From the weekly letter · 9 November 2026

Why cover this story this week?

A crossing restriction forces sparse networks

Imagine drawing a network in the plane while forbidding a sufficiently large collection of edges from all crossing one another. Must the network have only linearly many edges as its vertex count grows? A new preprint answers yes for every fixed forbidden collection size. The drawings can be complicated: edges may cross repeatedly, and even edges sharing an endpoint may cross.

The proof changes the setting rather than simplifying each crossing. Removing a small disk around every vertex turns edges into arcs on a sphere with boundary circles. The authors then use a theorem about patterns on surfaces to bound the number of arcs, supported by a triangulation whose size grows linearly with the original network. The result shows how a global restriction on crossings can control network density, and how topology can supply a counting tool for combinatorial geometry.

Editorial context written by AI from abstracts, not independent scientific validation. The kit and its verified quotes rely on the source paper, not on this letter.

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