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Dual and Hom transition functions define smooth bundles
Statement
If and are smooth vector bundles, then and are smooth vector bundles. In local bundle charts, the dual transition matrices are and the Hom transition matrices are .
Facts & Assumptions
Given: Smooth vector bundles and with local transition matrices and .
Vector bundle chart changes are fibrewise linear and smooth (Vector bundle charts and transition functions).
The matrix of the transpose linear map is the transpose matrix (In dual bases, the matrix of is the transpose of the matrix of ).
Finite products of second-countable spaces are second countable, and subspaces inherit second countability (The product set of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space, A product of two at most countable sets is at most countable, Second countability is hereditary).
An open surjection has the quotient topology on its target (The quotient topology of a surjection, quotient maps, saturated sets, and the quotient of a space by an equivalence relation with its canonical projection, A continuous open surjection, a continuous closed surjection, and a continuous surjection admitting a continuous section are all quotient maps).
Inversion is smooth on the open set of invertible matrices (Matrix inversion preserves regularity where the determinant is nonzero).
Proof
Put and , of ranks . For , let consist of tuples over a common base point , with a basis of . Give the finite-product subspace topology. It is second countable: choose one countable base of each supplied total space, take finite product boxes, and restrict them to . Only finitely many bases are chosen, and [L3] makes the resulting family countable.
Let as a set. Send a tuple in to the unique fibre map taking to . This map is onto: a basis can be chosen for each one fibre without selecting bases for all fibres simultaneously. Give the quotient topology. On a common local trivialization over , the open saturated set has coordinates , and the composite of with the proposed Hom matrix chart is . This is a continuous open surjection: is a homeomorphism by [L5], followed by the open product projection. By [L4], the induced chart is a homeomorphism and is open.
The map is globally open because its restriction over every such open is open. Images of a countable base of then form a countable base of : for each open, choose one and a basic neighbourhood with , so . Distinct maps over distinct base points separate by base neighbourhoods; distinct maps over one point separate in one Hausdorff matrix chart. Thus is Hausdorff and second countable.
If the original bundle frames change by and , a Hom matrix changes to . These maps and their inverses are smooth by [L1] and [L5], so the charts in step 2.1 give a smooth vector-bundle atlas; a boundary base chart yields the corresponding half-space product chart. Fibre addition and scalar multiplication are linear in these coordinates. If or , is the zero bundle ; if is empty, it is empty.
Take in steps 1.1–4.1 to obtain as a smooth bundle. A functional with row coordinates changes to ; by [L2], its column transition matrix is . Step 4.1 gives the stated Hom transition matrix.
Depends on
- Dual and Hom vector bundles
- Vector bundle charts and transition functions
- Smooth vector bundles, rank, fibres, and trivial bundles
- The product set $\prod_{i \in I} X_i$ of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space
- A product of two at most countable sets is at most countable
- Second countability is hereditary
- The quotient topology of a surjection, quotient maps, saturated sets, and the quotient of a space by an equivalence relation with its canonical projection
- A continuous open surjection, a continuous closed surjection, and a continuous surjection admitting a continuous section are all quotient maps
- Matrix inversion preserves $C^k$ regularity where the determinant is nonzero
- The transpose or algebraic adjoint $T^*:W^*\to V^*$, $T^*(g)=g\circ T$, of a linear map $T:V\to W$
- In dual bases, the matrix of $T^*$ is the transpose of the matrix of $T$
- A finite square real matrix is invertible if and only if its determinant is nonzero
Used by
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Sources
- John M. Lee, Introduction to Smooth Manifolds (standard reference, not scraped)
- Will J. Merry, Differential Geometry (standard reference, not scraped)