Cone on an infinite coinfinite ground set
ID: cone-on-an-infinite-coinfinite-ground-set
For an infinite coinfinite set , let in the space of infinite subsets of the natural numbers. This cone is a nonempty clopen set in the Ellentuck topology, because it is the basic neighbourhood , and any has a finite initial segment containing a point outside and hence a basic neighbourhood disjoint from .
The cone is a closed set with empty interior in the ordinary topology on infinite subsets: membership of a point outside witnesses its complement as an open set, while every ordinary finite-stem neighbourhood can be extended by an arbitrarily large point outside . Thus is an ordinary nowhere dense set, but it is neither ordinary open nor Ellentuck nowhere dense. It provides both distinctions between these two topologies.
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