Finite-Defect Rigidity and the Minimum Spherical 4-Design on the Two-Sphere

Abstract

We prove that every equal-weight spherical $4$-design on ${\mathbb{S}}^2$ has at least twelve points. Since the regular icosahedron is a spherical $5$-design, this determines the exact minimum$$N_4({\mathbb{S}}^2)=12;$$equivalently, no such design has $9$, $10$ or $11$ points.The proof is part of a finite-defect theory. If a spherical $2m$-design has corank $c=N-\dim P_m$, its Naimark complement consists of unit vectors $u_x\in {\mathbb{S}}^{c-1}$ forming a spherical $2$-design and satisfying the exact coupling$$u_x\cdot u_y=-\frac{K_m^{(d)}(x\cdot y)}{c}(x\ne y).$$This gives a pairwise kernel bound, an antipodal lower bound, Cayley-Bacharach information, and uniform lower bounds for multiplicative-relation spaces. In corank one the design splits into two equal spherical $m$-designs. We derive a residue formula for its signed Schoenberg coefficients; the coefficient of degree $m+3$ is negative exactly when $3\le d\le m+1$, excluding corank one throughout that range. At strengths four and six the only examples in any dimension are the regular hexagon and octagon, respectively.In corank two the complement is a circle Gale frame. Multiplication by its phase forces at least $d-1$ linear-quadratic aliases and yields exact norm and socle identities in every dimension. For eleven nodes on ${\mathbb{S}}^2$, two aliases produce a real harmonic cubic and a Hermitian quartic matrix. A matrix-valued Cayley-Bacharach argument eliminates the generic branch; the exceptional branch reduces to a Pauli normal form and contradicts the second moments. In dimensions $d\ge 4$ the corank-two problem remains open; we identify a forced quadratic socle as the obstruction to extending the present argument.

arXiv comment39 pages