In commutative algebra, the Fitting ideals of a finitely generated module over a commutative ring describe the obstructions to generating the module by a given number of elements. They were introduced by Hans Fitting (1936).

Definition

If M is a finitely generated module over a commutative ring R generated by elements m1,...,mn with relations

then the ith Fitting ideal of M is generated by the minors (determinants of submatrices) of order of the matrix . The Fitting ideals do not depend on the choice of generators and relations of M.

Some authors defined the Fitting ideal to be the first nonzero Fitting ideal .

Properties

The Fitting ideals are increasing

If M can be generated by n elements then Fittn(M) = R, and if R is local the converse holds. We have Fitt0(M)  Ann(M) (the annihilator of M), and Ann(M)Fitti(M)  Fitti1(M), so in particular if M can be generated by n elements then Ann(M)n  Fitt0(M).

Examples

If M is free of rank n then the Fitting ideals are zero for i<n and R for i  n.

If M is a finite abelian group of order (considered as a module over the integers) then the Fitting ideal is the ideal .

The Alexander polynomial of a knot is a generator of the Fitting ideal of the first homology of the infinite abelian cover of the knot complement.

Fitting image

The zeroth Fitting ideal can be used also to give a variant of the notion of scheme-theoretic image of a morphism, a variant that behaves well in families. Specifically, given a finite morphism of noetherian schemes , the -module is coherent, so we may define as a coherent sheaf of -ideals; the corresponding closed subscheme of is called the Fitting image of f.[1]

References

  1. Eisenbud, David; Harris, Joe. The Geometry of Schemes. Springer. p. 219. ISBN 0-387-98637-5.
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