Generalisations exist for any given reductive G-structure.
Introduction
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., tangent to) . 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 subspace of 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
Definition
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 subbundle of . 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 on to 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 lift of , and a transverse vector field .
^Fatibene L., Ferraris M., Francaviglia M.
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^Godina M. and Matteucci P. (2003), Reductive G-structures and Lie derivatives,
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^Fatibene L. and Francaviglia M. (2011), General theory of Lie derivatives for Lorentz tensors, Communications in Mathematics 19, 11–25
^Kobayashi, Shoshichi; Nomizu, Katsumi (1996), Foundations of Differential Geometry, vol. Vol. 1, Wiley-Interscience, ISBN0-470-49647-9{{citation}}: |volume= has extra text (help) (Example 5.2) pp. 55-56