English

Vector field cycles in the tangent bundle

Differential Geometry 2025-08-26 v1

Abstract

Given a closed Riemannian manifold (Mm,g)(M^m,g) and a vector field vv on MM, we form the Sasaki metric gSg_S on TMTM, and restrict it to the image of the cross section map of MM into TMTM defined by vv, whose pull back to MM defines a new metric g(v)g(v) on MM. We then view the cross section as an isometric embedding fg(v):(M,g(v))(TM,gS)f_{g(v)}: (M,g(v))\rightarrow (TM,g_S), which when vg=1\| v\|_g=1, ranges into the unit sphere bundle (S1(TM),gS)(S^1(TM),g_S). vv is minimal or minimal unit if these embeddings have null mean curvature vectors, conditions that occur if, vv is in the kernel or is an eigenvector, respectively, of a first order perturbation of a weighted rough Laplacian, the weights and perturbation determined by the covariant derivatives egv\nabla^g_{e}v along unit directions ee in suitable normal frames that include vv when vg=1\| v\|_g=1, and curvature tensor of gg. A minimal unit field must be Killing, and other than parallel fields, v=0v=0 is the only minimal one. We characterize the minimal unit vector fields on the standard sphere (\mbS2n+1,g)(\mbR2n+2,2)(\mb{S}^{2n+1},g) \hookrightarrow (\mb{R}^{2n+2},\| \, \|^2) as those defining contact strictly pseudoconvex CR structures whose Levi form and sign are determined by gg and the orientation. If Θfg(v)(M)\Theta_{f_{g(v)}}(M) and Φfg(v)(M)\Phi_{f_{g(v)}}(M) are the total exterior scalar curvature and squared L2L^2 norm of the mean curvature vector functionals, and m>2m>2, a canonical cycle fg(v)(M)f_{g(v)}(M) is a critical point of the functional (m/m1)Θfg(v)(M)+Φfg(v)(M)(m/m-1) \Theta_{f_{g(v)}}(M) +\Phi_{f_{g(v)}}(M) under conformal deformations, notion conveniently defined also when m2m\leq 2. The zero section of TMTM is a canonical cycle if, and only if, the scalar curvature of gg is constant. We describe some examples of these vector fields and cycles, and analyze their deformations under dilations of the field.

Keywords

Cite

@article{arxiv.2508.17441,
  title  = {Vector field cycles in the tangent bundle},
  author = {Santiago R. Simanca},
  journal= {arXiv preprint arXiv:2508.17441},
  year   = {2025}
}
R2 v1 2026-07-01T05:03:36.893Z