OFFSET
0,5
COMMENTS
Equivalently a(n) is the number of topologies on an n-set X such that for all x in X there is a y in X such that x and y have exactly the same neighborhoods.
LINKS
Pontus von Brömssen, Table of n, a(n) for n = 0..37
Wikipedia, Topological indistinguishability.
FORMULA
EXAMPLE
a(4) = 10 because letting X = {a,b,c,d} we have the trivial topology; {{},{b,c},{a,d},X} * 3; and {{},{a,b},X} *6.
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, Dec 28 2016
EXTENSIONS
a(19)-a(21) from Pontus von Brömssen, Apr 05 2023
STATUS
approved