Continuous bundle homomorphisms and their inverses #
For a bundle with finite-dimensional model source, continuity of a fibrewise linear map on total spaces is equivalent to continuity as a section of Mathlib's Hom bundle. Consequently a continuous fibrewise linear equivalence gives a homeomorphism of total spaces: continuity of the inverse need not be supplied separately.
We also prove local continuity of inversion for Hom-bundle sections with a Banach model
source. This local result does not require finite rank or invertibility away from the
point under consideration. The inverse is Mathlib's ContinuousLinearMap.inverse,
which is zero at noninvertible maps. In the same generality, invertibility of the fibre
maps is an open condition along a continuous Hom-bundle section, composition of
continuous Hom-bundle sections is continuous, and a σ-semilinear Hom-bundle section whose
preferred-frame coordinate expression is locally constant around each frame centre is continuous
(TauCeti.continuous_hom_bundle_of_inCoordinates_eq_const). Read in the trivializations at a
fixed point, a continuous Hom-bundle section is a continuous family of operators on their common
base set (TauCeti.continuousOn_inCoordinates).
For two vector bundle cores, a section of Mathlib's Hom bundle between them is continuous
exactly when it is continuous in every pair of a source chart and a target chart, on their
common domain, with chart expressions VectorBundleCore.inChartCoordinates
(VectorBundleCore.continuous_hom_bundle_iff_forall_continuousOn). Both directions read the
section through the local trivializations of the two charts
(VectorBundleCore.inChartCoordinates_eq_localTriv_comp in
TauCeti.Topology.VectorBundle.Coordinates).
The coordinate argument uses ContinuousLinearMap.inCoordinates_inverse and parallels
the smooth inverse argument in TauCeti.Geometry.Manifold.VectorBundle.Hom.
For the classical finite-rank bundle-isomorphism criterion see Hatcher,
Vector Bundles and K-Theory, §1.1.
Fibre maps between moving base points #
A family of fibre maps ϕ m : V (b₁ m) →L[𝕜] W (b₂ m) over two base maps is read through the
preferred trivializations centred at b₁ m₀ and b₂ m₀, as in Mathlib's
ContinuousAt.clm_apply_of_inCoordinates. Continuity of this coordinate expression within a set
at m₀ is preserved by composition and inversion, and makes invertibility an open condition at
m₀ relative to the set. The Hom-bundle lemmas below are the case of a single base map.
If two families of fibre maps are continuous within s at m₀ in coordinates, so is their
composite, provided the intermediate base map is continuous within s at m₀.
If two families of fibre maps are continuous at m₀ in coordinates, so is their composite,
provided the intermediate base map is continuous at m₀.
Along a family of fibre maps continuous within s at m₀ in coordinates and invertible at
m₀, the fibre maps at nearby points of s are invertible, provided both base maps are
continuous within s at m₀.
Along a family of fibre maps continuous at m₀ in coordinates and invertible at m₀, the
fibre maps at nearby points are invertible, provided both base maps are continuous at m₀.
Inversion preserves continuity within s at m₀ in coordinates of a family of fibre maps
invertible at m₀. No continuity of the base maps is needed. Only the model source must be
complete.
Inversion preserves continuity at m₀ in coordinates of a family of fibre maps invertible at
m₀. No continuity of the base maps is needed. Only the model source must be complete.
Inverse Hom-frame coordinates conjugate a model operator by the inverse target frame and forward source frame. No square identity or equality of the two model fibres is required, and both sides vanish outside the common base set.
For a finite-dimensional model source, a fibrewise continuous linear map is continuous on total spaces exactly when it is continuous as a section of the Hom bundle. The target need not have finite rank.
On the common base set of the trivializations at x₀, the coordinate expressions of a
continuous Hom-bundle section in these fixed trivializations form a continuous family of
operators.
Along a Hom-bundle section continuous within a set at a point where its fibre map is invertible, the fibre maps at nearby points of the set are invertible too. Only the model source must be complete.
Along a Hom-bundle section continuous at a point where its fibre map is invertible, the fibre maps at nearby points are invertible too.
The points at which a continuous Hom-bundle section is invertible form an open set.
Inversion of a continuous Hom-bundle section is continuous within a set at any point where the fibre map is invertible. Only the model source must be complete.
Inversion of a continuous Hom-bundle section is continuous at a point where its fibre map is invertible.
Inversion preserves continuity on a set of fibrewise invertible Hom-bundle maps.
Inversion preserves continuity of a fibrewise invertible Hom-bundle section.
Composition of Hom-bundle sections continuous within a set is continuous within it.
Composition of Hom-bundle sections continuous at a point is continuous there.
Composition of Hom-bundle sections continuous on a set is continuous on it.
Composition of continuous Hom-bundle sections is continuous.
A continuous fibrewise linear equivalence with finite-dimensional model source has a continuous inverse on total spaces. No compactness or separation of the base is needed.
The homeomorphism of total spaces induced by a continuous family of fibrewise linear equivalences. Continuity of the inverse follows from finite dimensionality.
Instances For
A continuous fibrewise bijective linear map between vector bundles with a finite-dimensional model source is induced by a homeomorphism of total spaces.
A section of the Hom bundle between two cores that is continuous on the common domain of a source chart and a target chart is continuous in those charts there.
A family of fibre maps is a continuous section of the Hom bundle between two cores exactly when it is continuous in every pair of charts.
A section of the σ-semilinear Hom bundle is continuous whenever its coordinate expression in
each preferred frame pair is locally constant around the frame centre. Topological counterpart of
TauCeti.contMDiff_hom_bundle_of_inCoordinates_eq_const.