Cubature formulae, polytopes, and spherical designs

The construction of a cubature formula of strength t for the unit sphere Ω d in ℝ d amounts to finding finite sets X 1,..., X N ⊂ Ω d and coefficients a 1,..., a N ∈ ℝ such that
|Ωd|−1∫Ωdf(ξ)dω(ξ)=∑i=1Nai|Xi|−1∑x∈Xιf(x),
(1.1)
for all functions f represented on Ω d by polynomials of degree ⩽ t; cf. [16], [15], [11]. Sobolev [14,15] introduced group theory into the construction of cubature formulae by considering orbits X ι under a finite subgroup G of the orthogonal group O(d). Thus spherical polytopes and root systems (cf. Coxeter [3]) enter the discussion. There are further relations to Coxeter’s work, since the obstruction to higher strength for a cubature formula is caused essentially by the existence of certain invariants. For finite groups generated by reflections, the theory of exponents and invariants goes back to Coxeter [4].