Smooth scalar rescaling of vector bundles #
Multiplying a vector in a bundle by a smooth scalar preserves smoothness even when its base
point varies. A nowhere-zero scalar function on the base therefore defines a diffeomorphism
of the total space, with inverse given by reciprocal multiplication. This construction fixes
the zero section and preserves the projection, without a choice of global trivialization.
More generally, over a 𝕜-charted base, scalar multiplication by 𝕜'-valued maps along a normed
field extension 𝕜 ⊆ 𝕜' preserves C^n regularity and equips C^n sections with compatible
Module 𝕜' and IsScalarTower 𝕜 𝕜' instances (ContMDiffSection.instModuleOfNormedAlgebra);
multiplication by a constant of 𝕜' also preserves differentiability of sections
(MDifferentiableAt.const_smul_section).
For normal bundles, positive rescaling converts unit-radius fibre coordinates to a radius varying over the submanifold. It is the rescaling step in the variable-radius tubular neighbourhood construction of J. M. Lee, Introduction to Smooth Manifolds, second edition, Theorem 6.24.
Multiplying a bundle-valued map by a scalar-valued map preserves C^n regularity within
a set at a point. The base point of the bundle-valued map may vary.
Multiplying a bundle-valued map by a scalar-valued map preserves C^n regularity at
a point, with the base point of the vector allowed to vary.
Multiplying a bundle-valued map by a scalar-valued map preserves C^n regularity
on a set.
Multiplying a bundle-valued map by a scalar-valued map preserves C^n regularity.
Multiplying a section of a 𝕜'-linear vector bundle by a constant of 𝕜' preserves
differentiability at a point, where differentiability is taken over 𝕜.
A nowhere-zero C^n scalar function on the base rescales each fibre by a C^n
diffeomorphism of the total space. Its inverse multiplies by the reciprocal scalar.
Instances For
Scalar rescaling acts fibrewise by the given scalar.
Rescaling by the constant one function is the identity diffeomorphism.
Successive fibre rescalings multiply the scalar functions on the base.
The inverse rescaling is the rescaling associated to the reciprocal function.
Rescaling sends a variable-radius open ball bundle to the open ball bundle whose radii are multiplied by the norms of the scalars. No positivity assumption on the old radii is needed.
Rescaling sends closed ball bundles to closed ball bundles with rescaled radii.
Rescaling sends sphere bundles to sphere bundles with rescaled radii.
Low-priority SMul 𝕜' instance on C^n sections of a 𝕜'-linear vector bundle over a
𝕜-charted base; when 𝕜' = 𝕜, ContMDiffSection.instSMul takes precedence and is definitionally
equal to this action.
Low-priority Module 𝕜' instance on C^n sections of a vector bundle carrying compatible 𝕜-
and 𝕜'-vector-bundle structures (in particular 𝕜 = ℝ, 𝕜' = ℂ).