Generalized nil-Coxeter algebras, cocommutative algebras, and the PBW property

Abstract: Poincare-Birkhoff-Witt (PBW) Theorems have attracted significant attention since the work of Drinfeld (1986), Lusztig (1989), and Etingof-Ginzburg (2002) on deformations of skew group algebras $H \ltimes {\rm Sym}(V)$, as well as for other cocommutative Hopf algebras $H$. In this paper we show that such PBW theorems do not require the full Hopf algebra structure, by working in the more general setting of a "cocommutative algebra", which involves a coproduct but not a counit or antipode. Special cases include infinitesimal Hecke algebras, as well as symplectic reflection algebras, rational Cherednik algebras, and more generally, Drinfeld orbifold algebras. In this generality we identify precise conditions that are equivalent to the PBW property, including a Yetter-Drinfeld type compatibility condition and a Jacobi identity. We also characterize the graded deformations that possess the PBW property. In turn, the PBW property helps identify an analogue of symplectic reflections in general cocommutative bialgebras. Next, we introduce a family of cocommutative algebras outside the traditionally studied settings: generalized nil-Coxeter algebras. These are necessarily not Hopf algebras, in fact, not even (weak) bialgebras. For the corresponding family of deformed smash product algebras, we compute the center as well as abelianization, and classify all simple modules.