Friday, 13 March 2009

geometric rep theory - Can the Quantum Torus be realized as a Hall Algebra?

Background



The Quantum Torus



Let q be an arbitrary complex number, and define (the algebra of) the quantum torus to be
Tq:=mathbbClanglexpm1,ypm1rangle/xyqyx


For q=1, this is the commutative ring of functions on the torus mathbbCtimestimesmathbbCtimes; hence, for general q, this is regarded as a quantization of the torus.



Hall Algebras



Consider a small abelian category A, with the property that HomA(M,N) and ExtiA(M,N) are always finite sets for any M,NinA and iinmathbbZ. Let overlineA denote the set of isomorphism classes in A, and let
H(A)=oplus[M]inoverlineAmathbbC[M]


denote the complex vector space spanned by overlineA. Endow H(A) with a multiplication by the formula
[M]cdot[N]=sqrtlangle[M],[N]rangle)sum[R]inoverlineAfracaRMN|Aut(M)||Aut(N)|[R]

where aRMN is the number of short exact sequences
0rightarrowNrightarrowRrightarrowMrightarrow0

and
langle[M],[N]rangle=sum(1)i|ExtiA(M,N)|

is the Euler form. This multiplication makes H(A) into an associate algebra called the Hall algebra of A; the proof can be found e.g. here.



Finite Fields and Quantization



The categories A appearing in the construction of a Hall algebra are usually linear over some finite field mathbbFq. Often, it is possible to simultaneously define a category Aq for each finite field mathbbFq; usually by considering modules on the mathbbFq-points of some scheme over mathbbZ. The corresponding Hall algebras H(Aq) will then usually be closely related, and can often be defined by relations that are functions in q.



The Question



I know that there are cases where an algebra is deformed by a parameter q, and then the resulting family of algebras `magically' coincides with a family of Hall algebras H(Aq) in the special cases when q is a prime power. I think this happens in the case of the Hecke algebra (discussed here), and the case of quantum universal enveloping algebras (discussed here). I somewhat understand that this is a symptom of a related convolution algebra on the scheme used to define Aq.



Is there a family of categories Aq such that the corresponding Hall algebras H(Aq) are isomorphic to the Quantum Torus Tq for all q a prime power? If so, is there a convolution algebra realization of the Quantum Torus?

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