − ddt .
Let ϕt be the time t flow ofV (as long as it is defined). Then
ϕt : π−1(1)→ π−1(1− t). (∗)
for t ∈ [0, 1).
Technical facts: This flow (∗)existsand is unique for t ∈ [0, 1);is continuous;
preserves the symplecticform (stratum-wise).
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Stratified gradient-Hamiltonian flow
Technical fact (partly following Harada-Kaveh): ϕt can be extended to acontinuous map
ϕ1 : G � N ∼= π−1(1)→ π−1(0) ∼= XCi
which, when restricted to
G � N ∩ hw−1(σ)
is a smooth symplectic isomorphism on an open dense subset.
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Step 5
ET ∗K G � N XCi R≥0 · Ci RN
←→step 2 ←→ϕ1 ←→ ←↩ →
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Step 5
ET ∗K G � N XCi R≥0 · Ci RN
k∗←→step 2
←→
←→ϕ1 ←→ ←↩ →←
→µ
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Collective Hamiltonians
Let (M, ω, µ) be a compact connected Hamiltonian K -manifold.
M is multiplicity free if µ−1(Oλ)/K is 0-dimensional, for all λ ∈ t∗+.
An integrable system on M is collective if its moment map is of the form
Mµ−→ k∗ → RN .
Question: When does (M, ω, µ) have a collective integrable torus action?
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Collective Hamiltonians
Theorem (H.-Lane)
Let (M, ω, µ) be any compact connected Hamiltonian K -manifold. Then,
the composition MΦ−→ k∗ µ−→ RN is continuous, proper, smooth on an open
dense subset of M, and generates a Hamiltonian (S1)N action on itssmooth locus.
In particular, if M is multiplicity free, then this is a completely integrabletorus action on an open dense subset of M.
Proof:
M → Msc = (EM × ET ∗K ) � T → (EM × XCi) � T → RN
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Gelfand-Zeitlin systemsCanonical bases and their parametrizationsGeneralized Gelfand-Zeitlin systemsProof of the main result