(For this question, all matrices are real).
According to the ancient paper "Über die Darstellbarkeit einer Matrix als Produkt yon
zwei symmetrischen Matrizen, als Produkt yon zwei
alternierenden Matrizen und als Produkt yon einer symmetrisehen and einer alternieenden Matrix" by H. Stenzel in Göttingen (1922)
which I cannot really read fully, since it is in German, any square matrix can be written as a product of two symmetric matrices (one of which is non-singular). If we strengthen the conditions, such that one of the factors must be positive-(semi)definite, what can we say? Any way of characterizing square matrices which can be written as a product of a symmetric and a symmetric positive-semidefinit matrix?
If $A$ is symmetric positive-definite and $B$ is symmetric, then the product $AB$ is similar to a symmetric matrix, so has real eigenvalues. So if any square matrix could be written such, all square matrices would have real eigenvalues, which is absurd. So there must be some restriction.
A version of the paper is here.
And, any more recent references for this problem?
Best Answer
The only restriction is to be diagonalizable with real eigenvalue. For if $M=PDP^{-1}$ with $P,D$ real and $D$ diagonal, then $M=AB$ with $B=P^{-T}DP^{-1}$ symmetric and $A=PP^T$ is PSD. And conversely, such a product is similar to the symmetric matrix $A^{1/2}BA^{1/2}$, hence is diagonalizable with real eigenvalues.
About the fact that every square real matrix is the product of two Hermitian matrices (complex counterpart of what you mention), see Factorization of a real matrix into Hermitian x Hermitian. Is it stable ? .