From Sidelnikov-Welch bounds to projection constants

Abstract: In the paper, we prove a recursive version of the weighted Sidelnikov-Welch inequality for real and complex unit vectors. Unlike the classical form, which gives a direct lower bound for a fixed even power sum, our inequality relates two consecutive even power sums. Iteration yields the usual weighted Sidelnikov-Welch bound. We apply this estimate to maximal relative projection constants. If $\mathbb{K}<sup>m$ admits a maximal equiangular tight frame with $M_{\mathbb K}$ vectors, then for every integer $k\geq1$, $$λ<em>{\mathbb K}(kM</em>{\mathbb K}-m,kM_{\mathbb K}) = λ<em>{\mathbb K}(m)-\frac{2m}{kM</em>{\mathbb K}}+1.$$ Moreover, the maximal value is realized by an equiangular tight frame when $k=1$ and by a biangular tight frame when $k\geq2$.