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The generic Jordan Hall regular module carries the normalized Heisenberg Fock action
Statement
Use the category of finite-dimensional nilpotent Jordan-quiver representations from Nilpotent representations of the Jordan quiver. Let be an indeterminate, , and be the generic Jordan-quiver Hall algebra extended to . Extend the exact isomorphism of The generic Jordan-quiver Hall algebra is the symmetric-function ring to ; its regular module is identified with . For a partition , let be the multiplicity of , set and , and put . Write for the partition obtained by removing one part when ; define when . Transport the Jordan Green pairing of Jordan Hall classes are Hall–Littlewood functions with the transported Green pairing to . On power sums it is where if and otherwise, and the product runs over the parts of with multiplicity. For , let be multiplication by and define Then is the adjoint of for , , and for each finite the assignments , , and give a representation of the finite Heisenberg Lie algebra from Finite-dimensional Heisenberg Lie algebra. The constant is the vacuum: , and the vectors , with nonnegative integer exponents, span the subalgebra ; as varies, they span .
Facts & Assumptions
Given: The generic Jordan Hall algebra over , the power-sum basis of , its transported Green pairing, and the finite Heisenberg Lie algebra.
The generic Jordan-quiver Hall algebra is the symmetric-function ring: the generic Hall algebra is a free polynomial algebra and extends to a graded algebra isomorphism with the symmetric-function ring.
Jordan Hall classes are Hall–Littlewood functions with the transported Green pairing: on power sums the transported Green pairing is diagonal with , and it is nondegenerate after extension to .
Power sums form a rational but not integral stable basis: and the monomials form a basis.
Finite-dimensional Heisenberg Lie algebra and The Heisenberg bracket satisfies the Lie algebra axioms: has generators with and all same-type and central brackets zero, and these relations define a Lie algebra.
A unital Hall algebra is a left module over itself: a unital associative Hall algebra acts on its underlying -module by left multiplication.
Proof
By [F1] and [F5], scalar extension of the generic Hall algebra acts on its regular module by multiplication; extending the isomorphism [F1] from to identifies this action with multiplication by when the Hall element acts. Thus is the transported regular action. By [F2]–[F3], the power sums form an orthogonal basis with nonzero norms, since and in for every part . For partitions , both adjoint pairings vanish unless is obtained by adjoining one part to ; in that case, if , then and . Thus the displayed operator is the adjoint on every pair of basis elements, hence on all of .
The formula in the Statement gives , the formal derivative by [F3], with value zero when . Therefore the commute, the commute, and .
For any and basis monomial , the polynomial product rule gives . Since the form a basis by [F3], . Together with the commuting relations of step 1.2 these are precisely the generator brackets in [F4].
For , send in to multiplication by , , and , respectively. Step 2.1 and the commuting relations in step 1.2 preserve every defining bracket, so the assignments give a Lie algebra representation. For any nonnegative integers , the monomial is ; hence these vectors span and, as varies, generate .
Remarks
The adjoint used here is the Jordan Green pairing from [F2]. It is not the ordinary Hall inner product defined in The Hall inner product on symmetric functions, whose power-sum norms are computed in Power sums are orthogonal for the Hall form. The scalar normalization of the positive modes accounts for the difference.
Depends on
- Modules over a Hall algebra are unital algebra representations
- Nilpotent representations of the Jordan quiver
- A unital Hall algebra is a left module over itself
- The generic Jordan-quiver Hall algebra is the symmetric-function ring
- Jordan Hall classes are Hall–Littlewood functions with the transported Green pairing
- Power sums form a rational but not integral stable basis
- Finite-dimensional Heisenberg Lie algebra
- The Heisenberg bracket satisfies the Lie algebra axioms
- The Hall inner product on symmetric functions
- Power sums are orthogonal for the Hall form
Used by
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Sources
- Olivier Schiffmann, Lectures on Hall Algebras, arXiv:math/0611617v2, §§2.3–2.4 (standard reference, not scraped)
- I. G. Macdonald, Symmetric Functions and Hall Polynomials, 2nd ed., Chapter I §4, equation (4.5) and Remark 8 (standard reference, not scraped)