The nullity of a
graph in the
mathematical subject of
graph theory can mean either of two unrelated numbers. If the graph has n vertices and m edges, then:
In the
matrix theory of graphs, the nullity of the graph is the nullity of the
adjacency matrixA of the graph. The nullity of A is given by n − r where r is the
rank of the adjacency matrix. This nullity equals the multiplicity of the
eigenvalue 0 in the spectrum of the adjacency matrix. See Cvetkovič and Gutman (1972), Cheng and Liu (2007), and Gutman and Borovićanin (2011).
In the
matroid theory the nullity of the graph is the nullity of the oriented
incidence matrixM associated with the graph. The nullity of M is given by m − n + c, where, c is the number of components of the graph and n − c is the
rank of the oriented incidence matrix. This name is rarely used; the number is more commonly known as the cycle rank, cyclomatic number, or circuit rank of the graph. It is equal to the rank of the co
graphic matroid of the graph. It also equals the nullity of the
Laplacian matrix of the graph, defined as L = D − A, where D is the diagonal matrix of vertex degrees; the Laplacian nullity equals the cycle rank because L = MMT (M times its own transpose).
Bo Cheng and Bolian Liu (2007), On the nullity of graphs. Electronic Journal of Linear Algebra, vol. 16, article 5, pp. 60–67.
Dragoš M. Cvetkovič and Ivan M. Gutman (1972), The algebraic multiplicity of the number zero in the spectrum of a bipartite graph. Matematički Vesnik (Beograd), vol. 9, pp. 141–150.
Ivan Gutman and Bojana Borovićanin (2011), Nullity of graphs: an updated survey. Zbornik Radova (Beograd), vol. 14, no. 22 (Selected Topics on Applications of Graph Spectra), pp. 137–154.