Completeness conjecture for obtuse rational lattice triangles

From papers

Let TT be a rational triangle, meaning that its angles are rational multiples of π\pi. The triangle TT is a lattice triangle if its unfolding is a translation surface (XT,ωT)(X_T,\omega_T) whose SL2(R)\mathrm{SL}_2(\mathbb{R})-orbit is closed in its stratum. An obtuse triangle has one angle in (π/2,π)(\pi/2,\pi).

Completeness conjecture. The known list of obtuse rational lattice triangles is complete: every obtuse rational lattice triangle is either a member of one of the two infinite families

(πn,πn,(n2)πn)\left(\frac{\pi}{n},\frac{\pi}{n},\frac{(n-2)\pi}{n}\right)

and

(π2n,πn,(2n3)π2n),\left(\frac{\pi}{2n},\frac{\pi}{n},\frac{(2n-3)\pi}{2n}\right),

or is Hooper's triangle with angles

(π12,π3,7π12).\left(\frac{\pi}{12},\frac{\pi}{3},\frac{7\pi}{12}\right).

The acute and right-angled cases are described as fully resolved, whereas the classification of obtuse scalene triangles, those with distinct interior angles, remains the primary open challenge.

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Sources & referencesView supporting material

Primary source

David Kurniadi Angdinata, Evan Chen, Ken Ono, Jiaxin Zhang and Jujian Zhang, “On the paucity of lattice triangles”, arXiv:2603.23928 (2026).

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