## Workshop on Pseudo-reductive groups at Newcastle University

### 2017 09 09 - [math] [algebraic-groups] [workshop] My colleague David
Stewart organized a
workshop on Pseudo-reductive
groups in
September 2017, which was partially funded by the Heilbronn Institute.

In this workshop, Gopal Prasad gave a mini-course on his work with
Conrad and Gabber on pseudo-reductive groups.

I contributed a lecture on *Reductive subgroups of parahoric group schemes*.
Here is the abstract for my talk:

Let K be the field of fractions of a complete discrete valuation ring
A with residue field k, and let G be a connected reductive algebraic
group over K. Suppose P is a parahoric group scheme attached to G. In
particular, P is a smooth affine A-group scheme having generic fiber
P_K = G; the group scheme P is in general not reductive over A. Assume
that G splits over an unramified extension of K.

The talk will give an overview of two results about G.

First, there is a closed and reductive A-subgroup scheme M of P for
which the special fiber M_k is a Levi factor of P_k. Moreover, the
reductive subgroups of G=P_K of the form M_K may be characterized.

Second, let X be a nilpotent section in Lie(P). We say that X is
balanced if the fibers C_K and C_k are smooth group schemes of the
same dimension, where C=C_P(X) is the scheme theoretic centralizer of
X. If X_0 is a given nilpotent element in the Lie algebra of the
reductive quotient of the special fiber P_k, we give results on the
possible *lifts* of X_0 to a balanced nilpotent section X of Lie(P).

My colleague David Stewart organized a workshop on Pseudo-reductive groups in September 2017, which was partially funded by the Heilbronn Institute.

In this workshop, Gopal Prasad gave a mini-course on his work with Conrad and Gabber on pseudo-reductive groups.

I contributed a lecture on *Reductive subgroups of parahoric group schemes*.
Here is the abstract for my talk:

Let K be the field of fractions of a complete discrete valuation ring A with residue field k, and let G be a connected reductive algebraic group over K. Suppose P is a parahoric group scheme attached to G. In particular, P is a smooth affine A-group scheme having generic fiber P_K = G; the group scheme P is in general not reductive over A. Assume that G splits over an unramified extension of K.

The talk will give an overview of two results about G.

First, there is a closed and reductive A-subgroup scheme M of P for which the special fiber M_k is a Levi factor of P_k. Moreover, the reductive subgroups of G=P_K of the form M_K may be characterized.

Second, let X be a nilpotent section in Lie(P). We say that X is balanced if the fibers C_K and C_k are smooth group schemes of the same dimension, where C=C_P(X) is the scheme theoretic centralizer of X. If X_0 is a given nilpotent element in the Lie algebra of the reductive quotient of the special fiber P_k, we give results on the possible

liftsof X_0 to a balanced nilpotent section X of Lie(P).