arXiv:2609.33788v1
Abstract
We study the Siegel--Veech transform on the Poincaré section for the horocycle flow consisting of lattice surfaces with a visible horizontal holonomy vector of length at most one. We compute its first moment and derive formulas for higher and factorial moments with respect to the section's induced ergodic invariant probability measure, which is not supported on a periodic orbit. We emphasize these formulas in the case of the square torus of unit area and, as an application, use its first-moment formula to recover the Boca--Zaharescu pair-correlation density for Farey fractions.
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Detailed mathematical audit
01Statements4 reported findingsNo statement issues found
The first, higher, and factorial moment formulas follow from the parabolic-orbit decomposition of visible holonomy vectors. Their modular specializations are correct, and the first-moment formula yields the stated Farey pair-correlation density. One auxiliary finite- identity needs the harmless qualifier “for all sufficiently large .”
The cross-sectional first and second moments have the stated densities
PDF pp. 3–5 and 9–18 · arXiv:2609.33788v1
At each height , visible vectors split into residue classes under the cusp translation. Summing the common parabolic shift tiles the horizontal line, and the change supplies the factor . For two vectors, fixing their heights, residue classes, and relative shift fixes their determinant and leaves the same one-dimensional unfolding. In the modular case, regrouping primitive residue classes by gives exactly the coefficient .
The higher and factorial moment formulas follow from the same determinant parametrization
PDF pp. 18–22 · arXiv:2609.33788v1
Using the first vector as the integration coordinate leaves one common parabolic shift and records every other vector by its height and determinant with the first. Lemma 5.2 correctly expresses every pairwise determinant as , so the displayed nonvanishing conditions are exactly pairwise distinctness for visible positive-height vectors. The modular congruences count precisely the admissible primitive residue tuples.
The first moment recovers the Boca–Zaharescu pair-correlation density
PDF pp. 22–27 · arXiv:2609.33788v1
Future section returns at the periodic BCZ point encode the normalized positive Farey differences. The test transforms are uniformly bounded, Riemann integrable, and converge uniformly as , so periodic-orbit equidistribution applies. Inserting their defining function into the modular first-moment identity and changing to produces the claimed totient sum and logarithm.
The orbit-average identity is eventual, not valid for every
PDF p. 25 · arXiv:2609.33788v1
The periodic BCZ orbit average also contains wrap-around pairs with , whereas sums only pairs inside one copy of . The proof itself shows these extra terms vanish once exceeds the upper endpoint of . Thus equation (80) should read “for all sufficiently large .” For example, at a function with makes the orbit average nonzero while the within-period pair sum is empty. The corrected eventual identity is all that Theorem 6.3 uses.
02Proofs3 reported findingsNo proof issues found
The cusp unfolding, determinant bookkeeping, integrability arguments, factorial restrictions, and Farey/BCZ limiting calculation were checked through their proof-critical steps. Apart from the local quantifier correction in Lemma 6.2, the proofs are complete.
The first-moment unfolding is exact
PDF pp. 9–12 · arXiv:2609.33788v1
For fixed , consecutive cusp translates produce adjacent intervals of length , so their -integrals cover exactly once. Then cancels the prefactor . Compact support bounds the contributing heights and proves absolute integrability for signed test functions. For the square torus, , , and , yielding the density .
Relative shifts and determinant data are counted without multiplicity
PDF pp. 12–18 · arXiv:2609.33788v1
The change is bijective, and determinant invariance determines the second transformed vector from the first. In the modular regrouping, a residue pair contributes to determinant exactly when ; after that congruence, one free integer shift remains. Compact support bounds both heights and the determinant, so the signed formulas are absolutely integrable.
The Farey limit proof is complete after the stated eventual-identity correction
PDF pp. 24–27 · arXiv:2609.33788v1
A vector with transformed coordinates and returns after time , and the sum of BCZ return times is . Only finitely many primitive vectors can contribute uniformly for scaling constants near , which proves boundedness, Riemann integrability, and uniform convergence of the transforms. Wrap-around terms disappear for all sufficiently large , exactly the regime needed before taking the weak limit.
03Novelty0 reported findingsNo novelty concerns found
No non-novelty findings.