Number of strings over Z_4 of length n with trace 2 and subtrace 2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =40a(5) =240a(6) =1120a(7) =4608a(8) =17280a(9) =65280a(10) =253440a(11) =1013760a(12) =4126720a(13) =16773120a(14) =67624960a(15) =270532608a(16) =1077903360a(17) =4294901760a(18) =17146183680a(19) =68584734720a(20) =274608947200