Well-covered Unit Graphs of Finite Rings

Let $R$ be a finite ring with identity. The unit graph (unitary Cayley graph) of $R$ is the graph with vertex set $R$, where two distinct vertices $x$ and $y$ are adjacent exactly whenever $x+y$ is a unit in $R$ ($x-y$ is a unit in $R$). Here, we study independent sets of unit graphs of matrix rings over finite fields and use them to characterize all finite rings for which the unit graph is well-covered or Cohen-Macaulay. Moreover, we show that the unit graph of $R$ is well-covered if and only if the unitary Cayley graph of $R$ is well-covered and the characteristic of $R\mathrm{/}J(R)$