In general, given a subbundle of a fiber bundle over and a vector field on, its restriction to is a vector field "along" not on (i.e., tangentto) . If one denotes by the canonical embedding, then is a section of the pullback bundle, where
and is the tangent bundle of the fiber bundle .
Let us assume that we are given a Kosmann decomposition of the pullback bundle , such that
i.e., at each one has where is a vector subspaceof and we assume to be a vector bundle over , called the transversal bundle of the Kosmann decomposition. It follows that the restriction to splits into a tangent vector field on and a transverse vector field being a section of the vector bundle
Let be the oriented orthonormal frame bundle of an oriented -dimensional
Riemannian manifold with given metric . This is a principal -subbundle of , the tangent frame bundle of linear frames over with structure group .
By definition, one may say that we are given with a classical reductive -structure. The special orthogonal group is a reductive Lie subgroup of . In fact, there exists a direct sum decomposition , where is the Lie algebra of , is the Lie algebra of , and is the -invariant vector subspace of symmetric matrices, i.e. for all
One then can prove that there exists a canonical Kosmann decomposition of the pullback bundle such that
i.e., at each one has being the fiber over of the subbundleof. Here, is the vertical subbundle of and at each the fiber is isomorphic to the vector space of symmetric matrices .
From the above canonical and equivariant decomposition, it follows that the restriction of an -invariant vector field onto splits into a -invariant vector field on, called the Kosmann vector field associated with, and a transverse vector field .
In particular, for a generic vector field on the base manifold , it follows that the restriction to of its natural lift onto splits into a -invariant vector field on, called the Kosmann liftof, and a transverse vector field .
^Fatibene, L.; Ferraris, M.; Francaviglia, M.; Godina, M. (1996). "A geometric definition of Lie derivative for Spinor Fields". In Janyska, J.; Kolář, I.; Slovák, J. (eds.). Proceedings of the 6th International Conference on Differential Geometry and Applications, August 28th–September 1st 1995 (Brno, Czech Republic). Brno: Masaryk University. pp. 549–558. arXiv:gr-qc/9608003v1. Bibcode:1996gr.qc.....8003F. ISBN80-210-1369-9.