Source-indexed orientation lifts #
An orientation lift consists of an orientation of the model vector space and locally constant
signs defined only on the source of each preferred chart. The signs compensate for the sign of
the transition derivative within the model range, including boundary and corner points.
Simultaneously reversing the model orientation and every chart sign leaves the induced tangent
orientations unchanged, and TauCeti.Manifold.OrientationLift.congrModel casts lifts along an
equality of models on the same model vector space and chart carrier.
References #
- Michael Lee's Mathlib PR #35376: the chart-sign lift and diagonal-quotient design, adapted here to source-indexed signs and actual tangent-bundle trivializations.
- J. M. Lee, Introduction to Smooth Manifolds, second edition, Chapter 15.
- TauCeti PR #15107: the earlier formalization of the chart-sign lift and diagonal-flip quotient.
A model orientation with source-indexed chart signs. Compatibility is expressed through
actual transition Jacobians within range I, including boundary and corner points.
- modelOrientation : Orientation ℝ E ι
Orientation of the model vector space.
The index type has the dimension of the model space.
A sign only at points lying in the chosen preferred chart's source.
- locallyConstant_sign (α : M) : IsLocallyConstant (self.sign α)
Chart signs are locally constant on their actual domains.
- compatible (α β x : M) (hα : x ∈ (chartAt H α).source) (hβ : x ∈ (chartAt H β).source) : 0 < ↑↑(self.sign α ⟨x, hα⟩) * ↑↑(self.sign β ⟨x, hβ⟩) * (fderivWithin ℝ (↑(extChartAt I β) ∘ ↑(extChartAt I α).symm) (Set.range ↑I) (↑(extChartAt I α) x)).det
The two chart signs compensate for the sign of the within-range transition Jacobian.
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Changing the atlas without changing its preferred charts leaves the lift data unchanged.
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Changing only the atlas preserves the model orientation.
Changing only the atlas transports each source sign along equality of preferred charts.
The product of the source signs is exactly the sign of the within-range transition Jacobian.
Signed transition determinants agree with the absolute Jacobian factor appearing in the change-of-variables theorem for top forms.
Simultaneously reverse the model orientation and all source signs. This changes only the representative lift, not any induced tangent orientation.
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Reverse the induced orientation while holding the chart signs fixed.
Reindex the model orientation of a lift along an equivalence of index types, keeping its chart signs.
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Reindexing commutes with the diagonal flip.
A total chart sign with the fixed value 1 off the chart source. Only the source-indexed
signs are part of the lift.
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The total sign is continuous when restricted to the actual preferred chart source.
Diagonal reversal negates the total sign on the chart source.
The signed model orientation read in a preferred chart at a point of its source.
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The chart orientation is the model orientation or its negative according to the chart sign.
The tangent orientation induced by a lift, using the actual inverse tangent trivialization of the preferred chart at the point.
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Reading the tangent orientation in any preferred chart gives precisely its signed model orientation. Thus the construction is independent of the chosen chart.
Diagonally flipping a lift preserves every actual tangent orientation.
The tangent orientation at a point is its signed model orientation in that point's preferred chart.
The tangent basis sign is the model basis sign multiplied by the preferred chart sign.
The chart sign and the absolute determinant of a linear equivalence carrying a basis b to
a basis of the chart orientation combine to the determinant itself.
The tangent orientations of a lift vary locally constantly in bundle trivializations.
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Negating the model orientation reverses every induced tangent orientation.
Reindexing a lift reindexes every induced tangent orientation.
Every orientation of the tangent bundle gives a source-indexed lift, after choosing a model orientation. The model choice disappears in the diagonal quotient.
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The lift reconstructed from a tangent bundle orientation recovers that orientation pointwise.
When the given atlas has positive transition derivatives, every model orientation is the
model orientation of the orientation lift all of whose chart signs are 1. On a model with
corners, whose preferred charts are all the identity, this is the constant model orientation.
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At every point, the tangent orientation of ofHasGroupoid model hι, read in the preferred
chart at that point, is the model orientation.
Two lifts with the same model orientation and tangent orientations have the same signs.
Equality of the induced tangent orientations identifies precisely the diagonal flip.
The equivalence relation that forgets the simultaneous reversal of the model orientation and all chart signs.
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The diagonal equivalence relation is equality or simultaneous sign reversal.
Cast an orientation lift along an equality of models on the same model vector space and chart carrier.