Hyperbolic classification for isosceles triangles with prescribed apex angle

Determine the classification in the hyperbolic plane of triangles for which erecting isosceles hyperbolic triangles with prescribed apex angle $2\beta$ produces a hyperbolic analogue of the spherical Napoleonic configuration, taking into account that the Euclidean chord-angle definition of a $\beta$-point has no direct intrinsic hyperbolic analogue.

Background

The paper studies a spherical construction based on isosceles chord triangles and classifies the spherical input triangles for which the three resulting circumcentres form an equilateral triangle. It notes that the original Euclidean chord-angle definition of a β\beta-point does not transfer directly to the hyperbolic plane. A suggested hyperbolic replacement is to erect isosceles hyperbolic triangles with apex angle 2β2\beta; their base angles are automatically smaller than π/2−β\pi/2-\beta, removing the acute-versus-obtuse ambiguity present in the spherical setting. The corresponding classification, including comparison with the known β=π/3\beta=\pi/3 case, is explicitly left unresolved.

References

What is the corresponding classification in the hyperbolic plane? The Euclidean chord-angle definition of a $\beta$-point has no direct intrinsic analogue, but Remark~\ref{rem:equivalent} suggests erecting isosceles hyperbolic triangles with apex angle $2\beta$. Their base angles are automatically smaller than $\pi/2-\beta$, so that there is no ambiguity between acute and obtuse base angles. For $\beta=\pi/3$, this construction should be compared with .

— Napoleonic triangles on the sphere from similar chord triangles  (2610.21408 - Tran, 15 Oct 2026) in Section 6, Further questions, second Question