Embedding theorems for non-positively curved polygons of finite groups

AbstractThe fundamental groups of complete squared complexes are a class of groups, some of which are not residually finite. A method is given for embedding the fundamental group of a complete squared complex as a subgroup of a square of finite groups, all of whose (Gersten—Stallings) vertex angles are ≤ π/2. It is also shown that every square of finite groups, all of whose vertex angles are ≤ π/2, can be embedded in a non-positively curved triangle of finite groups. In this way, a non-residually finite, non-positively curved triangle of finite groups is obtained