Products of Nilpotent and Idempotent Matrices over Finite Local Rings

Let $R$ be a finite commutative local principal ring. We study products of nilpotent and idempotent matrices in $M_2(R)$. We show that every product of nilpotent and idempotent matrices is either a product of idempotents or a product of nilpotents. We then consider IN- and NI-matrices, that is, matrices which can be written as a product of an idempotent and a nilpotent matrix in the respective orders. We prove that the classes of IN- and NI-matrices in $M_2(R)$ coincide and give an explicit description of their common class. Finally, if $\mathrm{\mid }R\mathrm{\mid }=q^n$ and $R\mathrm{/}J(R)\cong GF(q)$, we show that $$\mathrm{\mid }\mathrm{IN}(M_2(R))\mathrm{\mid }=\mathrm{\mid }\mathrm{NI}(M_2(R))\mathrm{\mid }=q^{3n-2}(q^n+q^2-1)\mathrm{.}$$