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%I #4 Jun 17 2013 03:48:16
%S 1,2,6,60,840,18720,559440,22196160,1092631680,65768371200,
%T 4703596128000,393785778355200
%N Number of universes with n entities, exactly one being the empty set, which satisfy the Axiom of Extensionality (two sets are identical when they have the same elements) and are closed under taking subsets.
%C These universes need not satisfy the Power Set Axiom.
%e 1 entity = 1 universe that consists of the empty set. 2 entities = 2 universes: when a = {{}}, when a = {a} on the other hand. 3 entities = 6 universes: a = {{}}, b = {a}; a = {{}}, b = {b}; and so on. 4 entities = 60 universes...
%Y Cf. A145984
%K hard,nonn,nice,more
%O 1,2
%A Csabay Karoly (csabay58(AT)gmail.com), Feb 19 2009, Jun 14 2009, Mar 20 2010
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