Weakening of Hardy property for means

Abstract

The aim of this paper is to find a broad family of means defined on a subinterval of $I \subset [0,+\infty)$ such that $$ \sum_{n=1}^\infty \mathscr{M}(a_1,\dots,a_n) <+\infty \quad\text{ for all }\quad a \in \ell_1(I).$$ Equivalently, the averaging operator $$ (a_1,\,a_2,a_3\,,\dots) \mapsto \big( a_1,\,\mathscr{M}(a_1,a_2),\,\mathscr{M}(a_1,a_2,a_3), \dots\big) $$ is a selfmapping of $\ell_1(I)$. This property is closely related to so-called Hardy inequality for means (which additionally requires boundedness of this operator). In fact we prove that these two properties are equivalent in a family of Gini means and Gaussian product of Power means. Moreover it is shown that this is not the case for quasi-arithmetic means.