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%I #3 Mar 30 2012 17:38:02
%S 3,6,8,18,18,56,126,282,686,1806
%N Number of elements of rank n in the free lattice on three generators.
%C The rank of a lattice term is the number of occurrences of variables in that term. The rank of an element is the minimum rank of a term representing that element.
%e a(2) = 6 because the 6 elements of rank 2 are x + y, x + z, y + z, xy, xz and yz.
%K more,nonn
%O 1,1
%A _David Wasserman_, Mar 31 2002
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