The Limiting Eigenvalue Distribution of Iterated k-Regular Graph Cylinders

We explore the limiting empirical eigenvalue distributions arising from matrices of the form $$A_{n+1} = \begin{bmatrix} A_n & I\\ I & A_n \end{bmatrix} ,$$where $A_0$ is the adjacency matrix of a $k$-regular graph. We find that for bipartite graphs, the distributions are centered symmetric binomial distributions, and for non-bipartite graphs, the distributions are asymmetric. This research grew out of our work on neural networks in $k$-regular graphs. Our original question was whether or not the graph cylinder construction would produce an expander graph that is a suitable candidate for the neural networks being developed at Nousot. This question is answered in the negative for our computational purposes. However, the limiting distribution is still of theoretical interest to us; thus, we present our results here.