English

A universal characteristic class for vector bundles with a connection

Algebraic Geometry 2025-04-22 v2 Commutative Algebra Algebraic Topology Differential Geometry

Abstract

In the paper I introduce a new characteristic class c(E)c(E) for a finite rank vector bundle EE on an affine scheme S:=Spec(A)S:=Spec(A) - the fundamental class of EE. The class c(E)c(E) is not a characteristic class in the classical sense in the sense that it lives in a pointed cohomology torsor Ext1(L,EndA(E))\operatorname{Ext}^1(L, \operatorname{End}_A(E)). Most characteristic classes lives in a cohomology group. The pointed cohomology torsor Ext1(L,EndA(E))\operatorname{Ext}^1(L, \operatorname{End}_A(E)) is a torsor on the abelian group H2(L,Z(EndA(E)))\operatorname{H}^2(L, Z(\operatorname{End}_A(E))) where Z(EndA(E))Z(\operatorname{End}_A(E)) is the center of the ring of endomorphisms of EE and where the cohomology is the Lie-Rinehart cohomology of the center. The class c(E)c(E) is trivial if and only if EE has a flat algebraic connection. Hence the class c(TS)c(T_S) where TST_S is the tangent bundle, is an an obstruction for SS to be algebraically parallelizable. I use a connection \nabla to define c(E)c(E) and I also prove the class c(E)c(E) is independent of choice of connection, hence c(E)c(E) is an invariant of the vector bundle EE. The class generalize the Chern class, the Pontryagin class, the Euler class and the Teleman characteristic class. I prove using an explicit example that the class c(E)c(E) is stronger than the Chern class and the Euler class. I also give a new proof of a formula for the curvature of a connection \nabla in terms of an idempotent endomorphism ϕ\phi defining EE. This formula was claimed and proved in a paper put out on the arXiv in 2011, and in this paper I give a new proof that is easier to read. The class may be interesting in the study of the "cancellation problem" in affine algebraic geometry and the problem of giving algebraic formulas for the topological Euler characteristic. I also calculate the algebraic deRham cohomology of the complex two sphere and prove it is infinite dimensional.

Keywords

Cite

@article{arxiv.2503.22314,
  title  = {A universal characteristic class for vector bundles with a connection},
  author = {Helge Øystein Maakestad},
  journal= {arXiv preprint arXiv:2503.22314},
  year   = {2025}
}
R2 v1 2026-06-28T22:37:52.738Z