Spherical Designs on $S^1$ of Finite Harmonic Strength

Abstract

We study exact harmonic strengths of finite spherical designs on the unit circle. For a nonempty finite set \(X\subset S^1\), let \(\Hst(X)\) be the set of positive integers \(k\) for which the \(k\)-th complex moment \(P_k(X)=\sum_{x\in X}x^k\) vanishes. Equivalently, \(X\) is a spherical \(T\)-design precisely when \(T\subset \Hst(X)\). We consider the exact realization problem: given a finite set \(T\subset\mathbb N\), determine whether there exists a finite set \(X\subset S^1\) such that \(\Hst(X)=T\). We prove that every finite \(T\subset\mathbb N\) is realizable. More precisely, for each \(t\ge 1\) we construct uncountably many five-point sets with \(\Hst(X)=\{t\}\), and we prove that no smaller set can have this exact harmonic strength. A product construction then gives, for every finite \(T\subset\mathbb N\), a realization with \(|X|=5^{|T|}\). We also initiate the associated minimum-size problem \(N(T,2)\). We prove \(N(\{t\},2)=5\) for all \(t\ge1\), determine \(N(\{2,3\},2)=5\), and show that the optimal \(\{2,3\}\)-example is unique up to rotation. Finally, we discuss a rigid seven-point example related to \(T=\{2,3,4,10\}\).