Vol. 15, No. 3, 1965

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On some classes of nearly open sets

Olav Njȧstad

Vol. 15 (1965), No. 3, 961–970

The open sets in a topological space are those sets A for which A0 A. Sets for which A00 A -“α-sets”- or A0A -“β-sets”- may naturally be considered as more or less “nearly open”. In this paper the structure of these sets and classes of sets are investigated, and some applications are given.

Topologies determining the same class of α-sets also determine the same class of β-sets, and vice versa. The class of β-sets forms a topology if and only if the original topology is extremally disconnected. The class of α-sets always forms a topology, and topologies generated in this way-“α-topolgies”- are exactly those where all nowhere dense sets are closed.

The class of all topologies which determine the same α-sets is convex in the ordering by inclusion, the α-topology being its finest member. Most topologies ordinary met with are the coarsest members of their corresponding classes; in particular this is the case for all regular topologies.

All topologies determining the same α-sets also determine the same continuous mappings into arbitrary regular spaces.

Mathematical Subject Classification
Primary: 54.20
Received: 18 September 1963
Revised: 28 April 1964
Published: 1 September 1965
Olav Njȧstad