%I #6 Oct 22 2013 18:41:11
%S 1,4,28,304,4576,91072,2306176,71993344,2707268608,120392433664,
%T 6235322515456,371376015818752,25166910597922816,1922787147517394944,
%U 164313634414773403648
%N Number of permutations of 2 copies of 1..n avoiding adjacent step pattern up, down.
%K nonn
%O 1,2
%A _R. H. Hardin_, May 10 2010
%E a(12)-a(15) from _Alois P. Heinz_, Oct 22 2013