OFFSET
0,2
LINKS
Wikipedia, Clopen set.
FORMULA
EXAMPLE
a(2) = 10. Let X = {a,b}. There are four distinct topologies (A000798) that can be placed on X: {{},X} {{},{a},X} {{}, {b},X} {{},{a},{b},X}. These topologies have 2 + 2 + 2 + 4 sets respectively that are both open and closed.
MATHEMATICA
lg = Length[A001035];
A[x_] = Sum[A001035[[n + 1]] x^n/n!, {n, 0, lg - 1}];
CoefficientList[A[Exp[x] - 1]^2 + O[x]^lg, x]*Range[0, lg - 1]! (* Jean-François Alcover, Jan 01 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, Jan 23 2017
STATUS
approved