It is interesting to compare the preceding theorem with the corresponding results for the product of two positive matrices and the product of two Hermitian matrices: a matrix is the product of two positive matrices if and only if it is similar to a positive one (cf. [l, Theorem 21); it is the product of two Hermitian matrices if and only if it is similar to a matrix with real entries, and in this case one of the Hermitian matrices may be taken to be invertible (cf. [2] or [6, Theorem 11). Other related results are [7, Propositions 2.1 and
2.31 concerning the products of a (real) symmetric matrix and a nonnegative matrix. The next corollary will be needed in Section 3 (in the proof of Proposition 3.5).