The lower central series invariants M k of an associative algebra A are the two-sided ideals generated by k-fold iterated commutators; the M k provide a filtration of A. We study the relationship between the geometry of X = Spec A ab and the associated graded components N k of this filtration. We show that the N k form coherent sheaves on a certain nilpotent thickening of X, and that Zariski localization on X coincides with noncommutative localization of A. Under certain freeness assumptions on A, we give an alternative construction of N k purely in terms of the geometry of X (and in particular, independent of A). Applying a construction of Kapranov, we exhibit the N k as natural vector bundles on the category of smooth schemes.
We present a criterion for establishing Morita equivalence of monoidal categories, and apply it to the categorical representation theory of reductive groups G. We show that the "de Rham group algebra" DpGq (the monoidal category of D-modules on G) is Morita equivalent to the universal Hecke category DpN zG{N q and to its monodromic variant r DpBzG{Bq. In other words, de Rham G-categories, i.e., module categories for DpGq, satisfy a "highest weight theorem" -they all appear in the decomposition of the universal principal series representation DpG{N q or in twisted D-modules on the flag variety r DpG{Bq.
This paper develops the tools of formal algebraic geometry in the setting of noncommutative manifolds, roughly ringed spaces locally modeled on the free associative algebra. We define a notion of noncommutative coordinate system, which is a principal bundle for an appropriate group of local coordinate changes. These bundles are shown to carry a natural flat connection with properties analogous to the classical Gelfand-Kazhdan structure. Every noncommutative manifold has an underlying smooth variety given by abelianization. A basic question is existence and uniqueness of noncommutative thickenings of a smooth variety, i.e., finding noncommutative manifolds abelianizing to a given smooth variety. We obtain new results in this direction by showing that noncommutative coordinate systems always arise as reductions of structure group of the commutative bundle of coordinate systems on the underlying smooth variety; this also explains a relationship between D-modules on the commutative variety and sheaves of modules for the noncommutative structure sheaf.
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