The volume of a spherical antiprism

We consider a spherical antiprism. It is a convex polyhedron with $2n$ vertices in the spherical space ${\mathbb{S}}^3$. This polyhedron has a group of symmetries $S_{2n}$ generated by a mirror-rotational symmetry of order $2n$, i.e. rotation to the angle $\pi \mathrm{/}n$ followed by a reflection. We establish necessary and sufficient conditions for the existence of such polyhedron in ${\mathbb{S}}^3$. Then we find relations between its dihedral angles and edge lengths in the form of cosine rules through a property of a spherical isosceles trapezoid. Finally, we obtain an explicit integral formula for the volume of a spherical antiprism in terms of the edge lengths.