Delecroix–Lelièvre's orbit-count conjecture in \mathcal{H}44

Let nn be an integer greater than 88, and consider primitive nn-squared origamis in H(4)\mathcal{H}(4), with Hodd(4)\mathcal{H}^{\mathrm{odd}}(4) denoting the odd component and Hhyp(4)\mathcal{H}^{\mathrm{hyp}}(4) the hyperelliptic component. Delecroix–Lelièvre's orbit-count conjecture. The number of SL(2,Z)\operatorname{SL}(2,\mathbb{Z})-orbits is as follows: there are precisely two orbits in Hodd(4)\mathcal{H}^{\mathrm{odd}}(4) outside the Prym locus, distinguished by monodromy groups Alt(n)\operatorname{Alt}(n) and Sym(n)\operatorname{Sym}(n); for odd nn, there are precisely four orbits in Hhyp(4)\mathcal{H}^{\mathrm{hyp}}(4), distinguished by HLK-invariants (4,[1,1,1])(4,[1,1,1]), (2,[3,1,1])(2,[3,1,1]), (0,[5,1,1])(0,[5,1,1]), and (0,[3,3,1])(0,[3,3,1]); and for even nn, there are precisely three orbits in Hhyp(4)\mathcal{H}^{\mathrm{hyp}}(4), distinguished by HLK-invariants (3,[2,2,0])(3,[2,2,0]), (1,[4,2,0])(1,[4,2,0]), and (1,[2,2,2])(1,[2,2,2]). The source attributes this conjecture to Delecroix and Lelièvre, notes substantial computational evidence, and additionally records the implicit expected orbit growth Ω(n5)\Omega(n^5); the classification itself remains conjectural in the supplied text.

Sources & referencesView supporting material

Primary source

Luke Jeffreys and Carlos Matheus, “Euler characteristics of SL(2,Z)-orbit graphs of origamis”, arXiv:2602.21984 (2026).

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