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A first saddle lobe admits a collar-fixed center-saddle cancellation
Statement
Assume Countable Choice . Let be a cooriented codimension-one foliation of a smooth -manifold and let be a disk map with a regular outer collar. Suppose the characteristic foliation of has a compact embedded disk lobe whose regular leaves are the full nested circles about one nondegenerate center . Suppose its frontier is one embedded piecewise- separatrix circuit with one nondegenerate saddle and otherwise regular arcs; there are no other characteristic critical points in a neighborhood of . Let be the cooriented first integral on the full circle annulus, continued across the center/saddle block and its regular collar. For the standard Euclidean metric on the source disk, assume has exactly one unstable half-trajectory from entering with forward limit , while its other unstable half-trajectory exits through a regular transverse section before any other singularity. Assume lies in one ambient leaf and fix a leafwise smoothing collar from the saddle corner to a regular loop , together with one compact filling of in . After flattening the fixed collars, the leafwise cap has outer boundary exactly . Then one can choose a compact regular-neighborhood block containing the lobe and saddle and construct a replacement map with the same outer boundary map, equal to the original map on an open collar of and outside , and with no characteristic critical points in ; all characteristic singularities outside are unchanged, so exactly one center and one saddle are removed. No homotopy from the original map on the interior of is asserted.
Facts & Assumptions
Given: The disk map and data of the statement, with a lobe , saddle , center , first integral , the prescribed exit section and fixed cap data.
A cancelling disk triad has an exact C² boundary scalar cancels a smooth minimum/saddle disk triad with interval faces and trajectory sides while preserving an actual scalar on an open collar of the entire boundary. It uses an auxiliary annular slab only to obtain a localized nonzero field, not as an actual source block.
Adapted descending field near a compact morse band supplies smooth adapted fields; its regular-chart patching preserves the sign of the scalar derivative. Every Picard–Lindelöf initial value problem has one maximal solution on an open interval supplies uniqueness of regular trajectories.
A fixed leafwise cap gives a joint transverse product with exact collar data and A fixed cap product glues by unique transverse flow roots supply a jointly transverse product and exact pointwise trace matching when cap and trace use the same short transverse-flow segments. Its uniform interval is fixed before the collar range is checked. Identity holonomy of a leafwise-null frontier follows from The holonomy representation and the holonomy group of a leaf and Holonomy depends only on leafwise homotopy relative to endpoints.
Finite C2 surface carriers have smooth normal forms and relative cap approximations gives a leafwise cap equal to a prescribed collar germ. Convolution with a mollifier is smooth, and derivatives pass under the integral sign supplies smooth source approximations; for inputs their derivatives through order two converge uniformly on compact subsets by uniform continuity. Morse lemma supplies the smooth critical charts after approximation, and The Euclidean inverse function theorem gives local inverses.
The standing assumption is Countable Choice (The countable-choice principle used in the foliation pair).
Proof
Choose one smooth positively transverse field near the compact image of . In each foliation box fix the intrinsic plaque of through its arc of , with transverse coordinate . The equation has a unique short root, since its derivative is nonzero. Its projected point is , and the assigned plaque and common short orbit make the roots agree on overlaps. The specified smoothing collar and null filling make the frontier leafwise null, so its identity holonomy returns the same assigned plaque after a circuit; this constructs on a thin neighbourhood of all of , including . Choose a regular inner circle inside . The given rounding collar and cap supply a continuous filling of its projection in the intrinsic leaf. Prescribe on its whole boundary collar, apply the relative cap approximation in [F4] to preserve that germ on a smaller open collar, and extend by outside this inner circle to form an enlarged cap . Thus on a neighbourhood of and the eventual source boundary; no saddle jets are flattened in making this extension. Apply [F3] with the same on this fixed cap before choosing the final block. It gives a jointly product , and its actual section satisfies pointwise on . Since on compact , shrink so its compact section ranges lie in .
Near the center choose the pullback of a genuine transverse foliation coordinate. If is a local nonvanishing defining one-form, then with . At a characteristic zero its derivative is , so the nondegenerate-center hypothesis makes a genuine Morse minimum after choosing the increasing orientation. The same argument makes the actual section Morse at . On each regular full-circle annulus, , and factor through the regular quotient coordinate, with positive one-variable derivatives after fixing their common coorientation. Use only near , near the outer annulus and , and interpolate their positive derivatives on a compact middle quotient interval. The positive affine amplitude and offset of are free: choose its endpoint value below the fixed upper value and its amplitude sufficiently small, then choose a positive interpolating derivative with the required integral. Integrating this derivative joins the two primitives with matching germs. This gives a primitive , equal to actual near and its exterior collar, with just the minimum and saddle. It uses only on regular annuli and assumes no nondegenerate Hessian for at .
Raise the minimum of toward by a strictly increasing reparameterization on the inner circle component, equal to the identity on a neighbourhood of the frontier value ; equivalently choose the small inner amplitude and positive interpolation in step 2.1 to make its minimum as close to from below as needed. This preserves the regular circle leaves and the actual germ near and leaves the exterior negative branch unchanged. Choose in that exterior branch tube and , with , so all these boundary levels and their buffers lie in . Smooth in on a compact neighbourhood in the ambient source plane, without imposing boundary equality, to a smooth . For sufficiently small error it has precisely one minimum and one saddle near and no other critical points: outside small critical balls the gradient has a positive minimum, while in each ball its Hessian remains within half the least singular value of the original Hessian. The map is then a contraction on a smaller ball, with its displacement of the old zero smaller than the inward margin, so its iterates converge to the unique new zero; the Hessian inertia is unchanged. Keep and on the eventual boundary collar.
Build an adapted descending field for in small Morse charts and regular connecting tubes using [F2]. The original inward saddle branch crosses a finite regular tube into a compact center sublevel disk, and the outward branch crosses a finite tube through the prescribed transverse exit. Strict derivative signs on these compact tubes persist under the chosen approximation. Route the corresponding local saddle rays through these two tubes, patching positive scalar derivatives in regular charts; the inward ray enters the center disk, whose decreasing flow has only the minimum as possible limit, and the other ray exits below . Thus exactly one of the saddle's two descending branches limits on the minimum. Reverse sign to obtain a smooth upward adapted field ; on the boundary buffer choose its regular pieces sufficiently close to that , since there. These are strict signs on compact regular regions; no equality of perturbed orbits or derivative estimate for a cancelled scalar is assumed.
Construct the actual interval-face block inside . At take a short exterior incoming interval straddling the outward descending branch. Its two endpoints, chosen on opposite sides of that branch, have upward trajectories avoiding the saddle and passing through its two outgoing arms. Follow these trajectories to , and join them by the outgoing level interval that runs around the near-frontier circle component. The enclosed region is the incoming interval strip with the center zero handle and saddle one handle added: below the center there is the exterior interval strip, the center adds one disk component, and the saddle band attaches once to that disk and once to the strip. After this joining the outgoing face is one interval and the region is a disk, with precisely in its interior and two trajectory sides. This also describes its boundary without assuming fictitious circle faces. The regular frontier arcs, the saddle chart and the chosen exit tube form a finite compact collection. Choose , side widths and the approximation error sufficiently small that the whole boundary buffer remains in and is regular, disjoint from and the closed connecting orbit. Thus the actual section is defined on the entire boundary collar, there, and its face errors from are less than .
Apply [F1] to this actual disk triad, its smooth and its scalar on the full boundary collar. The unique orbit in step 4.1 is the sole attaching-belt intersection. The supplier glues an abstract regular rectangle to the two sides only for the two-circle-face Milnor cancellation theorem; the field modification is supported inside the actual , so unchanged tangent sides prevent a modified trajectory from leaving . Its smooth product coordinates then integrate a positive density with variable face values and with a transverse side blend. Integration by parts gives a primitive while preserving the entire boundary germ. Consequently there is with and on an open collar of all of . Round corners within this regular collar, retaining the scalar germ and the enclosed lobe and saddle.
Choose an open interval with the compact boundary section range , and choose inside with . A positive smooth function can equal near and have tail integrals and : join to positive exponential tails on arbitrarily short intervals and make the remaining tail integrals arbitrarily small. Set . Then , is the identity on , and . Hence is , , and on the exact boundary collar. This controls the whole cancelled scalar range without claiming it is close to the old scalar.
Define on and extend by on its open boundary collar and outside . On the collar, steps 1.1 and 7.1 give , so this is a map with the same outer boundary map. Since , the characteristic distribution of its graph is , which is regular everywhere in . Exactly the original one center and one saddle have been removed there; every singularity outside is unchanged. No interior homotopy is required. Only the standing choice in [F5] is inherited through the suppliers, and all extra collars, rectangles, cutoffs and tail parameters are finite choices.
Depends on
- The holonomy representation and the holonomy group of a leaf
- Holonomy depends only on leafwise homotopy relative to endpoints
- Convolution with a mollifier is smooth, and derivatives pass under the integral sign
- Morse lemma
- A cancelling disk triad has an exact C² boundary scalar
- Finite C2 surface carriers have smooth normal forms and relative cap approximations
- Adapted descending field near a compact morse band
- A fixed leafwise cap gives a joint transverse product with exact collar data
- Morse cancellation criterion via a unique connecting orbit
- The cancellation modification is supported in a trajectory neighbourhood
- The countable-choice principle used in the foliation pair
- The Euclidean inverse function theorem
- Every Picard–Lindelöf initial value problem has one maximal solution on an open interval
- A fixed cap product glues by unique transverse flow roots
Used by
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Sources
- Mark Brittenham, Foliations and the Topology of 3-manifolds, class 11, author-hosted lecture notes (standard reference, not scraped)