Maximal subgroups of the Mathieu group $M_{23}$ and symplectic   automorphisms of supersingular K3 surfaces
  We show that the Mathieu groups $M_{22}$ and $M_{11}$ can act on the supersingular $K3$ surface with Artin invariant 1 in characteristic 11 as symplectic automorphisms. More generally we show that all maximal subgroups of the Mathieu group $M_{23}$ with three orbits on 24 letters act on a supersingular $K3$ surface with Artin invariant 1 in a suitable characteristic. 
Shigeyuki Kondo