In this paper the problem of the existence of regular nut graphs is addressed. A generalization of Fowler?s Construction which is a local enlargement applied to a vertex in a graph is introduced to generate nut graphs of higher order. Let N (?) denote the set of integers n such that there exists a regular nut graph of degree ? and order n. It is proven that N (3) = {12} ? {2k : k ? 9} and that N (4) = {8, 10, 12} ? {n : n ? 14}. The problem of determining N (?) for ? > 4 remains completely open.

The nullity of a graph G is the multiplicity of zero as an eigenvalue in the spectrum of its adjacency matrix. From the interlacing theorem, derived from Cauchy’s inequalities for matrices, a vertex of a graph can be a core vertex if, on deleting the vertex, the nullity decreases, or a Fiedler vertex, otherwise. We adopt a graph theoretical approach to determine conditions required for the identification of a pair of prescribed types of root vertices of two graphs to form a cut-vertex of unique type in the coalescence. Moreover, the nullity of subgraphs obtained by perturbations of the coalescence G is determined relative to the nullity of G. This has direct applications in spectral graph theory as well as in the construction of certain ipso-connected nano-molecular insulators.