M[g] gives the number of edges in the graph g. M[g, Directed] is obsolete because M[g] works for directed as well as undirected graphs.

MultipleEdgesQ[g] yields True if g has multiple edges between pairs of vertices. It yields False otherwise.

MultiplicationTable[l, f] constructs the complete transition table defined by the binary relation function f on the elements of list l.

MycielskiGraph[k] returns a triangle-free graph with chromatic number k, for any positive integer k.

NecklacePolynomial[n, c, Cyclic] returns a polynomial in the colors in c whose coefficients represent numbers of ways of coloring an n-bead necklace with colors chosen from ...

Neighborhood[g, v, k] returns the subset of vertices in g that are at a distance of k or less from vertex v. Neighborhood[al, v, k] behaves identically, except that it takes ...

NetworkFlow[g, source, sink] returns the value of a maximum flow through graph g from source to sink. NetworkFlow[g, source, sink, Edge] returns the edges in g that have ...

NextBinarySubset[l, s] constructs the subset of l following subset s in the order obtained by interpreting subsets as binary string representations of integers.

NextComposition[l] constructs the integer composition that follows l in a canonical order.

NextGrayCodeSubset[l, s] constructs the successor of s in the Gray code of set l.