Published online by Cambridge University Press: 24 October 2008
1. Mazur(1) has shown that any normed algebra A over the real field in which the norm is multiplicative in the sense that
is equivalent (i.e. algebraically isomorphic and isometric under one and the same mapping) to one of the following algebras: (i) the real numbers, (ii) the complex numbers, (iii) the real quaternions, each of these sets being regarded as normed algebras over the real field. Completeness of A is not assumed by Mazur. A relevant discussion is given also in Lorch (2).