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International Journal of Mathematics and Mathematical Sciences

Volume 2010 (2010), Article ID 326247, 19 pages

http://dx.doi.org/10.1155/2010/326247

## Universal Verma Modules and the Misra-Miwa Fock Space

^{1}Department of Mathematics and Statistics, University of Melbourne, Parkville VIC 3010, Australia^{2}Department of Mathematics, MIT, 77 Massachusetts Ave, Cambridge, MA 02139, USA

Received 7 March 2010; Accepted 5 November 2010

Academic Editor: Alistair Savage

Copyright © 2010 Arun Ram and Peter Tingley. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The Misra-Miwa -deformed Fock space is a representation of the quantized affine algebra . It has a standard basis indexed by partitions, and the nonzero matrix entries of the action of the Chevalley generators with respect to this basis are powers of . Partitions also index the polynomial Weyl modules for as tends to infinity. We explain how the powers of which appear in the Misra-Miwa Fock space also appear naturally in the context of Weyl modules. The main tool we use is the Shapovalov determinant for a universal Verma module.

#### 1. Introduction

Fock space is an infinite dimensional vector space which is a representation of several important algebras, as described in, for example, [1, Chapter 14]. Here we consider the charge zero part of Fock space, which we denote by , and its -deformation . The space has a standard -basis and . Following Hayashi [2], Misra and Miwa [3] define an action of the quantized universal enveloping algebra on . The only nonzero matrix elements of the Chevalley generators in terms of the standard basis occur when is obtained by adding a single -colored box to , and these are powers of .

We show that these powers of also appear naturally in the following context: partitions with at most parts index polynomial Weyl modules for the integral quantum group . Let be the standard dimensional representation of . If the matrix element is nonzero then, for sufficiently large , contains the highest weight vector of weight. There is a unique such highest weight vector which satisfies a certain triangularity condition with respect to an integral basis of . We show that the matrix element is equal to , where is the Shapovalov form and is the valuation at the cyclotomic polynomial .

Our proof is computational, making use of the Shapovalov determinant [4–6]. This is a formula for the determinant of the Shapovalov form on a weight space of a Verma module. The necessary computation is most easily done in terms of the universal Verma modules introduced in the classical case by Kashiwara [7] and studied in the quantum case by Kamita [8]. The statement for Weyl modules is then a straightforward consequence.

Before beginning, let us discuss some related work. In [9], Kleshchev carefully analyzed the highest weight vectors in a Weyl module for and used this information to give modular branching rules for symmetric group representations. Brundan and Kleshchev [10] have explained that highest weight vectors in the restriction of a Weyl module to give information about highest weight vectors in a tensor product of a Weyl module with the standard -dimensional representation of . Our computations put a new twist on the analysis of the highest weight vectors in , as we study them in their “universal” versions and by the use of the Shapovalov determinant. Our techniques can be viewed as an application of the theory of Jantzen [11] as extended to the quantum case by Wiesner [12].

Brundan [13] generalized Kleshchev's [9] techniques and used this information to give modular branching rules for Hecke algebras. As discussed in [14, 15], these branching rules are reflected in the fundamental representation of and its crystal graph, recovering much of the structure of the Misra-Miwa Fock space. Using Hecke algebras at a root of unity, Ryom-Hansen [16] recovered the full action on Fock space. To complete the picture, one should construct a graded category, where multiplication by in the representation corresponds to a grading shift. Recent work of Brundan-Kleshchev [17] and Ariki [18] explains that one solution to this problem is through the representation theory of Khovanov-Lauda-Rouquier algebras [19, 20]. It would be interesting to explicitly describe the relationship between their category and the present work. Another related construction due to Brundan-Stroppel considers the case when the Fock space is replaced by , where is the natural module and are fixed natural numbers.

We would also like to mention very recent work of Peng Shan [21] which independently develops a similar story to the one presented here, but using representations of a quantum Schur algebra where we use representations of . The approach taken there is somewhat different and in particular relies on localization techniques of Beĭlinson and Bernstein [22].

This paper is arranged as follows. Sections 2 and 3 are background on the quantum group and the Fock space . Sections 4 and 5 explain universal Verma modules and the Shapovalov determinant. Section 6 contains the statement and proof of our main result relating Fock space and Weyl modules.

#### 2. The Quantum Group and Its Integral Form

This is a very brief review, intended mainly to fix notation. With slight modifications, the construction in this section works in the generality of symmetrizable Kac-Moody algebras. See [23, Chapters 6 and 9] for details.

##### 2.1. The Rational Quantum Group

is the associative algebra over the field of rational functions generated by with relations The algebra is a Hopf algebra with coproduct and antipode given by respectively, (see [23, Section 9.1]).

As a -vector space, has a triangular decomposition where the inverse isomorphism is given by multiplication (see [23, Proposition 9.1.3]). Here is the subalgebra generated by the for , is the subalgebra generated by the for , and is the subalgebra generated by the for .

##### 2.2. The Integral Quantum Group

Let . For and , let
in . The *restricted integral form * is the -subalgebra of generated by , , and for ,,. As discussed in [24, Section 6], this is an integral form in the sense that
As with , the algebra has a triangular decomposition
where the isomorphism is given by multiplication (see [23, Proposition 9.3.3]). In this case, is the subalgebra generated by the , is the subalgebra generated by the , and is generated by and for , , and .

##### 2.3. Rational Representations

The Lie algebra of matrices has standard basis , where is the matrix with 1 in position and 0 everywhere else. Let . Let be the weight of given by . Define
to be the set of *integral weights*, the set of *dominant integral weights*, the set of *dominant polynomial weights*, the set of *positive roots*, the *root lattice*, the *positive part of the root lattice*, and the *negative part of the root lattice*, respectively.

For an integral weight , the *Verma module * for of the highest weight is
where is the one dimensional vector space over with action given by

Theorem 2.1 (see [23, Chapter 10.1]). *If then has a unique finite dimensional quotient and the map is a bijection between , and the set of irreducible finite dimensional -modules. *

A *singular vector* in a representation of is a vector such that for all *i*.

##### 2.4. Integral Representations

The *integral Verma module * is the -submodule of generated by . The *integral Weyl module * is the -submodule of generated by . Using (2.6) and (2.4),
In general, is not irreducible as a module.

#### 3. Partitions and Fock Space

We now describe the -deformed Fock space representation of constructed by Misra and Miwa [3] following work of Hayashi [2]. Our presentation largely follows [25, Chapter 10].

##### 3.1. Partitions

A partition is a finite length nonincreasing sequence of positive integers. Associated to a partition is its Ferrers diagram. We draw these diagrams as in Figure 1 so that, if , then is the number of boxes in row (rows run southeast to northwest ). Say that is contained in if the diagram for fits inside the diagram for and let be the collection of boxes of that are not in . For each box , the *content * is the horizontal position of and the *color * is the residue of modulo . In Figure 1, the numbers are listed below the diagram. The *size * of a partition is the total number of boxes in its Ferrers diagram.

The set of dominant polynomial weights from Section 2.3 is naturally identified with partitions with at most parts. If , then as -modules. The diagram of is obtained from the diagram of by adding a box on row , and appears in the sum on the right side of (3.1) if and only if is a partition. See, for example, [26, Section 6.1, Formula 6.8] for the classical statement and [23, Proposition 10.1.16] for the quantum case.

##### 3.2. The Quantum Affine Algebra

Let be the quantized universal enveloping algebra corresponding to the -node Dynkin diagram

More precisely, is the algebra generated by , for , with relations See [23, Definition Proposition 9.1.1]. The algebra is the quantum group corresponding to the nontrivial central extension of the algebra of polynomial loops in .

##### 3.3. Fock Space

Define *-deformed Fock space* to be the vector space with basis . Our is only the charge 0 part of Fock space described in [27]. Fix and partitions such that is a single box. Define
to be the set of *addable boxes of color *, the set of *removable boxes of color *, the *left removable-addable difference*, and the *right removable-addable difference*, respectively.

Theorem 3.1 (see [25, Theorem 10.6]). *There is an action of on determined by
**
where denotes the color of and the sum is over partitions which differ from by removing (resp. adding) a single -colored box.*

As a -module, is isomorphic to an infinite direct sum of copies of the basic representation . Using the grading of where has degree , the highest weight vectors in occur in degrees divisible by , and the number of the highest weight vectors in degree is the number of partitions of . Then, , where has degree , and acts trivially on the second factor (see [27, Propositin 2.3]). Note that we are working with the “derived” quantum group , not the “full” quantum group , which is why there are no -shifts in the summands of .

*Comment 1. *Comparing with [25, Chapter 10], our is equal to Ariki's and our is equal to Ariki's . However, these numbers play a slightly different role in Ariki's work, which is explained by a different choice of conventions.

#### 4. Universal Verma Modules

The purpose of this section is to construct a family of representations which are universal Verma modules in the sense that each can be “evaluated" to obtain any given Verma module. This notion was defined by Kashiwara [7] in the classical case and was studied in the quantum case by Kamita [8].

##### 4.1. Rational Universal Verma Modules

Let . This field is isomorphic to the field of fractions of via the map For each , define a -linear automorphism by where is the inner product on defined by . Let be the one-dimensional vector space over with basis vector and action given by

The*-shifted rational universal Verma module * is the -module
The universal Verma module is actually a module over , where is the field of fractions of . However, if we identify with using the map , the action of on is not by multiplication, but rather is twisted by the automorphism . It is to keep track of the difference between the action of and multiplication that we use different notation for the generators of and (i.e., versus ).

##### 4.2. Integral Universal Verma Modules

The field contains an -subalgebra
which is isomorphic to via the restriction of the map in (4.1). The *integral universal Verma module * is the -submodule of generated by . By restricting (4.4),
where is the -submodule of spanned by . In particular, is a free -module.

##### 4.3. Evaluation

Let be the map defined by where is the inner product on defined by . There is a surjective -module homomorphism “evaluation at ”

For fixed , the maps and extend to a map from the subspace of and , respectively, where no denominators evaluate to 0. Where it is clear we denote both these extended maps by .

*Example 4.1. *Computing the action of on and ,

##### 4.4. Weight Decompositions

Let be a -module. For each , we define the *-weight space* of to be
The universal Verma module is a -module, where the second factor acts as multiplication. The weight space if and only if with in the positive part of the root lattice. These nonzero weight spaces and the weight decomposition of can be described explicitly by
Here, is defined using the grading of with .

##### 4.5. Tensor Products

Let be a -module and a -module. The tensor product is a -module, where the first factor acts via the usual coproduct and the second factor acts by multiplication on . In the case when and both have weight space decompositions, the weight spaces of are

We also need the following.

Proposition 4.2. *The tensor product of a universal Verma module with a Weyl module satisfies
*

*Proof. *Fix . In general, has a Verma filtration (see, e.g., [28, Theorem 2.2]) and if is dominant for all such that then
which can be seen by, for instance, taking central characters. The proposition follows since this is true for a Zariski dense set of weights .

#### 5. The Shapovalov Form and the Shapovalov Determinant

##### 5.1. The Shapovalov Form

The *Cartan involution * is the -algebra anti-involution of defined by
The map is also a coalgebra involution. An *-contravariant* form on a -module is a symmetric bilinear form such that

It follows by the same argument used in the classical case [4] that there is an -contravariant form (the Shapovalov form) on each Verma module and this is unique up to rescaling. The radical of is the maximal proper submodule of , so for all . In particular, descends to an -contravariant form on .

Since fixes , there is a well-defined notion of an -contravariant form on a module. In particular, the restriction of the Shapovalov form on to is -contravariant.

##### 5.2. Universal Shapovalov Forms

There are surjective maps of -algebras and defined by and , for . Using the triangular decomposition (2.7), there is an -linear surjection
The *standard universal Shapovalov form* is the -bilinear form defined by
for all . Here, and are as in (4.1) and (4.2). Since
for , the form is -contravariant. As with the usual Shapovalov form, distinct weight spaces are orthogonal, where weight spaces are defined as in Section 4.4.

Evaluation at gives an -valued -contravariant form on by The form can be extended by linearity to an -contravariant form on .

##### 5.3. The Shapovalov Determinant

Let be a -module with a chosen -contravariant form. Let be an basis for the -weight space of . Let be the determinant of the form evaluated on the basis . Changing the basis changes the determinant by a unit in , and we sometimes write to mean the determinant calculated on an unspecified basis ( which is only defined up to multiplication by unit in ). The *Shapovalov determinant* is

Define the *partition function * by
Then, for any , and implies that and .

Theorem 5.1 (see [5, Proposition 1.9A], [6, Theorem 3.4], [4]). *For any weight ,
**
where is a unit in .*

Proposition 5.2. *Fix with . Choose an -basis for . Consider the -bases for and for . Then .*

*Proof. *For ,
The result follows by taking determinants.

##### 5.4. Contravariant Forms on Tensor Products

If and are -modules with -contravariant forms and , define an -bilinear form by . Similarly, for a module with -bilinear -contravariant form , define a -bilinear form on by Since is a coalgebra involution (i.e., , for ), the forms and are -contravariant.

In the case when , evaluation of the -contravariant form at gives an -contravariant form : for and . As in Section 4.3, this form can be extended to the -submodule of the rational module where no denominators evaluate to zero.

#### 6. The Misra-Miwa Formula for from Representation Theory

Let us prepare the setting for our main result (Theorem 6.1). Fix and a partition . Let be a positive integer greater than the number of parts of . All calculations below are in terms of representations of .(1)Let be the standard -dimensional module. Since , (3.1) implies where the sum is over those indices for which is a partition. For ease of notation, let .(2)Fix an -basis of where has weight and . Recursively, define singular weight vectors in by (i)(ii)for each , the submodule of generated by contains all weight vectors of of weight greater than or equal to . Thus, using (6.1), for each there is a one-dimensional space of singular vectors of weight in , and this is not contained in (since ). This implies that there unique singular vector of weight in where we recall that . (3)There is a unique -contravariant form on normalized so that and a unique -contravariant form on normalized so that . As in Section 5.4, define a -contravariant form on by . For each , define an element by

Theorem 6.1. *The Misra-Miwa operators from Section 3.3 satisfy
**
where is the box , is the color of box as in Figure 1, is the th cyclotomic polynomial in , and is the number of factors of in the numerator of minus the number of factors of in the denominator of . *

The proof of Theorem 6.1 will occupy the rest of this section. We will first prove a similar statement, Proposition 6.6, where the role of the Weyl modules is played by the universal Verma modules from Section 4. For ease of notation, let denote the module from Section 4.2.

*Definition 6.2. *Recursively define singular weight vectors and elements for by(i),
(ii)since generates as a module, Proposition 4.2 implies that, for each , there is a unique singular vector in , where and the factor of acts by multiplication on .

Let .

The are quantized versions of the Jantzen numbers first calculated in [11, Section 5] and quantized in [12]. It follows immediately from the definition that .

Lemma 6.3. *For any weight , up to multiplication by a power of ,
**
where, as in Section 5.3, is the determinant of the Shapovalov form evaluated on an -basis for the weight space of .*

*Comment 2. *In order for Lemma 6.3 to hold as stated, for each , one must calculate the in the numerator and denominator with respect to the same -basis. The power of which appears depends on this choice of -bases.

*Proof of Lemma 6.3. *For each fix an -basis for . Consider the following three -bases for :
Let denote the determinant of calculated on , where is one of ,, or . Let denote calculated with respect to the basis .

By the definition of the -contravariant form on (see Section 4.5),
For , is one dimensional and is a power of . Hence, up to multiplication by a power of , (6.7) simplifies to

Notice that is isomorphic to , and is the union of -bases for each of these submodules. For each , and each define an basis of by
Using ,
where the last equality uses Proposition 5.2. Here, as in Section 5.3, is the Shapovalov determinant calculated with respect to the basis .

The change of basis from to is unitriangular and the change of basis from to is unitriangular. Thus, , and so the right sides of (6.8) and (6.10) are equal. The lemma follows from this equality by rearranging.

Lemma 6.4. *Up to multiplication by a power of ,
*

*Proof. *Fix . Setting in Lemma 6.3 and applying Theorem 5.1 we see that, up to multiplication by a power of ,
where, for each , is a unit in . The value is 0 unless and . If , then acts as the identity on , so the corresponding factors in the numerator and denominator cancel. Hence, we need only consider factors on the right hand side where , , and . If , then , and hence , so on the left hand since we only need to consider those factors where . Up to multiplication by a power of , the expression reduces to
The last two expressions are equal because they are each a product over pairs with , and the factors of have been dropped because they are powers of . Using the fact that and making the change of variables and on the right side, (6.13) becomes
For , the lemma now follows by induction. For , the result simply says that , which we already know.

Proposition 6.5. *Let be a partition. Let (resp. ) be the set of boxes which can be added to (resp. removed from) on rows with such that the result is still a partition. Let and let be as in Figure 1. Then, up to multiplication by a power of ,
*

*Proof. *For , let be the last box in row of . By Lemma 6.4, up to multiplication by a power of ,
where the last equality is a simple calculation from definitions. The denominator on the right side is never zero, and the numerator is zero exactly when , so that is no longer a partition. If for any , then there is cancellation, giving (6.15). See Figure 2.

Proposition 6.6. *Let be as in Section 3.3. For any partition ,
*

*Proof. *By Proposition 6.5, if is not a partition. If is a partition, then
where the notation is as in Section 3.3. Since
is divisible by if and only if is divisible by , and is never divisible by . The result now follows from Proposition 6.5.

*Proof of Theorem 6.1. *Fix and . From definitions, . Thus, using (5.12),
The result now follows from Proposition 6.6.

#### Acknowledgments

The authors thank M. Kashiwara, A. Kleshchev, T. Tanisaki, R. Virk, and B. Webster for helpful discussions. A. Ram was partly supported by the NSF Grant no. DMS-0353038 and the Australian Research Council Grants nos. DP0986774 and DP0879951. P. Tingley was partly supported by the Australia Research Council Grant no. DP0879951 and the NSF Grant no. DMS-0902649.

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