Orthonormal systems in linear spans

Abstract

We show that any $N$-dimensional linear subspace of $L^2\left(\mathbb{T}\right)$ admits an orthonormal system such that the $L^2$ norm of the square variation operator $V^2$ is as small as possible. When applied to the span of the trigonometric system, we obtain an orthonormal system of trigonometric polynomials with a $V^2$ operator that is considerably smaller than the associated operator for the trigonometric system itself.