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Novikov ring

Mathematical construct

In mathematics, given an additive subgroup \Gamma \subset \R, the Novikov ring \operatorname{Nov}(\Gamma) of \Gamma is the subring of \Z[![\Gamma]!] consisting of formal sums \sum n_{\gamma_i} t^{\gamma_i} such that \gamma_1 \gamma_2 \cdots and \gamma_i \to -\infty. 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 \operatorname{Nov}(\Gamma) is a principal ideal domain. Let S be the subset of \Z[\Gamma] consisting of those with leading term 1. Since the elements of S are unit elements of \operatorname{Nov}(\Gamma), the localization \operatorname{Nov}(\Gamma)[S^{-1}] of \operatorname{Nov}(\Gamma) with respect to S is a subring of \operatorname{Nov}(\Gamma) called the "rational part" of \operatorname{Nov}(\Gamma); it is also a principal ideal domain.

Novikov numbers

Given a smooth function f on a smooth manifold M with nondegenerate critical points, the usual Morse theory constructs a free chain complex C_(f) such that the (integral) rank of C_p is the number of critical points of f of index p (called the Morse number). It computes the (integral) homology of M (cf. Morse homology): :H^(C_(f)) \cong H^(M,\Z)

In an analogy with this, one can define "Novikov numbers". Let X be a connected polyhedron with a base point. Each cohomology class \xi \in H^1(X,\R) may be viewed as a linear functional on the first homology group H_1(X,\R); when composed with the Hurewicz homomorphism, it can be viewed as a group homomorphism \xi\colon \pi=\pi_1(X) \to \R. By the universal property, this map in turns gives a ring homomorphism, :\phi_\xi\colon \Z[\pi] \to \operatorname{Nov} = \operatorname{Nov}(\R), making \operatorname{Nov} a module over \Z[\pi]. Since X is a connected polyhedron, a local coefficient system over it corresponds one-to-one to a \Z[\pi]-module. Let L_\xi be a local coefficient system corresponding to \operatorname{Nov} with module structure given by \phi_\xi. The homology group H_p(X, L_\xi) is a finitely generated module over \operatorname{Nov}, 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 b_p(\xi). The number of cyclic modules in the torsion part is denoted by q_p(\xi). If \xi = 0, L_\xi is trivial and b_p(0) is the usual Betti number of X.

The analog of Morse inequalities holds for Novikov numbers as well (cf. the reference for now.)

Notes

References

S. P. Novikov, Multi-valued functions and functionals: An analogue of Morse theory. Soviet Mathematics - Doklady 24 (1981), 222–226.
S. P. Novikov: The Hamiltonian formalism and a multi-valued analogue of Morse theory. Russian Mathematical Surveys 35:5 (1982), 1–56.

References

Here, $\Z[![\Gamma]!]$ is the ring consisting of the formal sums $\sum_{\gamma \in \Gamma} n_\gamma t^\gamma$ , $n_\gamma$ integers and ''t'' a formal variable, such that the multiplication is an extension of a multiplication in the integral [[group ring]] $\Z[\Gamma]$ .

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commutative-algebra ring-theory morse-theory

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