Hereditary Lindelöf Degree

Abstract

We begin with some equivalent definitions. For one of them we need the following notion: a sequence (x _ξ : ξ < κ) of distinct elements of a topological space X is right-separated provided that for every ξ < κ the set {x _η : η < ξ} is open in {x _ξ : ξ < κ}.