On the optimal error bound for the first step in the method of cyclic alternating projections

Abstract: Let $H$ be a Hilbert space and $H_1,...,H_n$ be closed subspaces of $H$. Set $H_0:=H_1\cap H_2\cap...\cap H_n$ and let $P_k$ be the orthogonal projection onto $H_k$, $k=0,1,...,n$. The paper is devoted to the study of functions $f_n:[0,1]\to\mathbb{R}$ defined by $$ f_n(c)=\sup{|P_n...P_2 P_1-P_0|\,|c_F(H_1,...,H_n)\leqslant c},\,c\in[0,1], $$ where the supremum is taken over all systems of subspaces $H_1,...,H_n$ for which the Friedrichs number $c_F(H_1,...,H_n)$ is less than or equal to $c$. Using the functions $f_n$ one can easily get an upper bound for the rate of convergence in the method of cyclic alternating projections. We will show that the problem of finding $f_n(c)$ is equivalent to a certain optimization problem on a subset of the set of Hermitian complex $n\times n$ matrices. Using the equivalence we find $f_3$ and study properties of $f_n$, $n\geqslant 4$. Moreover, we show that $$ 1-a_n(1-c)-\widetilde{b}_n(1-c)^2\leqslant f_n(c)\leqslant 1-a_n(1-c)+b_n(1-c)^2 $$ for all $c\in[0,1]$, where $a_n=2(n-1)\sin^2(\pi/(2n))$, $b_n=6(n-1)^2\sin^4(\pi/(2n))$ and $\widetilde{b}_n$ is some positive number.