The one-sided-degenerate P2 disk model #
The ordinary P2 model glues two polygons along a marked side, with at least one old side on each child. Cancellation in the Gallier--Xu normalization also uses the limiting case in which one child is a monogon. This file supplies a concrete model for that case.
The local target is a polynomial teardrop. The map
z ↦ (1 - z) ^ 2
is injective on the closed unit disk: translating by 1 puts the disk in a closed half-plane,
on which squaring has no nontrivial antipodal pair. Its image is star-shaped at the cusp. A
scaled copy receives the monogon, while the other child fills the collar between that copy and
the full teardrop.
Every polygon cell has the closed-unit-disk norm bound.
The polynomial disk used as the target of the degenerate two-child gluing.
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A point of the closed unit disk with real part 1 is the cusp point 1.
Squaring after translation by 1 is injective on the closed unit disk.
The teardrop image, equipped with the subspace topology inherited from the plane.
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The polynomial map as a map into its exact range.
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The closed disk is homeomorphic to its polynomial teardrop image.
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Every radial contraction of a teardrop point is again a teardrop point.
The monogon and collar maps #
The monogon occupies the inner radial half of the teardrop.
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Half of a teardrop point is still in the full teardrop.
The nonnegative height of the upper circular boundary above a fixed real coordinate.
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The lower unit-circle point on the vertical chord through a digon point, squared so that the two chord endpoints sweep the teardrop boundary once.
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The collar's boundary-direction point, bundled as a point of the unit disk.
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The teardrop boundary value of the collar direction has an exact quadratic factor.
The digon fills the radial collar between the half-size and full teardrop boundaries.
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Away from the two chord tips, the collar map is a radial teardrop value with scale in
[1/2, 1].
Exact compatibility on the fresh seam #
On the upper circular boundary of the digon, the collar meets the half-size teardrop occupied by the monogon.
Side zero of the digon is the upper semicircle with its usual angle parameter.
The imaginary coordinate on side zero of the digon.
The chord height agrees with the imaginary coordinate on the upper digon side.
Squaring the lower endpoint of an upper digon chord gives the corresponding monogon boundary point.
The analytic child map respects the exact reversed fresh-side parameter used by P2.
Descending the base child pair to the teardrop #
The base one-sided-degenerate child pair: a monogon and a digon.
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Map both children into their complementary regions of the teardrop.
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- LeanEval.Topology.ClassificationOfSurfaces.P2DegenerateDisk.baseChildPairMap (Sum.inl z) = ⟨LeanEval.Topology.ClassificationOfSurfaces.P2DegenerateDisk.monogonMap z, ⋯⟩
- LeanEval.Topology.ClassificationOfSurfaces.P2DegenerateDisk.baseChildPairMap (Sum.inr z) = ⟨LeanEval.Topology.ClassificationOfSurfaces.P2DegenerateDisk.collarMap z, ⋯⟩
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The child-pair map is constant on the generating fresh seam.
The child-pair map is constant on the equivalence relation generated by the fresh seam.
The continuous analytic map induced on the one-sided-degenerate child quotient.
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The collar has no hidden interior identifications #
The nonnegative radial magnitude used by the collar map.
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Equal non-tip collar values have the same boundary direction.
The lower unit-circle representatives used by the squaring map.
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Squaring is injective on the open lower semicircle used by non-tip collar chords.
The collar is injective away from its two pinched tips.
The only zero-height digon points are the two endpoints of its upper side.
Either collar tip is identified with the monogon basepoint by the generated seam relation.
Equality inside the collar is exactly equality modulo the two endpoint seam identifications.
Every point on the upper circular boundary of the digon has a side-zero parameter.
A disk point cannot lie radially beyond a non-cusp boundary point of the teardrop.
The half-size teardrop map reaches the cusp only at the monogon basepoint.
A monogon point and a collar point have the same image exactly along the declared seam.
Equality under the analytic child-pair map is exactly the generated seam relation.
Surjectivity onto the teardrop #
Every radial scale between one half and one is realized by a unique vertical chord in the digon collar, for any non-cusp teardrop boundary direction.
Every non-cusp teardrop point lies on a unique radial segment from a non-cusp boundary
direction, at a scale in (0, 1].
A monogon glued along its entire side to one side of a digon is a closed disk.
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The complete base equivalence from the unsplit monogon to the one-sided-degenerate child quotient.
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Exact compatibility with the surviving digon side #
In the base one-sided-degenerate P2 equivalence, the source side is exactly the surviving digon side.
Transport to an arbitrary positive surviving side count #
Boundary weights which make side zero fill one digon semicircle and divide the other
semicircle equally among the r old sides.
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The common parameter occupied by old side i inside the surviving digon semicircle.
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Reparameterize the (r+1)-gon boundary as a digon boundary, assigning the entire first
semicircle to the fresh side.
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Radially extend the weighted boundary map from the surviving child to a digon.
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Forget the side subdivision on the unsplit source disk.
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Simultaneously straighten a one-sided-degenerate child pair to the base monogon--digon pair.
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The weighted child straightening identifies exactly the two generated seam relations.
The arbitrary one-sided-degenerate child quotient is identified with the base monogon--digon quotient.
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The local source-to-children equivalence for any ordinary-valid right one-sided-degenerate P2 cut.
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Every old source side is carried to the corresponding surviving-child side with its exact original parameter.