Multiplicity theory

In abstract algebra, multiplicity theory concerns the multiplicity of a module M at an ideal I (often a maximal ideal)

${\mathbf{𝐞}}_I(M).$

The notion of the multiplicity of a module is a generalization of the degree of a projective variety. By Serre's intersection formula, it is linked to an intersection multiplicity in the intersection theory.

The main focus of the theory is to detect and measure a singular point of an algebraic variety (cf. resolution of singularities). Because of this aspect, valuation theory, Rees algebras and integral closure are intimately connected to multiplicity theory.

Multiplicity of a module

Let R be a positively graded ring such that R is generated as an R₀-algebra and R₀ is Artinian. Note that R has finite Krull dimension d. Let M be a finitely generated R-module and F_M(t) its Hilbert–Poincaré series. This series is a rational function of the form

$\frac{P(t)}{(1-t{)}^d},$

where $P(t)$ is a polynomial. By definition, the multiplicity of M is

$\mathbf{𝐞}(M)=P(1).$

The series may be rewritten

$F(t)=\sum _1^d\frac{a_{d-i}}{(1-t{)}^d}+r(t).$

where r(t) is a polynomial. Note that $a_{d-i}$ are the coefficients of the Hilbert polynomial of M expanded in binomial coefficients. We have

$\mathbf{𝐞}(M)=a_0.$

As Hilbert–Poincaré series are additive on exact sequences, the multiplicity is additive on exact sequences of modules of the same dimension.

The following theorem, due to Christer Lech, gives a priori bounds for multiplicity.

Lech — Suppose R is local with maximal ideal $\mathfrak{𝔪}$. If an I is $\mathfrak{𝔪}$-primary ideal, then

$e(I)\le d!\mathrm{deg}(R)\lambda (R/\bar{I}).$

See also

• Dimension theory (algebra)
• j-multiplicity
• Hilbert–Samuel multiplicity
• Hilbert–Kunz function
• Normally flat ring