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There are classical surfaces of revolution, shaped like footballs, that have constant positive curvature, except for their two cone points. How about such a surface with three cone points?

To give some context, a collaborator and I are trying to understand the moduli space of labeled quadrilaterals in $\mathbb{R}^3$ whose side lengths are all equal but whose joints are allowed to rotate freely. Kapovich and Millson showed that this moduli space is a Riemannian 2-manifold (in fact, a Kähler manifold), except for singular points corresponding to the three polygons that lie entirely along a single line. A fairly lengthy computation shows$^*$ that this manifold has constant positive curvature $1$; that each of the three singular points is an order-2 cone point; and that the distance from any cone point to any other is $\frac{\pi}{2}$. So the space looks something like the picture below, except that it has constant curvature.

The moduli of quadrilaterals

It would be nice to have an isometric embedding of this space in $\mathbb{R}^3$. Since this is the simplest example of an elliptic orbifold with three cone points, we are hoping that there might be a known parametrization.

$^*$ For this, we scale the quadrilaterals to have unit perimeter, so their sides have length $\frac{1}{4}$.

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  • $\begingroup$ A reference that might be at least tangentially relevant, not sure: W.J. Lenhart, S.H. Whitesides Reconfiguring closed polygonal chains in Euclidean d-space Discrete Comput. Geom., 13 (1995), pp. 123-140 $\endgroup$ Commented Jun 29, 2022 at 23:40

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