On the dual of variable Lebesgue spaces with unbounded exponent

Abstract

We study the dual space of the variable Lebesgue space $\Lp$ with unbounded exponent function $\pp$ and provide an answer to a question posed in~[fiorenza-cruzuribe2013]. Our approach is to decompose the dual into a topological direct sum of Banach spaces. The first component corresponds to the dual in the bounded exponent case, and the second is, intuitively, the dual of functions that live where the exponent is unbounded (in a heuristic sense). The second space is extremely complicated, and we illustrate this with a series of examples. In the special case of the variable sequence space $\ell^\pp$, we show that this piece can be further decomposed into two spaces, one of which can be characterized in terms of a generalization of finitely additive measures. As part of our work, we also considered the question of dense subsets in $\Lp$ for unbounded exponents. We constructed two examples, one for general variable Lebesgue spaces $\Lp$ and one in the sequence space $\lp$. This gives an answer to another question from~[fiorenza-cruzuribe2013].