Nonsurjective zero product preservers between matrix spaces over an arbitrary field

Abstract

A map $Φ$ between matrices is said to be zero product preserving if $$ Φ(A)Φ(B) = 0 \quad \text{whenever}\quad AB = 0. $$ In this paper, we give concrete descriptions of an additive/linear zero product preserver $Φ: {\bf M}_n(\mathbb{F}) \rightarrow {\bf M}_r(\mathbb{F})$ between matrix algebras of different dimensions over an arbitrary field $\mathbb{F}$. In particular, we show that if $Φ$ is linear and preserves zero products then $$ Φ(A)= S\begin{pmatrix} R_1 \otimes A & 0 \cr 0 & Φ_0(A)\end{pmatrix} S^{-1}, $$ 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 $Φ_0: {\bf M}_n(\mathbb{F}) \rightarrow {\bf M}_{r-nk}(\mathbb{F})$ into nilpotent matrices. If $Φ(I_n)$ is invertible, then $Φ_0$ is vacuous. In general, the structure of $Φ_0$ could be quite arbitrary, especially when $Φ_0({\bf M}_n(\mathbb{F}))$ has trivial multiplication, i.e., $Φ_0(X)Φ_0(Y) = 0$ for all $X, Y$ in ${\bf M}_n(\mathbb{F})$. We show that if $Φ_0(I_n) = 0$ or $r-nk \le n+1$, then $Φ_0({\bf M}_n(\mathbb{F}))$ 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.