Limits and Periodicity of Metamour $2$-Distance Graphs

Given a finite simple graph G, let M(G) denote its 2-distance graph, in which two vertices are adjacent if and only if they have distance 2 in G. In this paper, we consider the periodic behavior of the sequence G, M(G), M²(G), M³(G), … obtained by iterating the 2-distance operation. In particular, we classify the connected graphs with period 3, and we partially characterize those with period 2. We then study two families of graphs whose 2-distance sequence is eventually periodic: namely, generalized Petersen graphs and complete m-ary trees. For each family, we show that the eventual period is 2, and we determine the pre-period and the two limit graphs of the sequence.