what happens to rank of matrices when singular matrix multiply by non-singular matrix?? I need to find the prove for any rectangular matrix $A$ and any non-singular matrices $P$, $Q$ of appropriate sizes,
rank($PAQ$)= rank($A$)
I know that when singular matrix multiply by non-singular the result will be singular matrix. However, It is not relevant to the rank of the matrix. If A is singular with rank 2, why rank($PAQ$)= 2 not any numbers.

Hint: multiplying matrices amounts to compose linear maps. In other words, suppose $A$ is an $n\times m$ matrix, i.e. a linear map $f_A:\mathbb R^m\to\mathbb R^n$. Then, to perform the product $PAQ$ is the same as to give a factorization of $f_{PAQ}$ as $$\mathbb R^m\overset{f_Q}{\longrightarrow}\mathbb R^m\overset{f_A}{\longrightarrow}\mathbb R^n\overset{f_P}{\longrightarrow}\mathbb R^n,$$ where $f_Q$ and $f_P$ are isomorphisms. Hence,
$$\textrm{rank}(PAQ):=\dim\,(\textrm{im}\,(f_P\circ f_A\circ f_Q))=\dim\,(\textrm{im}\,f_A)=:\textrm{rank}\,A.$$
