Schur elements for Hecke algebras Maria Chlouveraki

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Schur elements for Hecke algebras

Maria Chlouveraki

Universit´e de Versailles - St Quentin

Finite Chevalley groups, reflection groups and braid groups

A conference in honour of Professor Jean Michel

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Let

R be a commutative integral domain ;

A be an R-algebra, free and finitely generated as an R-module ; K be a splitting field for A.

We write KA := K ⌦^R A.

A symmetrising trace on the algebra A is a linear map ⌧ : A ! R such that

1 ⌧(ab) = ⌧(ba) for all a, b 2 A, and

2 the map b⌧ : A ! Hom_R(A,R), a 7! (x 7! ⌧(ax)) is an isomorphism of A-bimodules.

Example

Let G be a finite group. The linear map ⌧ : Z[G] ! Z, P

g2G r_gg 7! r₁ is the canonical symmetrising trace on Z[G].

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Let

1 ⌧(ab) = ⌧(ba) for all a,b 2 A, and

2 the map ⌧b : A ! Hom_R(A,R), a 7! (x 7! ⌧(ax)) is an isomorphism of A-bimodules.

Example

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Let

1 ⌧(ab) = ⌧(ba) for all a,b 2 A, and

2 the map ⌧b : A ! Hom_R(A,R), a 7! (x 7! ⌧(ax)) is an isomorphism of A-bimodules.

Example

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Let E be a simple KA-module and let _E be the corresponding character. The map ⌧ can be extended to KA by extension of scalars. We have:

b

⌧ ¹( _E) 2 Z(KA).

The above element acts on E as a scalar; we call this scalar the Schur element of E with respect to ⌧ and denote it by s_E.

We have s_E 2 R_K, where R_K denotes the integral closure of R in K.

Example

Let G be a finite group and let ⌧ be the canonical symmetrising form on A := Z[G]. Let K be an algebraically closed field of characteristic 0, and let E 2 Irr(KA). We have s_E = |G|/ _E(1) 2 Z^K \ Q = Z.

We have thus shown that _E(1) divides |G|.

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b

⌧ ¹( _E) 2 Z(KA).

Example

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b

⌧ ¹( _E) 2 Z(KA).

Example

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b

⌧ ¹( _E) 2 Z(KA).

Example

Let G be a finite group and let ⌧ be the canonical symmetrising trace on A := Z[G]. Let K be an algebraically closed field of characteristic 0, and let E 2 Irr(KA). We have s_E = |G|/ _E(1) 2 Z^K \ Q = Z.

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1 If E 2 Irr(KA), then E is projective if and only if s_E 6= 0.

2 The algebra KA is semisimple if and only if s_E 6= 0 for all E 2 Irr(KA). If this is the case, then:

I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

I The blocks of A are the non-empty subsets B of Irr(KA) that are minimal with respect to the property:

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

3 Let ✓ : R ! L := Frac(✓(R)) be a ring homomorphism. Assume that R_K = R and that the algebra LA is split. Then:

I LA is semisimple if and only if ✓(s_E) 6= 0 for all E 2 Irr(KA).

I If ✓(s_E) 6= 0 for some E 2 Irr(KA), then E forms a block of defect 0.

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2 The algebra KA is semisimple if and only if s_E 6= 0 for all E 2 Irr(KA).

If this is the case, then:

I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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I We have

⌧ = X

E2Irr(KA)

1

s_E ^E.

X

E2B

1

s_E ^E(a) 2 R 8a 2 A.

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Let V be a finite dimensional complex vector space. A complex reflection group W is a finite subgroup of GL(V) generated by pseudo-reflections, i.e., elements whose vector space of fixed points is a hyperplane.

Theorem (Shephard-Todd)

Let W ⇢ GL(V) be an irreducible complex reflection group (i.e., W acts irreducibly on V). Then one of the following assertions is true:

(W, V) ⇠= (S_r,C^r ¹).

(W, V) ⇠= (G(de, e,r), C^r), where G(de, e,r) is the group of all r ⇥ r

monomial matrices whose non-zero entries are de-th roots of unity, while the product of all non-zero entries is an d-th root of unity.

(W, V) is isomorphic to one of the 34 exceptional groups G_n, n = 4, . . . ,37.

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Let V be a finite dimensional complex vector space. A complex reflection group W is a finite subgroup of GL(V) generated by pseudo-reflections, i.e., elements whose vector space of fixed points is a hyperplane.

Theorem (Shephard-Todd)

Let W ⇢ GL(V) be an irreducible complex reflection group (i.e., W acts irreducibly on V). Then one of the following assertions is true:

(W,V) ⇠= (S_r,C^r ¹).

(W,V) ⇠= (G(de, e, r), C^r), where G(de, e, r) is the group of all r ⇥ r

monomial matrices whose non-zero entries are de-th roots of unity, while the product of all non-zero entries is a d-th root of unity.

(W,V) is isomorphic to one of the 34 exceptional groups G_n, n = 4, . . . ,37.

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Every complex reflection group W has a presentation “`a la Coxeter” given by generators and relations. The generic Hecke algebra H(W) of W can be viewed as a deformation of the group algebra of W.

Example

The generic Hecke algebra of G₆ = ⌦

s,t | ststst = tststs, s² = t³ = 1 ↵ is

H(G₆) =

*

T_s, T_t

T_sT_tT_sT_tT_sT_t = T_tT_sT_tT_sT_tT_s, (T_s u_s_,0)(T_s u_s_,1) = 0,

(T_t u_t_,0)(T_t u_t,1)(T_t u_t,2) = 0 +

.

Set ⇣_d := exp(2⇡i/d). The algebra H(G₆) specialises to Z[G₆] when u_s_,0 7! 1, u_s_,0 7! 1,u_t,0 7! 1,u_t_,1 7! ⇣₃,u_t_,2 7! ⇣₃².

More generally, H(W) is a Z[u,u ¹]-algebra, where u := (u_s_,j)_s_,_j_=1,...,e_s_:=order_(s₎. The algebra H(W) specialises to Z[W] when u_s_,j 7! ⇣_e^j_s for all s, j.

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Example

s, t | ststst = tststs, s² = t³ = 1 ↵ is

H(G₆) =

*

T_s, T_t

(T_t u_t_,0)(T_t u_t_,1)(T_t u_t,2) = 0 +

.

Set ⇣_d := exp(2⇡i/d). The algebra H(G₆) specialises to Z[G₆] when u_s_,0 7! 1, u_s_,0 7! 1,u_t,0 7! 1,u_t_,1 7! ⇣₃,u_t_,2 7! ⇣₃².

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Example

H(G₆) =

*

T_s, T_t

(T_t u_t_,0)(T_t u_t_,1)(T_t u_t,2) = 0 +

.

Set ⇣_d := exp(2⇡i/d). The algebra H(G₆) specialises to Z[G₆] when u_s_,0 7! 1, u_s_,0 7! 1, u_t,0 7! 1,u_t,1 7! ⇣₃,u_t,2 7! ⇣₃².

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Example

H(G₆) =

*

T_s, T_t

(T_t u_t_,0)(T_t u_t_,1)(T_t u_t,2) = 0 +

.

More generally, H(W ) is a Z[u, u ¹]-algebra, where u := (u_s_,j)_s_,_j_=0,...,e_s ₁.

The algebra H(W) specialises to Z[W] when u_s_,j 7! ⇣_e^j_s for all s, j.

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Example

H(G₆) =

*

T_s, T_t

(T_t u_t_,0)(T_t u_t_,1)(T_t u_t,2) = 0 +

.

More generally, H(W ) is a Z[u, u ¹]-algebra, where u := (u_s_,j)_s_,_j_=0,...,e_s ₁. The algebra H(W) specialises to Z[W] when u_s_,j 7! ⇣_e^j_s for all s,j.

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1 Representation theory of finite reductive groups.

2 “Spetses” programme.

3 Representation theory of Cherednik algebras.

4 Knot theory.

5 Quantum groups.

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4 Knot theory.

5 Quantum groups.

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4 Knot theory.

5 Quantum groups.

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4 Knot theory.

5 Quantum groups.

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4 Knot theory.

5 Quantum groups.

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Conjectures (Brou´e-Malle-Michel-Rouquier)

1 Freeness: The algebra H(W) is a free Z[u, u ¹]-module of rank |W|.

2 Trace: There exists a canonical symmetrising trace ⌧ on H(W ) that satisfies certain canonicality conditions; the map ⌧ specialises to the

canonical symmetrising trace on the group algebra of W when u_s_,j 7! ⇣_e^j_s.

The Freeness Conjecture is verified for: all finite Coxeter groups ;

G(de, e, r) (Ariki-Koike, Brou´e-Malle-Michel) ;

all exceptional groups except for G₁₇, . . . , G₂₁ (Chavli, Marin, Marin-Pfei↵er). The Trace Conjecture is verified for:

all finite Coxeter groups ;

G(de, e, r) (BMM, Bremke-Malle, Malle-Mathas, Geck-Iancu-Malle) ; the exceptional groups G₄, G₁₂, G₂₂ and G₂₄ (Malle-Michel).

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Conjectures (Brou´e-Malle-Michel-Rouquier)

1 Freeness: The algebra H(W) is a free Z[u, u ¹]-module of rank |W|.

2 Trace: There exists a canonical symmetrising trace ⌧ on H(W ) that satisfies certain canonicality conditions; the map ⌧ specialises to the

canonical symmetrising trace on the group algebra of W when u_s_,j 7! ⇣_e^j_s. The Freeness Conjecture is verified for:

G(de, e, r) (Ariki-Koike, Brou´e-Malle-Michel) ;

all exceptional groups except for G₁₇, . . . , G₂₁ (Chavli, Marin, Marin-Pfei↵er).

The Trace Conjecture is verified for:

G(de, e, r) (BMM, Bremke-Malle, Malle-Mathas, Geck-Iancu-Malle) ; the exceptional groups G₄, G₁₂, G₂₂ and G₂₄ (Malle-Michel).

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Let K ✓ C be the field generated by the traces of the elements of W.

Theorem (Benard, Bessis)

The field K is a splitting field for W; it is called the field of definition of W.

We can always find N_W 2 Z^>0 such that if we take u_s_,j = ⇣_e^j_sv_s^N_,j^W

and set v := (v_s_,j)_s_,j, then the K(v)-algebra K(v)H(W) is split semisimple.

Example

For W = G₆, we have K = Q(⇣₁₂) and we can take N_W = 2.

H(G₆) =

*

T_s, T_t

T_sT_tT_sT_tT_sT_t = T_tT_sT_tT_sT_tT_s, (T_s v_s²_,0)(T_s + v_s²_,1) = 0,

(T_t v_t²_,0)(T_t ⇣₃v_t,1² )(T_t ⇣₃²v_t²_,2) = 0 +

Tits’s Deformation Theorem ) Irr(K(v)H(W)) $ Irr(W).

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Theorem (Benard, Bessis)

The field K is a splitting field for W; it is called the field of definition of W. We can always find N_W 2 Z^>0 such that if we take

u_s_,j = ⇣_e^j_sv_s^N_,j^W

Example

H(G₆) =

*

T_s, T_t

(T_t v_t²_,0)(T_t ⇣₃v_t,1² )(T_t ⇣₃²v_t²_,2) = 0 +

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Theorem (Benard, Bessis)

Example

H(G₆) =

*

T_s, T_t

(T_t v_t²_,0)(T_t ⇣₃v_t,1² )(T_t ⇣₃²v_t²_,2) = 0 +

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Theorem (Benard, Bessis)

Example

H(G₆) =

*

T_s, T_t

(T_t v_t²_,0)(T_t ⇣₃v_t,1² )(T_t ⇣₃²v_t²_,2) = 0 +

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The Schur elements of H(W) have been explicitly calculated for all finite Coxeter groups :

I for type A_n by Steinberg,

I for type B_n by Hoefsmit,

I for type D_n by Benson and Gay,

I for dihedral groups I₂(m) by Kilmoyer and Solomon,

I for F₄ by Lusztig,

I for E₆ and E₇ by Surowski,

I for E₈ by Benson,

I for H₃ by Lusztig,

I for H₄ by Alvis and Lusztig ;

G(d, 1, r) by Geck-Iancu-Malle and Mathas ; G(2d,2,2) by Malle ;

for the non-Coxeter exceptional complex reflection groups by Malle.

With the use of Cli↵ord theory, we obtain the Schur elements for G(de,e, r) from those of G(de,1, r) when r > 2 or r = 2 and e is odd. The Schur elements for G(de,e,2) when e is even derive from those of G(de,2,2).

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Theorem (C.)

Let E 2 Irr(W). The Schur element s_E is an element of Z^K[v,v ¹] of the form

s_E = ⇠_EN_E Y

i2IE

E,i(M_E_,i)

where

⇠_E is an element of Z^K, N_E = Q

s,j v_s^b_,j^s,j is a monomial in Z^K[v,v ¹] with Pes 1

j=0 b_s_,j = 0 for all s, I_E is an index set,

( _E_,i)_i₂_I_E is a family of K-cyclotomic polynomials in one variable, (M_E_,i)_i₂_I_E is a family of monomials in Z^K[v, v ¹] such that if

M_E_,i = Q

s,j v_s^a_,j^s,j, then gcd(a_s_,j) = 1 and Pes 1

j=0 a_s_,j = 0 for all s.

This is the factorisation of s_E into irreducible factors. The monomials (M_E_,i)_i₂_I_E are unique up to inversion, and we will call them potentially essential for W.

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Theorem (C.)

Let E 2 Irr(W). The Schur element s_E is an element of Z^K[v,v ¹] of the form

s_E = ⇠_EN_E Y

i2IE

E,i(M_E_,i)

where

⇠_E is an element of Z^K, N_E = Q

s,j v_s^b_,j^s,j is a monomial in Z^K[v,v ¹] with Pes 1

j=0 b_s_,j = 0 for all s, I_E is an index set,

( _E_,i)_i₂_I_E is a family of K-cyclotomic polynomials in one variable, (M_E_,i)_i₂_I_E is a family of monomials in Z^K[v, v ¹] such that if

M_E_,i = Q

s,j v_s^a_,j^s,j, then gcd(a_s_,j) = 1 and Pes 1

j=0 a_s_,j = 0 for all s.

This is the factorisation of s_E into irreducible factors. The monomials (M_E_,i)_i₂_I_E are unique up to inversion.

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Example

Take K = Q and ₄(x) = x² + 1. Then

4(ab ¹) = a²b ² + 1 = a²b ²(1 + a ²b²) = a²b ² ₄(a ¹b).

CHEVIE (C.-Michel)

SchurModels, SchurData, FactorizedSchurElement, FactorizedSchurElements.

H(G₆) =

*

T_s, T_t

T_sT_tT_sT_tT_sT_t = T_tT_sT_tT_sT_tT_s, (T_s a²)(T_s + b²) = 0,

(T_t c²)(T_t ⇣₃d²)(T_t ⇣₃²e²) = 0 +

gap > W := ComplexReflectionGroup(6); ;

gap > H := Hecke(W, [[a², b²], [c², E(3) ⇤ d², E(3)² ⇤ e²]]); ; gap > FactorizedSchurElement(H,[[1,0]]);

P₄(ab ¹)P₃⁰⁰P₆⁰(cd ¹)P₃⁰P₆⁰⁰(ce ¹)P₄P₁₂⁰⁰⁰(ab ¹cd ¹)P₄P₁₂⁰⁰⁰⁰(ab ¹ce ¹) P₄(ab ¹c²d ¹e ¹)

where P₄ = x² + 1, P₃⁰ = (x ⇣₃), P₃⁰⁰ = (x ⇣₃²), etc.

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Example

4(ab ¹) = a²b ² + 1 = a²b ²(1 + a ²b²) = a²b ² ₄(a ¹b).

CHEVIE (C.-Michel)

H(G₆) =

*

T_s, T_t

(T_t c²)(T_t ⇣₃d²)(T_t ⇣₃²e²) = 0 +

where P₄ = x² + 1, P₃⁰ = (x ⇣₃), P₃⁰⁰ = (x ⇣₃²), etc.

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Example

4(ab ¹) = a²b ² + 1 = a²b ²(1 + a ²b²) = a²b ² ₄(a ¹b).

CHEVIE (C.-Michel)

H(G₆) =

*

T_s, T_t

(T_t c²)(T_t ⇣₃d²)(T_t ⇣₃²e²) = 0 +

where P₄ = x² + 1, P₃⁰ = (x ⇣₃), P₃⁰⁰ = (x ⇣₃²), etc.

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Example

4(ab ¹) = a²b ² + 1 = a²b ²(1 + a ²b²) = a²b ² ₄(a ¹b).

CHEVIE (C.-Michel)

H(G₆) =

*

T_s, T_t

(T_t c²)(T_t ⇣₃d²)(T_t ⇣₃²e²) = 0 +

gap > H := Hecke(W, [[a², b²], [c², E(3) ⇤ d², E(3)² ⇤ e²]]); ; gap > FactorizedSchurElement(H,[[1, 0]]);

where P₄ = x² + 1, P₃⁰ = (x ⇣₃), P₃⁰⁰ = (x ⇣₃²), etc.

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For W = G(d,1, r) ⇠= (Z/dZ) o S_r ⇠= (Z/dZ)^r o S_r, H(W) is generated by elements

T₀,T₁, . . . , T_r ₁ satisfying the braid relations of type B_r:

and the extra relations:

(T₀ Q₀)(T₀ Q₁)· · ·(T₀ Q_d ₁) = 0 and (T_i q₀)(T_i q₁) = 0 for all i = 1, . . . ,r 1.

Theorem (Ariki-Koike)

The algebra H(W) is split semisimple over the field K := Q(Q₀, Q₁, . . . , Q_d ₁,q). We have

Irr(KH(W)) $ Irr(W) $ {d-partitions of r}.

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(T₀ Q₀)(T₀ ⇣_dQ₁)· · ·(T₀ ⇣_d^d ¹Q_d ₁) = 0 and (T_i q₀)(T_i + q₁) = 0 for all i = 1, . . . ,r 1.

Theorem (Ariki-Koike)

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(T₀ Q₀)(T₀ Q₁)· · · (T₀ Q_d ₁) = 0 and (T_i q)(T_i + 1) = 0 for all i = 1, . . . ,r 1.

Theorem (Ariki-Koike)

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Theorem (Ariki-Koike)

The algebra H(W) is split semisimple over the field K := Q(Q₀, Q₁, . . . , Q_d ₁,q).

We have

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Theorem (Ariki-Koike)

The algebra H(W) is split semisimple over the field K := Q(Q₀, Q₁, . . . , Q_d ₁,q).

We have

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The Schur elements of Ariki–Koike algebras have been independently determined by Geck-Iancu-Malle and Mathas. They belong to Z[Q₀^±¹, Q₁^±¹, . . . ,Q_d^±¹₁,q^±¹].

Theorem (C.-Jacon)

Let = ( ⁽⁰⁾, ⁽¹⁾, . . . , ^(d ¹⁾) be a d-partition of r. Then

s = ( 1)^r^(d ¹⁾q ^m (q 1) ^r Y

0sd 1

Y

(i,j)2[ ^(s)]

Y

0td 1

(q^h

(s), (t)

i,j Q_sQ_t ¹ 1)

where

m 2 N,

[ ^(s⁾] = {(i, j)| i 1,1  j  ^(s_i ⁾} is the set of nodes of ^(s⁾,

h_i_,^(s)_j ^, ^(t⁾ := ^(s_i ⁾ i + ^(t)_j ⁰ j + 1 is the generalised hook length of the node (i,j) with respect to ( ^(s⁾, ^(t)).

CHEVIE (C.-Michel)

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Theorem (C.-Jacon)

s = ( 1)^r^(d ¹⁾q ^m (q 1) ^r Y

0sd 1

Y

(i,j)2[ ^(s)]

Y

0td 1

(q^h

(s), (t)

i,j Q_sQ_t ¹ 1)

where

m 2 N,

[ ^(s⁾] = {(i,j)| i 1,1  j  ^(s_i ⁾} is the set of nodes of ^(s⁾,

h_i_,^(s)_j ^, ^(t) := ^(s_i ⁾ i + ^(t)_j ⁰ j + 1 is the generalised hook length of the node (i,j) with respect to ( ^(s⁾, ^(t)).

CHEVIE (C.-Michel)

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Theorem (C.-Jacon)

s = ( 1)^r^(d ¹⁾q ^m (q 1) ^r Y

0sd 1

Y

(i,j)2[ ^(s)]

Y

0td 1

(q^h

(s), (t)

i,j Q_sQ_t ¹ 1)

where

m 2 N,

[ ^(s⁾] = {(i,j)| i 1,1  j  ^(s_i ⁾} is the set of nodes of ^(s⁾,

h_i_,^(s)_j ^, ^(t) := ^(s_i ⁾ i + ^(t)_j ⁰ j + 1 is the generalised hook length of the node (i,j) with respect to ( ^(s⁾, ^(t)).

CHEVIE (C.-Michel)

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Consider H(W) defined over Z^K[v, v ¹], and let q be an indeterminate.

A cyclotomic specialisation is a Z^K-algebra homomorphism ' : Z^K[v, v ¹] ! Z^K[q,q ¹], v_s_,j 7! q^m^s,j , where m_s_,j 2 Z for all s,j.

The algebra H^'(W) obtained as a specialisation of H(W) via ' is called a cyclotomic Hecke algebra. It has the following properties:

H^'(W) is a free Z^K[q, q ¹]-module of rank |W| ; K(q)H^'(W ) is split semisimple ;

Irr(K(v)H(W)) $ Irr(K(q)H^'(W)) $ Irr(W) ;

H^'(W) is symmetric with Schur elements ('(s_E))_E₂_Irr(W₎ 2 Z^K[q,q ¹] . We define a : Irr(W) ! Z and A : Irr(W) ! Z such that

a(E) := Valuation_q('(s_E)) and A(E) := Degree_q('(s_E)).

Example

If '(s_E) = q ¹ ₅(q) = q ¹ + 1 + q + q² + q³, then a(E) = 1 and A(E) = 3.

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Consider H(W) defined over Z^K[v, v ¹], and let q be an indeterminate.

A cyclotomic specialisation is a Z^K-algebra homomorphism ' : Z^K[v, v ¹] ! Z^K[q,q ¹], v_s_,j 7! q^m^s,j ,

where m_s_,j 2 Z for all s,j. The algebra H^'(W) obtained as a specialisation of H(W) via ' is called a cyclotomic Hecke algebra.

It has the following properties: H^'(W) is a free Z^K[q, q ¹]-module of rank |W| ;

K(q)H^'(W ) is split semisimple ;

Irr(K(v)H(W)) $ Irr(K(q)H^'(W)) $ Irr(W) ;

H^'(W) is symmetric with Schur elements ('(s_E))_E₂_Irr(W₎ 2 Z^K[q,q ¹] . We define a : Irr(W) ! Z and A : Irr(W) ! Z such that

a(E) := Valuation_q('(s_E)) and A(E) := Degree_q('(s_E)).

Example

If '(s_E) = q ¹ ₅(q) = q ¹ + 1 + q + q² + q³, then a(E) = 1 and A(E) = 3.