In mathematics, particularly differential topology, the secondary vector bundle structure refers to the natural vector bundle structure (TE, p∗, TM) on the total space TE of the tangent bundle of a smooth vector bundle (E, p, M), induced by the push-forward p∗ : TE → TM of the original projection map p : E → M. This gives rise to a double vector bundle structure (TE,E,TM,M). In the special case (E, p, M) = (TM, πTM, M), where TE = TTM is the double tangent bundle, the secondary vector bundle (TTM, (πTM)∗, TM) is isomorphic to the tangent bundle (TTM, πTTM, TM) of TM through the canonical flip.
Construction of the secondary vector bundle structure Let (E, p, M) be a smooth vector bundle of rank N. Then the preimage (p∗)−1(X) ⊂ TE of any tangent vector X in TM in the push-forward p∗ : TE → TM of the canonical projection p : E → M is a smooth submanifold of dimension 2N, and it becomes a vector space with the push-forwards
+ ∗ : T ( E × M E ) → T E , λ ∗ : T E → T E {\displaystyle +_{*}:T(E\times _{\!M}\!E)\to TE,\qquad \lambda _{*}:TE\to TE}
of the original addition and scalar multiplication
+ : E × M E → E , λ : E → E {\displaystyle +:E\times _{\!M}\!E\to E,\qquad \lambda :E\to E}
as its vector space operations. It becomes clear + ∗ {\displaystyle +_{*}} actually defines addition on the fibers of p ∗ {\displaystyle p_{*}} as T ( E × M E ) = T E × T M T E {\displaystyle T(E\times _{\!M}\!E)=TE\times _{TM}TE} . The triple (TE, p∗, TM) becomes a smooth vector bundle with these vector space operations on its fibres.
Proof Let (U, φ) be a local coordinate system on the base manifold M with φ(x) = (x1, ..., xn) and let
{ ψ : W → φ ( U ) × R N ψ ( v k e k | x ) := ( x 1 , … , x n , v 1 , … , v N ) {\displaystyle {\begin{cases}\psi :W\to \varphi (U)\times \mathbf {R} ^{N}\\\psi \left(v^{k}e_{k}|_{x}\right):=\left(x^{1},\ldots ,x^{n},v^{1},\ldots ,v^{N}\right)\end{cases}}}
be a coordinate system on W := p − 1 ( U ) ⊂ E {\displaystyle W:=p^{-1}(U)\subset E} adapted to it. Then
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