# Matrix equation with positive semi-definite matrixes

Consider a matrix question as $Q_{m \times m}X_{m \times n} S_{n \times n} = Z_{m \times n}$ where we know that $Q$ and $S$ are symmetric positive semi-definite parametric matrices. Does there exist any $X_{m \times n}$ matrix satisfying this equation (at least one)?

I don't even know how to start. Can someone give some hints?

Thanks

$U^+$ denotes the Moore-Penrose inverse of $U$. Let $Y=XS$. Then Part 1: $QY=Z$; if $QQ^+Z\not= Z$, then no solutions. Otherwise $Y=Q^+Z+(I-Q^+Q)W$ where $W$ is an arbitrary $m\times n$ matrix.

Part 2. One has $Y=XS$. If $YS^+S\not= Y$, then no solutions. Otherwise $X=YS^++V(I-SS^+)$, where $V$ is an arbitrary $m\times n$ matrix.

EDIT. Answer to m0_as. We can also write the equation $(Q\otimes S^T)X=Z$ where we stack the matrix row by row. cf. https://en.wikipedia.org/wiki/Kronecker_product

In the same way as above, using $(Q\otimes S^T)^+$, we can solve the equation.

Remark 1. $rank(Q\otimes S^T)=rank(Q)rank(S)$.

Remark 2. If your matrices are parametric, then make this clear in your question. It is more difficult. Use a formal calculation.

• Thanks for your answer. But can you tell me how I can check whether there exists any $Q^+$ that $Q Q^+ Z =Z$? My Q matrix is a parametric matrix and not a numerical one. – m0_as Oct 16 '15 at 4:52