Suppose that <i></i>₀ is an unknotted simple closed curve contained in the 3-sphere which happens to be invariant under a subgroup <i>G</i> of the Mbius group of <i>S³</i> = the group (generated by inversions in 2-spheres). It is shown that there is an equivariant isotopy <i></i>_<i>t</i>, 0 <i>t</i> 1, from <i></i>₀ to a round circle <i></i>₁.

We study the spherical cone metrics on surfaces from the point of view of inner angles. A rigidity result is obtained. The existence of spherical cone metric of Delaunay type is also established.

The goal of this chapter is an attempt to relate some ideas of Grothendieck in his Esquisse dun programme [10] and some of the recent results on 2-dimensional topology and geometry. Especially, we shall discuss Teichmller theory, the mapping class groups, the SL (2, C) representation variety of surface groups, and Thurstons theory of measured laminations.

We generalise work of Young-Eun Choi to the setting of ideal triangulations with vertex links of arbitrary genus, showing that the set of all (possibly incomplete) hyperbolic cone-manifold structures realised by positively oriented hyperbolic ideal tetrahedra on a given topological ideal triangulation and with prescribed cone angles at all edges is (if non-empty) a smooth complex manifold of dimension the sum of the genera of the vertex links. Moreover, we show that the complex lengths of a collection of peripheral elements give a local holomorphic parameterisation of this manifold.

Several identities similar to the Schlaefli formula are established for tetrahedra in a space of constant curvature.