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 .
If M is a finitely generated module over a commutative ring R generated by elements m<sub>1</sub>,...,m<sub>n</sub> 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 .
The Fitting ideals are increasing
If M can be generated by n elements then Fitt<sub>n</sub>(M) = R, and if R is local the converse holds. We have Fitt<sub>0</sub>(M) â Ann(M) (the annihilator of M), and Ann(M)Fitt<sub>i</sub>(M) â Fitt<sub>i−1</sub>(M), so in particular if M can be generated by n elements then Ann(M)<sup>n</sup> â Fitt<sub>0</sub>(M).
If M is free of rank n then the Fitting ideals are zero for i<n and R for i âÂÂ¥ 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.
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.