In mathematics, given an additive subgroup , the Novikov ring of is the subring of [1] consisting of
formal sums such that and . The notion was introduced by
Sergei Novikov in the papers that initiated the generalization of
Morse theory using a closed one-form instead of a function. The notion is used in
quantum cohomology, among the others.
The Novikov ring is a
principal ideal domain. Let S be the subset of consisting of those with leading term 1. Since the elements of S are unit elements of , the
localization of with respect to S is a subring of called the "rational part" of ; it is also a
principal ideal domain.
In an analogy with this, one can define "Novikov numbers". Let X be a connected polyhedron with a base point. Each cohomology class may be viewed as a linear functional on the first homology group ; when composed with the
Hurewicz homomorphism, it can be viewed as a group homomorphism . By the universal property, this map in turns gives a ring homomorphism,
,
making a module over . Since X is a
connected polyhedron, a
local coefficient system over it corresponds one-to-one to a -module. Let be a local coefficient system corresponding to with module structure given by . The homology group is a finitely generated module over which is, by the
structure theorem, the direct sum of its free part and its torsion part. The rank of the free part is called the Novikov Betti number and is denoted by . The number of cyclic modules in the torsion part is denoted by . If , is trivial and is the usual Betti number of X.
The analog of
Morse inequalities holds for Novikov numbers as well (cf. the reference for now.)
Notes
^Here, is the ring consisting of the formal sums , integers and t a formal variable, such that the multiplication is an extension of a multiplication in the integral
group ring.