RéGA
# Matilde Maccan - 15h45 - Algebraic groups and their actions: a few anecdotes in positive characteristic

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Salle Grisvard, IHP, Paris
#### Salle Grisvard, IHP, Paris

Description

By a surprising construction due to Chevalley, simple algebraic groups and their root systems are defined over the integers. Over algebraically closed fields,

one might thus think that the geometry of homogeneous varieties does not depend much on the characteristic. However, this is far from being true: in positive

characteristics, the behavior is often more pathological (if one is pessimistic) or richer (if one is optimistic). In both cases, it can be useful to keep in

mind a few examples; among them, we will introduce infinitesimal groups, non-Fano rational homogeneous varieties, and very special isogenies.