English

Riemannian metric representatives of the Stiefel-Whitney classes

Differential Geometry 2026-01-14 v2

Abstract

If MM is a closed manifold, and KK is a smooth triangulation of MM, Whitney proved that all of the Stiefel-Whitney classes are specified as cochains on the dual cell complex (K)(K')^* assigning the value 11 mod 22 to each dual cell. We provide the pair (M,K)(M,K) with an arbitrary Riemannian metric gg, and use Whitney's criteria to show that there are associated representatives of all the Stiefel-Whitney classes w1(M),,wn(M)w_1(M), \ldots , w_n(M). The representative of w1(M)w_1(M) is determined by detgij\det{g_{ij}}, the gijg_{ij}s computed in a frame that is locally defined at each dual 11-cell; the representatives of the even classes w2k(M)w_{2k}(M) are determined by the Chern-Gauss-Bonnet density 2k2k-form of locally defined totally geodesic oriented 2k2k manifolds with boundary associated to each dual 2k2k-cell; and the representatives of the odd classes w2k+1(M)w_{2k+1}(M) are determined by the hypersurface area form of the boundary sphere of a locally defined totally geodesic oriented (2k+1)(2k+1) manifold with boundary associated to each dual (2k+1)(2k+1)-cell. If (M,J,g)(M,J,g) is Hermitian, we prove that the metric representative of w2k(M)w_{2k}(M) so obtained is the Z/2\mathbb{Z}/2 reduction of the kk-th Chern class ck(M,J)c_k(M,J) induced by the coefficient homomorphism, and that the metric representative of any odd degree class w2k+1(M)w_{2k+1}(M) so obtained is trivial in cohomology.

Keywords

Cite

@article{arxiv.2004.05719,
  title  = {Riemannian metric representatives of the Stiefel-Whitney classes},
  author = {Santiago R Simanca},
  journal= {arXiv preprint arXiv:2004.05719},
  year   = {2026}
}

Comments

This version contains a number of editorial corrections to the original, and includes several changes prompted by comments of a referee that improve significantly the presentation