In mathematics, a group is said to be almost simple if it contains a non-abelian simple group and is contained within the automorphism group of that simple group – that is, if it fits between a (non-abelian) simple group and its automorphism group. In symbols, a group is almost simple if there is a (non-abelian) simple group S such that , where the inclusion of in is the action by conjugation, which is faithful since is has trivial center.^[1]

• Trivially, non-abelian simple groups and the full group of automorphisms are almost simple. For or the symmetric group is the automorphism group of the simple alternating group so is almost simple in this trivial sense.
• For there is a proper example, as sits properly between the simple and due to the exceptional outer automorphism of Two other groups, the Mathieu group and the projective general linear group also sit properly between and

The full automorphism group of a non-abelian simple group is a complete group (the conjugation map is an isomorphism to the automorphism group),^[2] but proper subgroups of the full automorphism group need not be complete.

By the Schreier conjecture, now generally accepted as a corollary of the classification of finite simple groups, the outer automorphism group of a finite simple group is a solvable group. Thus a finite almost simple group is an extension of a solvable group by a simple group.

• Quasisimple group
• Semisimple group

• Almost simple group at the Group Properties wiki