Realizing enveloping algebras via moduli stacks

preprint2015arXivOpen access

Abstract

Let CF(ObjA)\mathop{\rm CF}\nolimits(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) denote the vector space of Q\mathbb{Q}-valued constructible functions on a given stack ObjA\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A} for an exact category A\mathcal{A}. By using the Ringel--Hall algebra approach, Joyce proved that CF(ObjA)\mathop{\rm CF}\nolimits(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) is an associative Q\mathbb{Q}-algebra via the convolution multiplication and the subspace CFindObjA)\mathop{\rm CF}\nolimits^{\rm ind}\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) of constructible functions supported on indecomposables is a Lie subalgebra of CF(ObjA)\mathop{\rm CF}\nolimits(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) in [10]. In this paper, we show that there is a subalgebra CFKS(ObjA)\mathop{\rm CF}\nolimits^{\text{KS}}(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) of CF(ObjA)\mathop{\rm CF}\nolimits(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) isomorphic to the universal enveloping algebra of CFind(ObjA)\mathop{\rm CF}\nolimits^{\rm ind}(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}). Moreover we construct a comultiplication on CFKS(ObjA)\mathop{\rm CF}\nolimits^{\text{KS}}(\mathop{\mathfrak{Obj}\kern .05em}\nolimits_\mathcal{A}) and a degenerate form of Green's theorem. This generalizes Joyce's work, as well as results of [3].

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