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Legendrian barriers in prequantization bundles

T0 review · 2 major / 4 minor · reviewed 2026-08-03 · deepseek-v4-flash

Pith's one-line read Prequantization bundles carry explicit Legendrian barriers: removing one makes long embedded contact flows impossible.

desk verdict Generalizes the S^3 Legendrian barrier to prequantization bundles, but the main theorem is proved only for quasi-holomorphic-section skeleta, not for all polarizations, and k=2 degenerates. read the letter →

arxiv 2512.18808 v2 pith:LHTZWM4T submitted 2025-12-21 math.SG

classification math.SG MSC 53D3553D1253D10
keywords LegendrianbarrierprequantizationbundleisotropicskeletoncontactHamiltonianLagrangiansymplecticdiscquasi-holomorphicsectionrigidity
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper proves that every prequantization bundle—the contact circle bundle over a closed symplectic manifold with integral symplectic class—contains explicit Legendrian barriers. For a polarization of degree $k\ge 2$, the barrier is the minimal Legendrian lift of the polarization's isotropic skeleton: it meets each fiber over the skeleton in $k$ equally spaced points. The main theorem says that any closed Legendrian submanifold moved by any contact Hamiltonian $H\ge 1$ either hits the barrier within time $1/k$, or its $1/k$-time sweep fails to be embedded. In other words, deleting the barrier destroys the possibility of long embedded Reeb or Hamiltonian cylinders. This gives a quantitative contact rigidity phenomenon in all dimensions and recovers the earlier three-dimensional sphere result as a special case.

What carries the argument

The proof's load-bearing object is a Liouville polarization of the negative symplectization, identified with a symplectic disc bundle over $N$. Starting from a quasi-holomorphic section $\sigma$ of the $k$th power of the tautological line bundle whose norm has non-degenerate critical points, the paper extends $\sigma$ to a compactifying projective sphere bundle via $s_\varepsilon=(1-R)^{k/2}P_R\sigma+\varepsilon R^{k/2}P_R\sigma_\infty$. The zero set of $s_\varepsilon$ is a smooth divisor, and the associated Liouville form has a skeleton equal to $\Phi^{-1}(\Lambda_k \times (0,1))$—the radial product of the Legendrian barrier, where $\Phi$ is the identification between the disc bundle and the negative symplectization. A decomposition theorem for symplectic disc bundles then identi

What would settle it

A direct test: in the standard prequantization $S^{2n+1} \to \mathbb{CP}^n$ with the degree-$2$ skeleton made of half great circles, look for a closed Legendrian $\Lambda$ and a contact Hamiltonian $H\ge 1$ such that $\Phi^t_H(\Lambda)$ stays embedded for $t\in [0,1/2]$ and avoids the two-point Legendrian lift $\Lambda_2$. Finding one would disprove the $1/k$ bound. Alternatively, exhibiting a degree-$2$ polarization whose skeleton is not realizable by any $\kappa < 1$ quasi-holomorphic section with non-degenerate norm logarithm would show the proof does not cover the theorem's stated hypotheses.

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Extended reading notes

Core claim

The central discovery is that the Legendrian lift of a Lagrangian barrier is itself a Legendrian barrier. Let $(N,\tau)$ be a closed symplectic manifold with integral symplectic class, $\Gamma_k$ an isotropic skeleton of a polarization of degree $k\ge 2$, and $\Lambda_k$ its minimal Legendrian lift to the prequantization bundle. For every closed Legendrian $\Lambda$ and every smooth contact Hamiltonian $H\ge 1$, either there is an $H$-chord from $\Lambda$ to $\Lambda_k$ of length at most $1/k$, or the union of $\Phi^t_H(\Lambda)$ over $t\in [0,1/k]$ is not embedded. When $H$ is autonomous this becomes a chord from $\Lambda$ to itself or to $\Lambda_k$ of length $\le 1/k$. A direct consequence is that no contact form $\alpha' \le \alpha$ admits a strong contact embedding of $D(1/k)^n \times S^1$ into $P\setminus \Lambda_k$.

Load-bearing premise

The theorem is stated for every polarization of degree $k\ge 2$, but the proof needs the skeleton to be the skeleton of a quasi-holomorphic section of the $k$th power line bundle with control constant $\kappa < 1$, transverse zeros, and non-degenerate critical points of the norm's logarithm; such sections are only known to exist for large $k$.

Editorial extensions

If this is right

  • For any contact form α'≤α, there is no strong contact embedding of D(1/k)^n×S^1 into P\Λ_k; the largest round cylinder that fits contactomorphically has radius at most 1/k.
  • Any Legendrian isotopy in P\Λ_k generated by a contact Hamiltonian H≥1 has discriminant length at least k, so the barrier forces every long isotopy to pass through non-embedded stages.
  • The area class of an isotropic skeleton of degree k is discrete with values in (1/k)Z; this rationality is exactly what makes the k-fold Legendrian lift well-defined.
  • For any smooth polarization of degree k, every Lagrangian immersion in M\L_k bounds a symplectic disc of area <1/k, so the Lagrangian capacity of the complement is at most 1/k.
  • For autonomous Hamiltonians the statement reduces to a chord from Λ to itself or to Λ_k of length ≤1/k, giving a concrete realization of the paper's notion of universal interlinking.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The proof requires the skeleton to be realizable by a quasi-holomorphic section with control constant κ<1; known existence results only guarantee such sections for large k. If some degree-2 polarization fails this condition, the theorem as stated would need a separate treatment for small k.
  • The same strategy, using a degenerate symplectic form on a projective compactification, may produce barriers in other open symplectic manifolds with rational symplectic class, not only negative symplectizations of prequantization bundles.
  • The paper's (δ,δ')-universal interlinker notion suggests that the constants 1/k could be sharp for contact capacities; testing whether δ and δ' can be tuned independently would give finer quantitative invariants.
  • The k=1 case appears to be genuinely exceptional—the statement already fails for complex projective space with the Fubini–Study form—so the rational-area mechanism underlying the proof seems to be the decisive feature.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The paper studies Legendrian barriers in contact prequantization bundles. The main result, Theorem 1, asserts that for a closed symplectic manifold (N,τ) with integral symplectic class, any isotropic skeleton Γ_k of a polarization of degree k≥2 admits a k-fold Legendrian lift Λ_k in the prequantization bundle (P,α), and this lift is a Legendrian barrier: for any closed Legendrian Λ and any smooth contact Hamiltonian H≥1, either there is an H-chord from Λ to Λ_k of length ≤1/k, or the 1/k-sweep of Λ is not embedded. The proof uses Mohnke's trick to convert an embedded long sweep into a Lagrangian submanifold in the negative symplectization, then identifies the negative symplectization with a symplectic disc bundle over N. The technical core constructs, from a quasi-holomorphic section σ of L^k, a Liouville form whose skeleton is exactly the preimage of Λ_k×(0,1), and then applies Biran's decomposition and a ruled-manifold capacity bound. The paper also proves a Lagrangian analogue (Theorem 3) bounding the Cieliebak–Mohnke capacity of complements of polarizations. The proof is largely computational and self-contained, but there is a significant gap between the statement of Theorem 1 and the hypotheses actually used in its proof.

Significance. If the result holds in the stated generality, it is a substantial contribution: it generalizes the Legendrian barrier phenomenon from S^3 (proved in [OS24]) to all prequantization bundles, with the sharp constant 1/k and an explicit, geometrically meaningful barrier Λ_k. The construction via quasi-holomorphic sections and the degenerate compactification of the symplectic disc bundle are useful tools that may find further applications. The paper also gives a clean proof of a Lagrangian barrier theorem (Theorem 3) and provides a detailed account of Mohnke's trick. However, the central dichotomy is proved only for skeleta associated to quasi-holomorphic sections, while the theorem claims all degree-k polarizations. This mismatch, together with an apparent degeneration of the construction at k=2, prevents the main theorem from being established as stated.

major comments (2)
  1. [Theorem 1 vs. Section 8] Theorem 1 quantifies over all isotropic skeleta associated to a polarization of degree k≥2 (Definition 3.4), but Section 8 begins with 'Let Γ_k ⊂ N be an isotropic skeleton associated to a quasi-holomorphic section of L^k → (N,τ).' No argument is given that every polarization degree k (especially k=2,3) admits a quasi-holomorphic section σ with transverse vanishing and ln|σ| Morse. Section 5 explicitly says such sections 'might exist when k≫1 by Donaldson-Giroux theory ... or for any other reason.' Since Proposition 7.4 and the subsequent Biran-decomposition step rely on the explicit s_ε construction, the proof covers only section-associated skeleta. The theorem should be restated in that restricted class, or a reduction from arbitrary polarizations to this class must be supplied.
  2. [Section 5, k=2 degeneration] The critical-point analysis in Section 5 produces interior critical points at R = (1 + (ε/|σ(z)|)^{2/(k−2)})^{-1}. For k=2 this exponent is undefined; solving the critical-point equations forces |σ(z)|=ε, so for generic ε there are no interior critical points. Correspondingly, in Lemma 5.4 the Hessian term −k(k−2)ε^2 R^{k−3}/(2(1−R)) dR^2 vanishes, so −ln|s_ε| is not Morse in the radial direction, and Corollary 6.3 cannot identify stable manifolds as claimed. Lemma 6.5's later remark that k=2 can be handled by taking ε≫1 does not restore the missing critical points. Consequently Lemma 6.4 and Proposition 7.4 do not establish the skeleton equality Skel(s_ε)=Φ^{-1}(Λ_k×(0,1)) for k=2, so Theorem 1 is not proved in the stated k≥2 range.
minor comments (4)
  1. [Abstract/Organization] The text 'theomem 1' in the Organization paragraph is a typo for 'Theorem 1'. Also 'Aknowledgements' should be 'Acknowledgements'.
  2. [Notation] The notation L^k and L^{⊗k} is used interchangeably across Sections 3.5, 4.4, and 5; this may confuse readers. Consistency is recommended.
  3. [Section 3.4] In the proof of Lemma 3.7, the unexplained symbol κ′(κ) is introduced but never used; either remove it or clarify its role.
  4. [Definition 3.4] 'a s tuple' should read 'a tuple'. Also 'Poincaré-Dual' should be 'Poincaré dual' in several places.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular reduction; main risk is a scope gap between the stated theorem and the quasi-holomorphic-section proof, not circularity.

full rationale

Walking the derivation chain, I find no step in which a claimed prediction is equal, by construction or by a fitted parameter, to an input. The barrier constant 1/k is the degree of the polarization / residue of the Liouville form constructed in Section 7, not a fitted value; Proposition 3.5 derives the k-equipartition from the area class (1/k)Z, and the main estimate T = A_min(L) < 1/k comes from Theorem 3.1 after Biran-compactifying the basin supplied by Proposition 7.4. Self-citations are present but not circularly load-bearing: [OS24] is the motivating S^3 theorem, [Ops13] is used as an independent theorem identifying basins of tame Liouville forms with disc bundles, and [Ops15] supplies a residue computation; none of these assumes the conclusion of Theorem 1. The real weakness is a statement/proof mismatch: Theorem 1 quantifies over all polarizations of degree k ≥ 2, while Section 8 opens with “an isotropic skeleton associated to a quasi-holomorphic section”, and Section 5 concedes “Such sections might exist when k≫1 by Donaldson-Giroux theory”. The k = 2 case also degenerates in the displayed critical-radius formula (exponent 2/(k−2) and the vanishing −k(k−2) coefficient in Lemma 5.4). These are unproved generality and technical gaps, not circular reductions: an unsupported bridge from polarizations to quasi-holomorphic sections does not make the theorem's output an input. Score 2 reflects the minor self-citations; no circular step is scored.

Assumptions & free parameters 1 free parameters · 6 assumptions · 0 invented entities

No speculative entities are introduced: Lambda_k and Z_k are constructed from the input data (polarization/section and degree k), not postulated. Free parameters are limited to the deformation parameter epsilon and the implicit choice of quasi-holomorphic section; constants 1/k and the fiber area are structural inputs, not fit parameters. The main external inputs are the SFT compactness toolkit, Donaldson-Giroux quasi-holomorphic sections (only asymptotic in k), the author's own [Ops13] and [Ops15], and standard Gromov/positivity theory.

free parameters (1)
  • epsilon (deformation parameter in s_epsilon = s_0 + epsilon s_infinity) = epsilon > 2 max_N |sigma| when k = 2; small for k >= 3
    Hand-chosen deformation parameter controlling the zero set Z(s_epsilon) and the skeleton. The final bound 1/k is independent of epsilon, but the construction requires epsilon in a specific range, qualitatively different for k = 2 (Lemma 6.5).
assumptions (6)
  • domain assumption SFT compactness (Bourgeois-Eliashberg-Hofer-Wysocki-Zehnder) applies to the stretched-neck degenerations used in Theorem 3.1
    Invoked in the proof of Lemma 3.2/Theorem 3.1 to produce the holomorphic building B with a sphere in the fiber class through p; standard but heavy machinery, assumed rather than proved.
  • domain assumption Existence of a kappa-quasi-holomorphic section sigma of L^k, transverse to zero, with ln|sigma| Morse and kappa < 1
    Opening of Section 5. Guaranteed by Donaldson-Giroux only for k >> 1; for small k (in particular k = 2) the input is an assumption not delivered by the cited theory, while Theorem 1 is stated for all polarizations of degree k >= 2.
  • domain assumption The basin of attraction of a tame Liouville form with residue -1/k is a symplectic disc bundle SDB(Z_k, 1/k)
    Used in Section 8 via the author's own [Ops13]. A published external result, but self-cited, and the degenerate context of Section 4.3 (non-symplectic compactification) is not identical to the standard setting.
  • domain assumption The Liouville form lambda on N\Sigma has residue -1/k along Sigma (rationality of Lagrangian skeleta)
    Proposition 3.5; the proof is sketched and completed by reference to the author's own [Ops15]. Load-bearing for Corollary 3.6 (holonomy in Z_k) and for the kZ arithmetic in Section 8.
  • standard math Positivity of intersections and Gromov compactness for the fiber class F in the sphere bundle
    Used in Lemma 3.2's compactness argument (only one bubble in class F, no H_2(Hur)(Y) bubbles); standard symplectic-topology input.
  • domain assumption Morse-theory dichotomy: for the gradient-like flow X_epsilon, every forward trajectory either reaches Z(s_epsilon) in finite time or converges to a critical point of phi_epsilon
    End of Section 6: 'classical arguments of Morse theory', after Lemma 6.5 shows X_epsilon points away from N_infinity and is tangent to N_0. Requires Lyapunov/properness control of phi_epsilon = -ln|s_epsilon| on the open manifold SDB(N, tau).

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Pith. "Pith review of Legendrian barriers in prequantization bundles." pith.science (2026). https://pith.science/paper/LHTZWM4T

@misc{pith2026251218808,
  author       = {Pith},
  title        = {Pith review of: Legendrian barriers in prequantization bundles},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/LHTZWM4T}},
  note         = {Machine review of arXiv:2512.18808}
}
read the original abstract

We show that prequantization bundles have explicit Legendrian barriers, whose removal obstruct the embedding of long cylinders over Legendrian submanifolds.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Lengths of Reeb chords and Viterbo restriction

    math.SG 2026-06 accept novelty 6.5 of 10

    Products of spheres yield n-invertible cotangent bundles, so every compact Legendrian in ST*M has a Reeb chord of length at most the n-invertibility capacity.

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