Let R be a ring with 1, M2(R) the ring of 2 × 2 matrices over R,
and S the Sugano quaternion ring over R. Then, S is isomorphic with
a subring of M2(R) if and only if 2 is not a zero divisor in R, and
S
∼=
M2(R) if and only if 2 is invertible in R. Moreover, if 2 is invertible
in R, then M2(R) is a Galois extension of R with an abelian inner Galois
group G of order 4. This implies that the rings of 2 × 2 matrices over
the real field and complex field are central Galois algebras induced by
the central Galois algebra of 2 × 2 matrices over the rational field with
Galois group G.