Graphs with Dense Constant Neigbourhoods

Let \(G\) be a finite graph and \(x\) be its vertex. The \({neighbourhood}\) of \(x\) in \(G\), denoted \(N_G(x)\), is a subgraph of \(G\) induced by all vertices adjacent to \(x\). \(G\) is a \({graph \; with \; a \; constant \; neighbourhood}\) if there exists a graph \(H\) such that \(N_G(x)\) is isomorphic to \(H\) for every vertex \(x\) of \(G\).

We completely characterize graphs with constant neighbourhoods isomorphic to complements of regular disconnected graphs.