The local obstruction to semi-stable reduction for abelian varieties

Grothendieck defined a group that represents the local obstruction for an abelian variety to have semi-stable reduction. These groups were studied by Silverberg and Zarhin and more recently by the author in order to give a group theoretic characterization of them depending only on the dimension. We give an overview of the developments since Grothendieck’s definition with the added novelty of the case of equal characteristic local fields.