Number of reversible strings with n beads of 4 colors.

Terms

a(0) =1a(1) =4a(2) =10a(3) =40a(4) =136a(5) =544a(6) =2080a(7) =8320a(8) =32896a(9) =131584a(10) =524800a(11) =2099200a(12) =8390656a(13) =33562624a(14) =134225920a(15) =536903680a(16) =2147516416a(17) =8590065664a(18) =34359869440a(19) =137439477760a(20) =549756338176