# On the number of harmonic frames

## Abstract:

There is a finite number $h_{n,d}$ of tight frames of $n$ distinct vectors for $\C^d$ which are the orbit of a vector under a unitary action of the cyclic group $\Z_n$. These cyclic harmonic frames (or geometrically uniform tight frames are used in signal analysis and quantum information theory, and provide many tight frames of particular interest. Here we investigate the conjecture that $h_{n,d}$ grows like $n^{d-1}$. By using a result of Laurent which describes the set of solutions of algebraic equations in roots of unity, we prove the asymptotic estimate $$h_{n,d} \approx {n^d \over \varphi(n)}\ge n^{d-1}, \qquad n\to\infty.$$ By using a group theoretic approach, we also give some exact formulas for $h_{n,d}$, and estimate the number of cyclic harmonic frames up to projective unitary equivalence.

Keywords: Finite tight frames, harmonic frames, finite abelian groups, character theory, Pontryagin duality, Mordell-Lang conjecture, projective unitary equivalence

Math Review Classification: Primary 42C15, 94A12; Secondary 11Z05, 20C15, 94A15, 20F28

Length: 22 pages

Last Updated: 22 November 2016