Classification conjecture for prime alternating links with crosscap number three

Let kk be a positive integer. For a link LL, write γ(L)\gamma(L) for its crosscap number and Γ(L)\Gamma(L) for its unoriented genus. A pretzel link P(2,2m,2n)P(2\ell,2m,2n) is specified by positive integers satisfying 0<mn0<\ell\leq m\leq n.

Crosscap-three classification conjecture. For each even crossing number 2k62k\geq 6, there are exactly

p(k,3)=k2+312p(k,3)=\left\lfloor\frac{k^2+3}{12}\right\rfloor

prime alternating links LL with Γ(L)<γ(L)=3\Gamma(L)<\gamma(L)=3, namely the pretzel links P(2,2m,2n)P(2\ell,2m,2n) with 0<mn0<\ell\leq m\leq n and +m+n=k\ell+m+n=k. Here p(k,3)p(k,3) denotes the number of partitions of kk into three positive parts.

The claim gives an explicit classification and count for the observed links with crosscap number exceeding unoriented genus. It is presented as a conjecture, and no resolution is supplied.

Sources & referencesView supporting material

Primary source

Isaias Bahena, Thomas Kindred and Jason Parsley, “Crosscap numbers of alternating links via state codes”, arXiv:2512.09887 (2025).

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