Delphi.science

Two-colour Zig-Zag Numberlink on rectangles

Source: github.com/zzyzek/R2K2ZZN/tree/6567375a4c4574d26237756819e2…

Z. Zzyzek (github: zzyzek)

Palomar Registrypalomar/PALOMAR-2026-11-04-000002Nov 4, 2026cs.DMscore 0

A Lean 4 proof that two-colour Zig-Zag Numberlink on rectangular grids (the paired 2-disjoint path cover problem: two vertex-disjoint paths s0 → t0 and s1 → t1 that together visit every cell) is decided by an explicit procedure, and that an explicit construction finds a solution whenever one exists. For both sides at least 11, an instance is solvable exactly when it passes a finite catalogue of forbidden patterns; for a side at most 10, a plug dynamic program decides it. The finite cases of the characterization are 21 boxes of slice certificates and 59 window certificates, all checked by the…

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