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Published online by Cambridge University Press: 20 November 2018
1. Let X be a topological space, and let X′ be the set of all non-convergent ultrafilters on X. If A ⊆ X, let , and A* = A ∪ A′. If
is a filter on X such that
for all
, then let.
be the filter on X* generated by
; let
be the filter on X* generated by
. If
exists then
; otherwise,
.
A convergence is defined on X* as follows: If x ∈ X, then a filter A → x in X* if and only if , where Vx(x) is the X neighborhood filter at x;
, then
in X* if and only if
.