Search: id:A072446
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%I A072446
%S A072446 1,2,12,420,254076,17199454920
%N A072446 Number of subsets S of the power set P{1,2,...,n} such that: {1}, {2},
               ..., {n} are all elements of S; if X and Y are elements of S and 
               X and Y have a non-empty intersection, then the union of X and Y 
               is an element of S.
%H A072446 Wim van Dam, <a href="http://www.cs.berkeley.edu/~vandam/subpowersets/
               sequences.html">Sub Power Set Sequences</a>
%e A072446 a(3)=12 because of the 12 sets: {{1}, {2}, {3}}; {{1}, {2}, {3}, {1, 
               2}}; {{1}, {2}, {3}, {1, 3}}; {{1}, {2}, {3}, {2, 3}}; {{1}, {2}, 
               {3}, {1, 2, 3}}; {{1}, {2}, {3}, {1, 2}, {1, 2, 3}}; {{1}, {2}, {3}, 
               {1, 3}, {1, 2, 3}}; {{1}, {2}, {3}, {2, 3}, {1, 2, 3}}; {{1}, {2}, 
               {3}, {1, 2}, {1, 3}, {1, 2, 3}}; {{1}, {2}, {3}, {1, 2}, {2, 3}, 
               {1, 2, 3}}; {{1}, {2}, {3}, {1, 3}, {2, 3}, {1, 2, 3}}; {{1}, {2}, 
               {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}}.
%Y A072446 Cf. A072444, A072445, A072447.
%Y A072446 Sequence in context: A156509 A051009 A060942 this_sequence A015181 A012378 
               A012383
%Y A072446 Adjacent sequences: A072443 A072444 A072445 this_sequence A072447 A072448 
               A072449
%K A072446 nonn
%O A072446 1,2
%A A072446 Wim van Dam (vandam(AT)cs.berkeley.edu), Jun 18 2002

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