Nonsurjective zero product preservers between matrix spaces over an arbitrary field

A map $\Phi$ between matrices is said to be zero product preserving if $$\Phi(A)\Phi(B) = 0 \quad \text{whenever}\quad AB = 0.$$ In this paper, we give concrete descriptions of an additive/linear zero product preserver $\Phi: {\bf M}_n(\mathbb{F}) \rightarrow {\bf M}_r(\mathbb{F})$ between matrix algebras of different dimensions over an arbitrary field $\mathbb{F}$. In particular, we show that if $\Phi$ is linear and preserves zero products then $$\Phi(A)= S\begin{pmatrix} R_1 \otimes A & 0 \cr 0 & \Phi_0(A)\end{pmatrix} S^{-1},$$ for some invertible matrices $R_1$ in ${\bf M}_k(\mathbb{F})$, $S$ in ${\bf M}_r(\mathbb{F})$ and a zero product preserving linear map $\Phi_0: {\bf M}_n(\mathbb{F}) \rightarrow {\bf M}_{r-nk}(\mathbb{F})$ into nilpotent matrices. If $\Phi(I_n)$ is invertible, then $\Phi_0$ is vacuous. In general, the structure of $\Phi_0$ could be quite arbitrary, especially when $\Phi_0({\bf M}_n(\mathbb{F}))$ has trivial multiplication, i.e., $\Phi_0(X)\Phi_0(Y) = 0$ for all $X, Y$ in ${\bf M}_n(\mathbb{F})$. We show that if $\Phi_0(I_n) = 0$ or $r-nk \le n+1$, then $\Phi_0({\bf M}_n(\mathbb{F}))$ indeed has trivial multiplication. More generally, we characterize subspaces ${\bf V}$ of square matrices satisfying $XY = 0$ for any $X, Y \in {\bf V}$. Similar results for double zero product preserving maps are obtained.