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arXiv · 2608.20934

Products of Nilpotent and Idempotent Matrices over Finite Local Rings

David Dolžan
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Abstract

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 $|R|=q^n$ and $R/J(R)\cong GF(q)$, we show that \[ |\operatorname{IN}(M_2(R))| = |\operatorname{NI}(M_2(R))| = q^{3n-2}(q^n+q^2-1). \]

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David Dolžan. 2026-08-21. Products of Nilpotent and Idempotent Matrices over Finite Local Rings. https://arxiv.org/abs/2608.20934

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