%I #6 Nov 02 2013 20:00:14

%S 1,6,84,2054,78231,4290094,320821079,31414709469,3903342922319,

%T 600255674485509,111949642485388109,24900064681761042208,

%U 6512262064364063116565,1978717544093002418592533

%N Number of permutations of 2 copies of 1..n avoiding adjacent step pattern up, down, down, up.

%K nonn

%O 1,2

%A _R. H. Hardin_, May 10 2010

%E a(11)-a(14) from _Alois P. Heinz_, Nov 02 2013