Modular Representations of Truncated current Lie algebras

In this paper we consider the structure and representation theory of truncated current algebras $\mathfrak{g}_m = \mathfrak{g}[t]/(t^{m+1})$ associated to the Lie algebra $\mathfrak{g}$ of a standard reductive group over a field of positive characteristic. We classify semisimple and nilpotent elements and describe their associated support varieties. Next, we prove various Morita equivalences for reduced enveloping algebras, including a reduction to nilpotent $p$-characters, analogous to a famous theorem of Friedlander--Parshall. We go on to give precise upper bounds for the dimensions of simple modules for all $p$-characters, and give lower bounds on these dimensions for homogeneous $p$-characters. We then develop the theory of baby Verma modules for homogeneous $p$-characters and, whenever the $p$-character has standard Levi type, we give a full classification of the simple modules. In particular we classify all simple modules with homogeneous $p$-characters for $\mathfrak{g}_m$ when $\mathfrak{g} = \mathfrak{gl}_n$. Finally, we compute the Cartan invariants for the restricted enveloping algebra $U_0(\mathfrak{g}_m)$ and show that they can be described by precise formulae depending on decomposition numbers for $U_0(\mathfrak{g})$.