adjacency matrix
adjacency matrix (connectivity matrix; reachability matrix) A matrix used as a means of representing an adjacency structure, which in turn represents a graph. If A is the adjacency matrix corresponding to a given graph G, then a_{ij} = 1
if there is an edge from vertex i to vertex j in G; otherwise a_{ij} = 0
If G is a directed graph then a_{ij} = 1
if there is an edge directed from vertex i to vertex j; otherwise a_{ij} = 0
If the vertices of the graph are numbered 1,2,…m, the adjacency matrix is of a type m×m. If A×A×…×A (p terms, p←m)
is evaluated, the nonzero entries indicate those vertices that are joined by a path of length p; indeed the value of the (i,j)th entry of A^{p} gives the number of paths of length p from the vertex i to vertex j. By examining the set of such matrices, p = 1,2,…, m–1
it can be determined whether two vertices are connected.
It is also possible for adjacency matrices to be formed from Boolean matrices.
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