Abstract:
Based on the study of the geometric properties of unconditional quasibasic sequences, we show that in an arbitrary symmetric space there exists no unconditional quasibasis consisting of nonnegative functions. Moreover, we demonstrate that in an arbitrary Banach function lattice $X$ of type $p>1$ one can introduce an equivalent norm such that there exists no monotone (with respect to the new norm) basis in $X$ that consists of nonnegative functions.
Keywords:basis, quasibasis, basic sequence, symmetric space, Rademacher system, type of a Banach space.
The work of the first author was supported within the development program of the Scientific and Educational Mathematical Center of the Volga Federal District (agreement no. 075-02-2022-878).
Citation:
S. V. Astashkin, P. A. Terekhin, “On Quasibases and Bases of Symmetric Spaces Consisting of Nonnegative Functions”, Approximation Theory, Functional Analysis, and Applications, Collected papers. On the occasion of the 70th birthday of Academician Boris Sergeevich Kashin, Trudy Mat. Inst. Steklova, 319, Steklov Math. Inst., Moscow, 2022, 20–28; Proc. Steklov Inst. Math., 319 (2022), 13–21