Isomorphisms of the linear groups GL2(R) over associative rings R

Isomorphisms of the linear groups GL2(R) over associative rings R with 1/2 and 1/3 are considered. In particular, we give a full description of automorphisms φ:GL2(R)→GL2(R), where R is any commutative associative ring with 1 and 1/2, 3 is non-zero-divisor, and R is generated by invertible elements.