The Automorphism Group of a Self-Dual Code Does Not Contain

It has been proven in a series of works that the order of the automorphism group of a self-dual binary [72,36,16] code does not exceed five. Up to equivalence, we obtain a parametrization of all self-dual binary codes of length 72 with automorphism of order four, which can be extremal. We use extensive computations in Magma and on a supercomputer to show that the automorphism group of an extremal self-dual binary code of length 72 does not have an element of order four.