The fixed point theorem for simplicial nonpositive curvature

We prove that for an action of a finite group G on a systolic complex X there exists a G–invariant subcomplex of X of diameter 5. For 7–systolic locally finite complexes we prove there is a fixed point for the action of any finite G. This implies that free products with amalgamation (and HNN extensions) of 7–systolic groups over finite subgroups are also 7–systolic. 1