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%I #3 Mar 31 2012 12:35:44
%S 1,6,74,1730,62340,3258982,232475188,21711360018,2576183815724,
%T 378267268113542
%N Number of permutations of 2 copies of 1..n avoiding adjacent step pattern up, down, up, down
%K nonn
%O 1,2
%A _R. H. Hardin_ May 10 2010