Multiplicity of a filtered module ($m(M)=d(M)![j^{d(M)}]{\dim }_k{F}_{j}M$)

With the fixed algebra-filtration convention in dimension of a filtered module, the multiplicity is factorial times the common leading coefficient of the cumulative Hilbert function for $M\ne 0$, and is zero for $M=0$. It is a positive integer with a standard graded algebra normalization, and can be a positive rational number for a weighted grading. Fixed shifts between good filtrations of a module preserve its leading Hilbert coefficient. Rescaling the algebra filtration itself can change multiplicity, so its normalization must be specified.

The subspace filtration and quotient filtration in a short exact sequence are good when the associated graded algebra is Noetherian. Their cumulative Hilbert functions add in every degree. Positive leading coefficients make the degree of the sum the larger degree; when both degrees agree, factorial times the leading coefficients add. Thus equal-dimensional nonzero factors consume positive multiplicity of a filtered module. With a standard graded algebra normalization these multiplicities are integral.