On the basis property of a trigonometric system arising in the Frankl problem

Abstract

We consider the function system {cos4nθ} n=0 ∞ , {sin(4n − 1)θ} n=1 ∞ , which arises in the Frankl problem in the theory of elliptic-hyperbolic equations. We show that this system is a Riesz basis in the space L 2(0, π/2) and construct the biorthogonal system.