This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A280202 #29 Apr 18 2023 07:36:34
%S A280202 1,0,1,1,10,31,361,2164,32663,313121,6199024,86219497,2225685925,
%T A280202 42396094690,1414152064833,35520966967269,1517860883350266,
%U A280202 48936884016265947,2659543345912283917,107827798819822505332,7409614386025588874195,371626299919138199117981
%N A280202 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 are topologically indistinguishable.
%C A280202 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.
%H A280202 Pontus von Brömssen, <a href="/A280202/b280202.txt">Table of n, a(n) for n = 0..37</a>
%H A280202 Wikipedia, <a href="https://en.wikipedia.org/wiki/Topological_indistinguishability">Topological indistinguishability</a>.
%F A280202 E.g.f.: A(exp(x) - 1 - x) where A(x) is the e.g.f. for A001035.
%F A280202 a(n) = Sum_{k=0..floor(n/2)} A008299(n,k)*A001035(k).
%e A280202 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.
%Y A280202 Column k=0 of A280192.
%Y A280202 Cf. A001035, A008299.
%K A280202 nonn
%O A280202 0,5
%A A280202 _Geoffrey Critzer_, Dec 28 2016
%E A280202 a(19)-a(21) from _Pontus von Brömssen_, Apr 05 2023