# Symmetric Crystals for gl

## Full text

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### Symmetric Crystals for gl

Dedicated to Professor Heisuke Hironaka on the occasion of his seventy-seventh birthday

By

NaoyaEnomotoand MasakiKashiwara∗∗

Abstract

In the preceding paper, we formulated a conjecture on the relations between certain classes of irreducible representations of aﬃne Hecke algebras of type B and symmetric crystals for gl. In the present paper, we prove the existence of the symmetric crystal and the global basis forgl.

§1. Introduction

Lascoux-Leclerc-Thibon ([LLT]) conjectured the relations between the rep- resentations of Hecke algebras oftype Aand the crystal bases of the aﬃne Lie algebras of type A. Then, S. Ariki ([A]) observed that it should be understood in the setting of aﬃne Hecke algebras and proved the LLT conjecture in a more general framework. Recently, we presented the notion of symmetric crystals and conjectured that certain classes of irreducible representations of the aﬃne Hecke algebras oftype Bare described by symmetric crystals forgl([EK]).

The purpose of the present paper is to prove the existence of symmetric crystals in the case ofgl.

Let us recall the Lascoux-Leclerc-Thibon-Ariki theory. Let HAn be the aﬃne Hecke algebra of type A of degreen. Let KAn be the Grothendieck group

Communicated by T. Kawai. Received May 14, 2007. Revised November 18, 2007.

2000 Mathematics Subject Classiﬁcation(s): Primary 17B37; Secondary 20C08.

Key words: crystal bases, aﬃne Hecke algebras, LLT conjecture.

The second author is partially supported by Grant-in-Aid for Scientiﬁc Research (B) 18340007, Japan Society for the Promotion of Science.

Research Institute for Mathematical Sciences, Kyoto University, Kyoto 606–8502, Japan.

e-mail: henon@kurims.kyoto-u.ac.jp

∗∗e-mail: masaki@kurims.kyoto-u.ac.jp

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Naoya Enomoto and Masaki Kashiwara

of the abelian category of ﬁnite-dimensional HAn-modules, and KA=n0KAn. Then it has a structure of Hopf algebra by the restriction and the induction.

The set I = C may be regarded as a Dynkin diagram with I as the set of vertices and with edges betweena∈Iandap21. Herep1is the parameter of the aﬃne Hecke algebra usually denoted byq. LetgI be the associated Lie algebra, andgI the unipotent Lie subalgebra. LetUI be the group associated to gI. Hence gI is isomorphic to a direct sum of copies of A(1)−1 if p21 is a primitive -th root of unity and to a direct sum of copies of gl if p1 has an inﬁnite order. ThenCKAis isomorphic to the algebraO(UI) of regular functions on UI. LetUq(gI) be the associated quantized enveloping algebra. ThenUq(gI) has an upper global basis{Gup(b)}b∈B(∞). By specializing

C[q, q−1]Gup(b) at q = 1, we obtain O(UI). Then the LLTA-theory says that the elements associated to irreducible HA-modules corresponds to the image of the upper global basis.

In [EK], we gave analogous conjectures for aﬃne Hecke algebras of type B. In the type B case, we have to replaceUq(gI) and its upper global basis with symmetric crystals (see§2.3). It is roughly stated as follows. Let HBn be the aﬃne Hecke algebra of type B of degree n. Let KBn be the Grothendieck group of the abelian category of ﬁnite-dimensional modules over HBn, and KB=

n0KBn. Then KB has a structure of a Hopf bimodule over KA. The group UI has the anti-involutionθinduced by the involutiona→a−1ofI=C. Let UIθ be the θ-ﬁxed point set of UI. Then O(UIθ) is a quotient ring of O(UI).

The action ofO(UI)CKA onCKB, in fact, descends to the action of O(UIθ).

We introduce Vθ(λ) (see§2.3), a kind of the q-analogue of O(UIθ). The conjecture in [EK] is then:

(i) Vθ(λ) has a crystal basis and a global basis.

(ii) KBis isomorphic to a specialization ofVθ(λ) atq= 1 as anO(UI)-module, and the irreducible representations correspond to the upper global basis ofVθ(λ) atq= 1.

Remark. In [KM], Miemietz and the second author gave an analogous conjecture for the aﬃne Hecke algebras of type D.

In the present paper, we prove thatVθ(λ) has a crystal basis and a global basis forg=gl andλ= 0.

More precisely, letI=Zodd be the set of odd integers. Letαi (i∈I) be

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the simple roots with

(αi, αj) =







2 ifi=j,

1 ifi=2, 0 otherwise.

Letθ be the involution ofI given byθ(i) =−i. Let Bθ(gl) be the algebra overK:=Q(q) generated byEi,Fi, and invertible elementsTi(i∈I) satisfying the following deﬁning relations:

(i) theTi’s commute with each other, (ii) Tθ(i)=Ti for anyi,

(iii) TiEjTi−1=qiθ(i)j)Ej andTiFjTi−1=qiθ(i),−αj)Fj fori, j∈I, (iv) EiFj=q−(αij)FjEi+ (δi,j+δθ(i),jTi) fori, j∈I,

(v) theEi’s and theFi’s satisfy the Serre relations (see Deﬁnition 2.1 (4)).

Then there exists a unique irreducibleBθ(gl)-moduleVθ(0) with a generator φsatisfyingEiφ= 0 andTiφ=φ(Proposition 2.11). We deﬁne the endomor- phismsEi andFi ofVθ(0) by

Eia=

n1

Fi(n−1)an, Fia=

n0

fi(n+1)an, when writing

a=

n0

Fi(n)an withEian= 0.

Here Fi(n) = Fin/[n]! is the divided power. Let A0 be the ring of functions a∈K which do not have a pole at q= 0. Let Lθ(0) be theA0-submodule of Vθ(0) generated by the elementsFi1· · ·Fiφ(0,i1, . . . , i∈I). Let Bθ(0) be the subset ofLθ(0)/qLθ(0) consisting of theFi1· · ·Fiφ’s. In this paper, we prove the following theorem.

Theorem (Theorem 4.15).

(i) FiLθ(0)⊂Lθ(0) andEiLθ(0)⊂Lθ(0), (ii) Bθ(0) is a basis ofLθ(0)/qLθ(0),

(iii) FiBθ(0)Bθ(0), andEiBθ(0)Bθ(0) {0},

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Naoya Enomoto and Masaki Kashiwara

(iv) FiEi(b) =bfor anyb∈Bθ(0)such thatEib= 0, andEiFi(b) =bfor any b∈Bθ(0).

By this theorem, Bθ(0) has a similar structure to the crystal structure.

Namely, we have operators Fi: Bθ(0) Bθ(0) and Ei: Bθ(0)Bθ(0) {0}, which satisfy (iv). Moreover εi(b) := max

n∈Z0|Einb∈Bθ(0) is ﬁnite.

We call it thesymmetric crystalassociated with (I, θ). Contrary to the usual crystal case,Eθ(i)b may coincide withEibin the symmetric crystal case.

Letbe the bar operator ofVθ(0). Namely,is a unique endomorphism of Vθ(0) such that φ= φ, av = ¯av¯ and Fiv =Fiv¯ fora K and v Vθ(0).

Here ¯a(q) = a(q−1). Let Vθ(0)A be the smallest submodule of Vθ(0) over A:=Q[q, q−1] such that it containsφand is stable by theFi(n)’s.

Then we prove the existence of global basis:

Theorem (Theorem 5.5).

(i) For any b∈ Bθ(0), there exists a unique Glowθ (b)∈Vθ(0)A∩Lθ(0) such thatGlowθ (b) = Glowθ (b)andb= Glowθ (b) modqLθ(0),

(ii) {Glowθ (b)}b∈Bθ(0) is a basis of theA0-moduleLθ(0), theA-moduleVθ(0)A and the K-vector space Vθ(0).

We call Glowθ (b) thelower global basis. The Bθ(gl)-module Vθ(0) has a unique symmetric bilinear form (, ) such that (φ, φ) = 1 and Ei andFi are transpose to each other. The dual basis to {Glowθ (b)}b∈Bθ(0) with respect to (,) is called anupper global basis.

Let us explain the strategy of our proof of these theorems. We ﬁrst con- struct a PBW type basis{Pθ(m)φ}m ofVθ(0) parametrized by theθ-restricted multisegments m. Then, we explicitly calculate the actions of Ei and Fi in terms of the PBW basis{Pθ(m)φ}m. Then, we prove that the PBW basis gives a crystal basis by the estimation of the coeﬃcients of these actions. For this we use a criterion for crystal bases (Theorem 4.1).

§2. General Deﬁnitions and Conjectures

§2.1. Quantized universal enveloping algebras and its reduced q-analogues

We shall recall the quantized universal enveloping algebraUq(g). LetIbe an index set (for simple roots), andQthe freeZ-module with a basisi}i∈I.

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Let (,) :Q×Q→Zbe a symmetric bilinear form such that (αi, αi)/2Z>0

for anyi and (αi, αj)Z0 for i=j where αi := 2αi/(αi, αi). Letq be an indeterminate and set K:=Q(q). We deﬁne its subrings A0, A and A as follows.

A0={f K|f is regular atq= 0}, A={f K|f is regular atq=∞},

A=Q[q, q−1].

Deﬁnition 2.1. The quantized universal enveloping algebra Uq(g) is theK-algebra generated by elements ei, fi and invertible elements ti (i I) with the following deﬁning relations.

(1) Theti’s commute with each other.

(2) tjeit−1j =qji)ei and tjfit−1j =q−(αji)fi for any i, j∈I. (3) [ei, fj] =δijti−t−1i

qi−qi−1 fori, j∈I. Hereqi:=qii)/2. (4) (Serre relation) Fori=j,

b k=0

(1)ke(k)i eje(b−k)i = 0, b k=0

(1)kfi(k)fjfi(b−k)= 0. Here b= 1(αi, αj) and

e(k)i =eki/[k]i!, fi(k)=fik/[k]i!,

[k]i = (qki −q−ki )/(qi−q−1i ), [k]i! = [1]i· · ·[k]i.

Let us denote byUq(g) (resp.Uq+(g)) theK-subalgebra ofUq(g) generated by thefi’s (resp. the ei’s).

Letei andei be the operators onUq(g) (see [K1, 3.4]) deﬁned by [ei, a] = (eia)ti−t−1i eia

qi−q−1i (a∈Uq(g)).

These operators satisfy the following formulas similar to derivations:

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Naoya Enomoto and Masaki Kashiwara

Note that in [K1], the operatorei was deﬁned. It satisﬁesei =−◦ei◦−, while ei satisﬁes ei =∗ ◦ei◦ ∗. They are related byei = Ad(ti)◦ei.

The algebraUq(g) has a unique symmetric bilinear form (,) such that (1,1) = 1 and

(eia, b) = (a, fib) for anya, b∈Uq(g).

It is non-degenerate and satisﬁes (eia, b) = (a, bfi). The left multiplication of fj, ei andei have the commutation relations

eifj =q−(αij)fjei+δij, eifj =fjei +δijAd(ti), and both theei’s and theei’s satisfy the Serre relations.

Deﬁnition 2.2. The reducedq-analogueB(g) ofgis theK-algebra gen- erated byei andfi.

§2.2. Review on crystal bases and global bases

Sinceeiandfi satisfy theq-boson relation, any elementa∈Uq(g) can be uniquely written as

a=

n0

fi(n)an witheian= 0.

Herefi(n)= fin [n]i!.

Deﬁnition 2.3. We deﬁne the modiﬁed root operators ei and fi on Uq(g) by

eia=

n1

fi(n−1)an, fia=

n0

fi(n+1)an. Theorem 2.4([K1]). We deﬁne

L() =

0, i1,...,i∈I

A0f˜i1· · ·f˜i ·1⊂Uq(g), B() =

f˜i1· · ·f˜i·1 modqL()|0, i1,· · ·, i∈I ⊂L()/qL().

Then we have

(i) eiL()⊂L() andfiL()⊂L(), (ii) B()is a basis of L()/qL(),

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(iii) fiB()B()andeiB()B()∪ {0}. We call(L(),B())the crystal basisof Uq(g).

Let be the automorphism of K sending q to q−1. Then A0 coincides withA.

Let V be a vector space overK,L0 anA0-submodule ofV, L anA- submodule, andVA anA-submodule. SetE:=L0∩L∩VA.

Deﬁnition 2.5 ([K1], [K2, 2.1]). We say that (L0, L, VA) isbalanced if each ofL0,L and VA generatesV as a K-vector space, and if one of the following equivalent conditions is satisﬁed.

(i) E→L0/qL0is an isomorphism, (ii) E→L/q−1Lis an isomorphism,

(iii) (L0∩VA)(q−1L∩VA)→VA is an isomorphism,

(iv) A0QE →L0,AQE→L, AQE →VA and KQE →V are isomorphisms.

Letbe the ring automorphism ofUq(g) sendingq,ti,ei,fito q−1,t−1i , ei,fi.

Let Uq(g)A be theA-subalgebra ofUq(g) generated by e(n)i , fi(n) and ti. Similarly we deﬁneUq(g)A.

Theorem 2.6. (L(), L(), Uq(g)A)is balanced.

Let

Glow:L()/qL()−→E :=L()∩L()∩Uq(g)A be the inverse ofE−→L ()/qL(). Then

Glow(b)|b∈B()

forms a basis ofUq(g). We call it a (lower)global basis. It is ﬁrst introduced by G. Lusztig ([L]) under the name of “canonical basis” for the A, D, E cases.

Deﬁnition 2.7. Let

{Gup(b)|b∈B()} be the dual basis of

Glow(b)|b∈B()

with respect to the inner product (,). We call it the upper global basis ofUq(g).

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Naoya Enomoto and Masaki Kashiwara

§2.3. Symmetric crystals

Let θ be an automorphism of I such that θ2 = id and (αθ(i), αθ(j)) = (αi, αj). Hence it extends to an automorphism of the root latticeQbyθ(αi) = αθ(i), and induces an automorphism ofUq(g).

Deﬁnition 2.8. LetBθ(g) be the K-algebra generated by Ei, Fi, and invertible elementsTi(i∈I) satisfying the following deﬁning relations:

(i) theTi’s commute with each other, (ii) Tθ(i)=Ti for anyi,

(iii) TiEjTi−1=qiθ(i)j)Ej andTiFjTi−1=qiθ(i),−αj)Fj fori, j∈I, (iv) EiFj=q−(αij)FjEi+ (δi,j+δθ(i),jTi) fori, j∈I,

(v) theEi’s and theFi’s satisfy the Serre relations (Deﬁnition 2.1 (4)).

We setEi(n)=Ein/[n]i! andFi(n)=Fin/[n]i!.

Lemma 2.9. IdentifyingUq(g)with the subalgebra ofBθ(g)by the mor- phismfi →Fi, we have

Tia=

Eia=

Ei+eia+

(2.3)

fora∈Uq(g).

Proof. The ﬁrst relation is obvious. In order to prove the second, it is enough to show that ifasatisﬁes (2.3), thenfjasatisﬁes (2.3). We have

Ei(fja) = (q−(αij)fjEi+δi,j+δθ(i),jTi)a

=q−(αij)fj(

Ei+eia+

= (

Ei+ei(fja) +

The following lemma can be proved in a standard manner and we omit the proof.

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Lemma 2.10. Let K[Ti±;i I] be the commutative K-algebra gener- ated by invertible elements Ti (i I) with the deﬁning relations Tθ(i) = Ti. Then the mapUq(g)K[Ti±;i∈I]⊗Uq+(g)→ Bθ(g)induced by the multipli- cation is bijective.

Let λ∈P+:={λ∈Hom(Q,Q)| αi, λ ∈Z0 for anyi∈I} be a domi- nant integral weight such thatθ(λ) =λ.

Proposition 2.11.

(i) There exists aBθ(g)-moduleVθ(λ)generated by a non-zero vectorφλsuch that

(a) Eiφλ= 0 for any i∈I, (b) Tiφλ=qi,λ)φλ for any i∈I,

(c) {u∈Vθ(λ)|Eiu= 0for any i∈I}=Kφλ.

Moreover such a Vθ(λ)is irreducible and unique up to an isomorphism.

(ii) there exists a unique symmetric bilinear form (,) on Vθ(λ) such that (φλ, φλ) = 1and (Eiu, v) = (u, Fiv)for any i∈I andu, v ∈Vθ(λ), and it is non-degenerate.

Remark 2.12. Set Pθ = {µ∈P|θ(µ) =µ}. Then Vθ(λ) has a weight decomposition

Vθ(λ) =

µ∈Pθ

Vθ(λ)µ, where Vθ(λ)µ =

u∈Vθ(λ)|Tiu=qi,µ)u

. We say that an element u of Vθ(λ) has aθ-weightµand write wtθ(u) =µifu∈Vθ(λ)µ. We have wtθ(Eiu) = wtθ(u) + (αi+αθ(i)) and wtθ(Fiu) = wtθ(u)(αi+αθ(i)).

In order to prove Proposition 2.11, we shall construct twoBθ(g)-modules, analogous to Verma modules and dual Verma modules.

Lemma 2.13. LetUq(g)φλbe a freeUq(g)-module with a generatorφλ. Then the following action gives a structure of aBθ(g)-module on Uq(g)φλ :





(2.4)

for anyi∈I anda∈Uq(g).

Moreover Bθ(g)/

i∈I

(Bθ(g)Ei+Bθ(g)(Ti−qi,λ)))→Uq(g)φλ is an iso- morphism.

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Naoya Enomoto and Masaki Kashiwara

Proof. We can easily check the deﬁning relations in Deﬁnition 2.8 except the Serre relations for theEi’s.

Fori=j∈I, setS =b

n=0(1)nEi(n)EjEi(b−n)whereb= 1− hi, αj. It is enough to show that the action ofSonUq(g)φλ is equal to 0. We can easily check thatSFk=q−(bαijk)FkS. Sinceλ= 0, we haveSUq(g)φλ= 0.

HenceUq(g)φλ has aBθ(g)-module structure.

The last statement is obvious.

Lemma 2.14. Let Uq(g)φλ be a free Uq(g)-module with a generator φλ. Then the following action gives a structure of aBθ(g)-module onUq(g)φλ:





Fi(λ) =

φλ (2.5)

for anyi∈I anda∈Uq(g). Moreover, there exists a non-degenerate bilinear form,: Uq(g)φλ×Uq(g)φλKsuch thatFiu, v=u, Eiv,Eiu, v= u, Fiv,Tiu, v=u, Tivforu∈Uq(g)φλ andv∈Uq(g)φλ, andφλ, φλ= 1.

Proof. There exists a unique symmetric bilinear form (,) on Uq(g) such that (1,1) = 1 and fi and ei are transpose to each other. Let us deﬁne ,:Uq(g)φλ×Uq(g)φλKbyλ, bφλ= (a, b) fora∈Uq(g) andb∈ Uq(g). Then we can easily checkFiu, v=u, Eiv,Tiu, v=u, Tiv. Since ei is transpose to the right multiplication of fi, we have Eiu, v =u, Fiv. Hence the action of Ei, Fi, Ti on Uq(g)φλ satisfy the deﬁning relations in Deﬁnition 2.8.

Proof of Proposition2.11. SinceEiφλ= 0 andφλhas aθ-weightλ, there exists a uniqueBθ(g)-linear morphismψ:Uq(g)φλ →Uq(g)φλ sendingφλ to φλ. LetVθ(λ) be its imageψ(Uq(g)φλ).

(i) (c) follows from

u∈Uq(g)|eiu= 0 for any i

=K andUq(g)φλ Vθ(λ). The other properties (a), (b) are obvious. Let us show that Vθ(λ) is irreducible. LetS be a non-zeroBθ(g)-submodule. ThenScontains a non-zero vector v such that Eiv = 0 for any i. Then (c) implies that v is a constant multiple ofφλ. HenceS=Vθ(λ).

Let us prove (ii). Foru, u ∈Uq(g)φλ, set ((u, u)) =u, ψ(u). Then it is a bilinear form onUq(g)φλ which satisﬁes

((φλ, φλ)) = 1, ((Fiu, u)) = ((u, Eiu)), ((Eiu, u)) = ((u, Fiu)), and ((Tiu, u)) = ((u, Tiu)).

(2.6)

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It is easy to see that a bilinear form which satisﬁes (2.6) is unique. Since ((u, u)) also satisﬁes (2.6), ((u, u)) is a symmetric bilinear form on Uq(g)φλ. Sinceψ(u) = 0 implies ((u, u)) = 0, ((u, u)) induces a symmetric bilinear form onVθ(λ). Since (,) is non-degenerate onUq(g), ((,)) is a non-degenerate symmetric bilinear form onVθ(λ).

Lemma 2.15. There exists a unique endomorphism of Vθ(λ) such thatφλ=φλ andav= ¯av¯,Fiv=Fiv¯for any a∈K andv∈Vθ(λ).

Proof. The uniqueness is obvious.

Let ξ be an anti-involution of Uq(g) such that ξ(q) = q−1 and ξ(fi) = fθ(i). Let ˜ρ be an element of Q⊗P such that ( ˜ρ, αi) = (αi, αθ(i))/2. Deﬁne c(µ) =

(µ+ ˜ρ, θ(µ+ ˜ρ))( ˜ρ, θ( ˜ρ))

/2 + (λ, µ) forµ∈P. Then it satisﬁes c(µ)−c(µ−αi) = (λ+µ, αθ(i)).

Hencec takes integral values onQ:=

ii.

We deﬁne the endomorphism Φ of Uq(g)φλby Φ(λ) =q−c(µ)ξ(a)φλ for a∈Uq(g)µ. Let us show that

Φ(Fi(λ)) =FiΦ(λ) for anya∈Uq(g).

(2.7)

Fora∈Uq(g)µ, we have Φ(Fi(λ)) = Φ

fia+qi,λ+µ)afθ(i)

φλ

=

q−c(µ−αi)ξ(a)fθ(i)+q−(αi,λ+µ)−c(µ−αθ(i))fiξ(a) φλ. On the other hand, we have

FiΦ(λ) =Fi

q−c(µ)ξ(a)φλ

=q−c(µ)

fiξ(a) +qi,λ+θ(µ))ξ(a)fθ(i)

φλ.

Therefore we obtain (2.7).

Hence Φ induces the desired endomorphism ofVθ(λ)⊂Uq(g)φλ. Hereafter we assume further that

there is noi∈I such thatθ(i) =i.

We conjecture thatVθ(λ) has a crystal basis under this assumption. This means the following. SinceEiandFisatisfy theq-boson relation, anyu∈Vθ(λ) can be

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Naoya Enomoto and Masaki Kashiwara

uniquely written asu=

n0Fi(n)un withEiun = 0. We deﬁne the modiﬁed root operatorsEi andFiby:

Ei(u) =

n1

Fi(n−1)un andFi(u) =

n0

Fi(n+1)un.

LetLθ(λ) be theA0-submodule ofVθ(λ) generated byFi1· · ·Fiφλ (0 and i1, . . . , i∈I), and let Bθ(λ) be the subset

Fi1· · ·FiφλmodqLθ(λ)|0,i1, . . . , i∈I

ofLθ(λ)/qLθ(λ).

Conjecture 2.16. For a dominant integral weightλsuch thatθ(λ) =λ, we have

(1) FiLθ(λ)⊂Lθ(λ) andEiLθ(λ)⊂Lθ(λ), (2) Bθ(λ) is a basis ofLθ(λ)/qLθ(λ),

(3) FiBθ(λ)Bθ(λ), andEiBθ(λ)Bθ(λ) {0},

(4) FiEi(b) =b for anyb∈Bθ(λ) such that Eib= 0, andEiFi(b) =bfor any b∈Bθ(λ).

As in [K1], we have

Lemma 2.17. Assume Conjecture2.16. Then we have (i) Lθ(λ) ={v∈Vθ(λ)|(Lθ(λ), v)A0},

(ii) Let (,)0 be the Q-valued symmetric bilinear form on Lθ(λ)/qLθ(λ) induced by (,). Then Bθ(λ) is an orthonormal basis with respect to (,)0.

Moreover we conjecture thatVθ(λ) has a global crystal basis. Namely we have

Conjecture 2.18. The triplet (Lθ(λ), Lθ(λ), Vθ(λ)lowA ) is balanced.

HereVθ(λ)lowA :=Uq(g)Aφλ. Its dual version is as follows.

Let us denote byVθ(λ)upA the dual space

v∈Vθ(λ)|(Vθ(λ)lowA , v)A . Then Conjecture 2.18 is equivalent to the following conjecture.

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Conjecture 2.19. (Lθ(λ), c(Lθ(λ)), Vθ(λ)upA) is balanced.

Here c is a unique endomorphism of Vθ(λ) such that c(φλ) = φλ and c(av) = ¯ac(v), c(Eiv) = Eic(v) for any a K and v Vθ(λ). We have (c(v), v) = (v,v¯) for anyv, v∈Vθ(λ).

Note thatVθ(λ)upA is the largestA-submoduleM ofVθ(λ) such thatM is invariant by theEi(n)’s andM Kφλ=Aφλ.

By Conjecture 2.19, Lθ(λ)∩c(Lθ(λ))∩Vθ(λ)upA Lθ(λ)/qLθ(λ) is an isomorphism. Let Gupθ be its inverse. Then{Gupθ (b)}b∈Bθ(λ)is a basis ofVθ(λ), which we call theupper global basisofVθ(λ). Note that{Gupθ (b)}b∈Bθ(λ)is the dual basis to{Glowθ (b)}b∈Bθ(λ)with respect to the inner product ofVθ(λ).

We shall prove these conjectures in the caseg=glandλ= 0.

§3. PBW Basis of Vθ(0) for g=gl

§3.1. Review on the PBW basis In the sequel, we set I=Zodd and

(αi, αj) =





2 fori=j,

1 for j=2, 0 otherwise,

and we consider the corresponding quantum group Uq(gl). In this case, we haveqi=q. We write [n] and [n]! for [n]i and [n]i! for short.

We can parametrize the crystal basis B() by the multisegments. We shall recall this parametrization and the PBW basis.

Deﬁnition 3.1. Fori, j∈I such that ij, we deﬁne a segmenti, j as the interval [i, j] I:=Zodd. A multisegment is a formal ﬁnite sum of segments:

m=

ij

miji, j

withmi,j Z0. We callmij the multiplicity of a segmenti, j. Ifmi,j >0, we sometimes say thati, jappears inm. We sometimes writemi,j(m) formi,j. We sometimes writeifor i, i. We denote by Mthe set of multisegments.

We denote by the zero element (or the empty multisegment) ofM.

Deﬁnition 3.2. For two segments i1, j1 and i2, j2, we deﬁne the orderingPBW by the following:

i1, j1PBWi2, j2 ⇐⇒



 j1> j2

or

j1=j2 andi1i2.

(14)

Naoya Enomoto and Masaki Kashiwara

We call this ordering thePBW-ordering.

Deﬁnition 3.3. For a multisegment m, we deﬁne the element P(m) Uq(gl) as follows.

(1) For a segmenti, j, we deﬁne the elementi, j ∈Uq(gl) inductively by i, i=fi,

i, j=i, j−2j, j −qj, ji, j−2 fori < j. (2) For a multisegmentm=

ij

miji, j, we deﬁne

P(m) =−→

i, j(mij). Here the product −→

is taken over segments appearing inm from large to small with respect to the PBW-ordering. The element i, j(mij) is the divided power of i, ji.e.

i, j(n)=







 1

[n]!i, jn forn >0,

1 forn= 0,

0 forn <0.

Hence the weight ofP(m) is equal to wt(m):=

ikjmi,jαk: tiP(m)t−1i = qi,wt(m))P(m).

Theorem 3.4([L]). The set of elements{P(m)|m∈ M}is aK-basis ofUq(gl). Moreover this is an A-basis ofUq(gl)A. We call this basis the PBW basisofUq(gl).

Deﬁnition 3.5. For two segments i1, j1 and i2, j2, we deﬁne the orderingcry by the following:

i1, j1cryi2, j2 ⇐⇒



 j1> j2

or

j1=j2 andi1i2. We call this ordering thecrystal ordering.

Example 3.6. The crystal ordering is diﬀerent from the PBW-ordering.

For example, we have1,1>cry1,1>cry1, while we have1,1>PBW

1,1>PBW 1.

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