Trivializations #
Main definitions #
Basic definitions #
-
Trivialization F p
: structure extending partial homeomorphisms, defining a local trivialization of a topological spaceZ
with projectionp
and fiberF
. -
Pretrivialization F proj
: trivialization as a partial equivalence, mainly used when the topology on the total space has not yet been defined.
Operations on bundles #
We provide the following operations on Trivialization
s.
Trivialization.compHomeomorph
: given a local trivializatione
of a fiber bundlep : Z → B
and a homeomorphismh : Z' ≃ₜ Z
, returns a local trivialization of the fiber bundlep ∘ h
.
Implementation notes #
Previously, in mathlib, there was a structure topological_vector_bundle.trivialization
which
extended another structure topological_fiber_bundle.trivialization
by a linearity hypothesis. As
of PR leanprover-community/mathlib#17359, we have changed this to a single structure
Trivialization
(no namespace), together with a mixin class Trivialization.IsLinear
.
This permits all the data of a vector bundle to be held at the level of fiber bundles, so that the
same trivializations can underlie an object's structure as (say) a vector bundle over ℂ
and as a
vector bundle over ℝ
, as well as its structure simply as a fiber bundle.
This might be a little surprising, given the general trend of the library to ever-increased bundling. But in this case the typical motivation for more bundling does not apply: there is no algebraic or order structure on the whole type of linear (say) trivializations of a bundle. Indeed, since trivializations only have meaning on their base sets (taking junk values outside), the type of linear trivializations is not even particularly well-behaved.
This structure contains the information left for a local trivialization (which is implemented
below as Trivialization F proj
) if the total space has not been given a topology, but we
have a topology on both the fiber and the base space. Through the construction
topological_fiber_prebundle F proj
it will be possible to promote a
Pretrivialization F proj
to a Trivialization F proj
.
- toFun : Z → B × F
- invFun : B × F → Z
- source : Set Z
- open_target : IsOpen self.target
- baseSet : Set B
- open_baseSet : IsOpen self.baseSet
- proj_toFun : ∀ p ∈ self.source, (↑self.toPartialEquiv p).1 = proj p
Instances For
Coercion of a pretrivialization to a function. We don't use e.toFun
in the CoeFun
instance
because it is actually e.toPartialEquiv.toFun
, so simp
will apply lemmas about
toPartialEquiv
. While we may want to switch to this behavior later, doing it mid-port will break a
lot of proofs.
Equations
- ↑e = ↑e.toPartialEquiv
Instances For
Equations
- Pretrivialization.instCoeFunPretrivializationForAllProd = { coe := Pretrivialization.toFun' }
If the fiber is nonempty, then the projection also is.
Composition of inverse and coercion from the subtype of the target.
Equations
- Pretrivialization.setSymm e = Set.restrict e.target ↑(PartialEquiv.symm e.toPartialEquiv)
Instances For
A fiberwise inverse to e
. This is the function F → E b
that induces a local inverse
B × F → TotalSpace F E
of e
on e.baseSet
. It is defined to be 0
outside e.baseSet
.
Equations
- Pretrivialization.symm e b y = if hb : b ∈ e.baseSet then cast ⋯ (↑(PartialEquiv.symm e.toPartialEquiv) (b, y)).snd else 0
Instances For
A structure extending partial homeomorphisms, defining a local trivialization of a projection
proj : Z → B
with fiber F
, as a partial homeomorphism between Z
and B × F
defined between
two sets of the form proj ⁻¹' baseSet
and baseSet × F
, acting trivially on the first coordinate.
- toFun : Z → B × F
- invFun : B × F → Z
- source : Set Z
- open_source : IsOpen self.source
- open_target : IsOpen self.target
- continuousOn_toFun : ContinuousOn (↑self.toPartialEquiv) self.source
- continuousOn_invFun : ContinuousOn self.invFun self.target
- baseSet : Set B
- open_baseSet : IsOpen self.baseSet
- proj_toFun : ∀ p ∈ self.source, (↑self.toPartialHomeomorph p).1 = proj p
Instances For
Coercion of a trivialization to a function. We don't use e.toFun
in the CoeFun
instance
because it is actually e.toPartialEquiv.toFun
, so simp
will apply lemmas about
toPartialEquiv
. While we may want to switch to this behavior later, doing it mid-port will break a
lot of proofs.
Equations
- ↑e = ↑e.toPartialEquiv
Instances For
Natural identification as a Pretrivialization
.
Equations
- Trivialization.toPretrivialization e = { toPartialEquiv := e.toPartialEquiv, open_target := ⋯, baseSet := e.baseSet, open_baseSet := ⋯, source_eq := ⋯, target_eq := ⋯, proj_toFun := ⋯ }
Instances For
Equations
- Trivialization.instCoeFunTrivializationForAllProd = { coe := Trivialization.toFun' }
Equations
- Trivialization.instCoeTrivializationPretrivialization = { coe := Trivialization.toPretrivialization }
The preimage of a subset of the base set is homeomorphic to the product with the fiber.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The source is homeomorphic to the product of the base set with the fiber.
Equations
Instances For
Each fiber of a trivialization is homeomorphic to the specified fiber.
Equations
- One or more equations did not get rendered due to their size.
Instances For
In the domain of a bundle trivialization, the projection is continuous
Composition of a Trivialization
and a Homeomorph
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Read off the continuity of a function f : Z → X
at z : Z
by transferring via a
trivialization of Z
containing z
.
Read off the continuity of a function f : X → Z
at x : X
by transferring via a
trivialization of Z
containing f x
.
A fiberwise inverse to e'
. The function F → E x
that induces a local inverse
B × F → TotalSpace F E
of e'
on e'.baseSet
. It is defined to be 0
outside e'.baseSet
.
Equations
Instances For
If e
is a Trivialization
of proj : Z → B
with fiber F
and h
is a homeomorphism
F ≃ₜ F'
, then e.trans_fiber_homeomorph h
is the trivialization of proj
with the fiber F'
that sends p : Z
to ((e p).1, h (e p).2)
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Coordinate transformation in the fiber induced by a pair of bundle trivializations. See also
Trivialization.coordChangeHomeomorph
for a version bundled as F ≃ₜ F
.
Equations
- Trivialization.coordChange e₁ e₂ b x = (↑e₂ (↑(PartialHomeomorph.symm e₁.toPartialHomeomorph) (b, x))).2
Instances For
Coordinate transformation in the fiber induced by a pair of bundle trivializations, as a homeomorphism.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Restrict a Trivialization
to an open set in the base.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given two bundle trivializations e
, e'
of proj : Z → B
and a set s : Set B
such that
the base sets of e
and e'
intersect frontier s
on the same set and e p = e' p
whenever
proj p ∈ e.baseSet ∩ frontier s
, e.piecewise e' s Hs Heq
is the bundle trivialization over
Set.ite s e.baseSet e'.baseSet
that is equal to e
on proj ⁻¹ s
and is equal to e'
otherwise.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given two bundle trivializations e
, e'
of a topological fiber bundle proj : Z → B
over a linearly ordered base B
and a point a ∈ e.baseSet ∩ e'.baseSet
such that
e
equals e'
on proj ⁻¹' {a}
, e.piecewise_le_of_eq e' a He He' Heq
is the bundle
trivialization over Set.ite (Iic a) e.baseSet e'.baseSet
that is equal to e
on points p
such that proj p ≤ a
and is equal to e'
otherwise.
Equations
- Trivialization.piecewiseLeOfEq e e' a He He' Heq = Trivialization.piecewise e e' (Set.Iic a) ⋯ ⋯
Instances For
Given two bundle trivializations e
, e'
of a topological fiber bundle proj : Z → B
over a
linearly ordered base B
and a point a ∈ e.baseSet ∩ e'.baseSet
, e.piecewise_le e' a He He'
is the bundle trivialization over Set.ite (Iic a) e.baseSet e'.baseSet
that is equal to e
on
points p
such that proj p ≤ a
and is equal to ((e' p).1, h (e' p).2)
otherwise, where
h = e'.coord_change_homeomorph e _ _
is the homeomorphism of the fiber such that
h (e' p).2 = (e p).2
whenever e p = a
.
Equations
- Trivialization.piecewiseLe e e' a He He' = Trivialization.piecewiseLeOfEq e (Trivialization.transFiberHomeomorph e' (Trivialization.coordChangeHomeomorph e' e He' He)) a He He' ⋯
Instances For
Given two bundle trivializations e
, e'
over disjoint sets, e.disjoint_union e' H
is the
bundle trivialization over the union of the base sets that agrees with e
and e'
over their
base sets.
Equations
- One or more equations did not get rendered due to their size.