Divisor criterion for the square-root class of Belyi functions

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Let nn be an odd positive integer, let XX be an algebraic curve, and let f:X→P1f:X\rightarrow\mathbb{P}^1 be a Belyi function with monodromy of cycle type (n,311⋯1,n)(n,311\cdots1,n). Let PP and OO be the locations on XX of the ramifications of order n−1n-1, and let TT be the location of the ramification of order 22. Let Sqct⁡(f)\operatorname{Sqct}(f) denote the square-root class appearing in the claim.

Square-root class conjecture. One has

Sqct⁡(f)={(22⋯2111,322⋯2,n)}\operatorname{Sqct}(f)=\{(22\cdots2111,322\cdots2,n)\}

if and only if

(T)∼n+12(P)−n−12(O)(T)\sim\frac{n+1}{2}(P)-\frac{n-1}{2}(O)

as divisors on XX.

The authors say that this was explicitly verified for g=1g=1 and n=5,7,9n=5,7,9, and present it as a conjectural criterion for adapting the square-root cycle-type class to a divisor-theoretic Galois invariant. The general assertion remains unresolved in the source.

References

Primary source

Ravi Jagadeesan, “A new Gal(Q/Q)-invariant of dessins d'enfants”, arXiv:1403.7690 (2014).

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