%I #3 Mar 31 2012 12:35:44
%S 1,3,12,108,1944,38718,994392,38570364,1447995258,79481475504,
%T 7085744965872
%N Number of permutations of 2 copies of 1..n with no element e[i>=2]<e[1+floor((i-2)/3)] (3-way heap)
%Y Simple 2-way heap A056971
%K nonn
%O 1,2
%A _R. H. Hardin_ May 17 2010