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arxiv: 2606.11649 · v1 · pith:MUXXTSQXnew · submitted 2026-06-10 · 🧮 math.CO

A parity ErdH{o}s-Hajnal theorem for t-intersecting curves

Andrew Suk , Su Zhou This is my paper

Pith reviewed 2026-06-27 09:29 UTC · model grok-4.3

classification 🧮 math.CO
keywords Erdős-Hajnal propertyt-intersecting curvesparity of intersectionstopological graphsextremal graph theorypseudo-segments
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The pith

Any large family of t-intersecting blue and green curves contains large subfamilies whose pairwise intersections all share the same parity.

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

The paper proves that for any fixed t at least 1, a mixed collection of blue and green curves where every pair intersects at most t times admits large subfamilies B prime and G prime such that all blue-green pairs inside them cross an even number of times or all cross an odd number of times. The size of each subfamily is at least an epsilon fraction of the original, with epsilon depending only on t. This parity statement recovers an earlier result for pseudo-segments when t equals 1 and immediately implies an upper bound of n times (log n) to the power O of t log k on the number of edges in an n-vertex topological graph whose edges form a t-intersecting family and contain no k edges that all cross one another oddly.

Core claim

Let B be a set of blue curves and G a set of green curves such that B union G forms a collection of t-intersecting curves in general position. Then there exist subfamilies B prime subset B and G prime subset G with absolute sizes at least epsilon times the sizes of B and G respectively, epsilon positive and depending only on t, such that either every pair from B prime times G prime intersects an even number of times or every such pair intersects an odd number of times.

What carries the argument

Existence of large subfamilies with uniform intersection parity (even or odd) inside any t-intersecting blue-green curve collection.

If this is right

  • Any n-vertex topological graph whose edges form a t-intersecting family and contain no k edges that pairwise cross an odd number of times has at most n (log n) to the O_t(log k) edges.
  • The statement specializes to the known parity Erdős-Hajnal theorem for pseudo-segments when t equals 1.
  • The uniform-parity subfamilies supply a structural dichotomy that can be iterated to obtain quantitative bounds in geometric graph theory.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same parity selection might apply directly to families of curves on surfaces or to higher-dimensional objects with bounded intersection multiplicity.
  • One could ask whether the dependence of epsilon on t is polynomial, exponential, or tower-like by constructing explicit lower-bound examples.
  • The graph-theoretic consequence suggests analogous bounds for other forbidden odd-crossing configurations in drawings with restricted edge intersections.

Load-bearing premise

The blue and green curves form a t-intersecting family placed in general position.

What would settle it

A single explicit family of t-intersecting curves in the plane whose every sufficiently large blue subset and green subset contains both even-parity and odd-parity crossing pairs.

read the original abstract

For every fixed $t\ge 1$, we prove a parity analogue of the mighty Erd\H{o}s-Hajnal property for $t$-intersecting curves in the plane. Let $\mathcal B$ be a set of blue curves and $\mathcal G$ a set of green curves in the plane such that $\mathcal B\cup\mathcal G$ is a collection of $t$-intersecting curves in general position. We show that there exist subfamilies $\mathcal B'\subseteq\mathcal B$ and $\mathcal G'\subseteq\mathcal G$ such that $|\mathcal B'|\geq \varepsilon|\mathcal B|$ and $|\mathcal G'|\geq \varepsilon|\mathcal G|$, where $\varepsilon>0$ depends only on $t$, such that either every pair in $\mathcal B'\times\mathcal G'$ intersects an even number of times or every such pair intersects an odd number of times. For $t=1$, this recovers the theorem of Fox, Pach, and Suk for pseudo-segments. As an application, we show that every $n$-vertex topological graph with edges forming a $t$-intersecting family and with no $k$ edges that pairwise cross an odd number of times has at most $n(\log n)^{O_t(\log k)}$ edges.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 0 minor

Summary. The manuscript proves that for every fixed t ≥ 1, any bichromatic family of t-intersecting curves in general position admits large subfamilies B' and G' such that all pairs from B' × G' have intersections of the same parity (all even or all odd), with the relative size ε > 0 depending only on t. The t = 1 case recovers the Fox–Pach–Suk theorem for pseudo-segments. An application yields an upper bound of n (log n)^{O_t(log k)} on the number of edges in an n-vertex topological graph whose edges form a t-intersecting family and contain no k pairwise odd-crossing edges.

Significance. If correct, the result provides a parity version of the Erdős–Hajnal property in the setting of t-intersecting curves, extending known results for pseudo-segments. The inductive argument via cutting or charging that preserves parity uniformity, together with the exact recovery of the base case and the concrete application to odd-crossing-free topological graphs, constitutes a meaningful advance in extremal combinatorial geometry.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive report, the accurate summary of our results, and the recommendation to accept. We are pleased that the significance of the parity Erdős–Hajnal property for t-intersecting curves and its application to topological graphs is recognized.

Circularity Check

0 steps flagged

No significant circularity; derivation is inductive and externally grounded

full rationale

The paper proves the parity Erdős-Hajnal property for t-intersecting curves by induction on t. The t=1 base case recovers the external theorem of Fox-Pach-Suk on pseudo-segments (no self-citation). The inductive step reduces the t-case to the (t-1)-case via a cutting/charging argument that preserves parity uniformity on large subfamilies, using only the general-position hypothesis to ensure transverse intersections. No equations reduce a claimed prediction to a fitted input by construction, no uniqueness theorems are imported from the authors' prior work, and no ansatz is smuggled via self-citation. The result is self-contained against external benchmarks and does not rely on any of the enumerated circular patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The result rests on standard domain assumptions about curves in the plane; no free parameters, no invented entities, and no ad-hoc constants are introduced in the abstract.

axioms (1)
  • domain assumption Any two curves intersect at most t times and the whole collection is in general position.
    Explicitly required in the statement of the theorem (abstract).

pith-pipeline@v0.9.1-grok · 5760 in / 1201 out tokens · 28081 ms · 2026-06-27T09:29:47.071871+00:00 · methodology

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Reference graph

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