Number of 2-sided strip polytans with n cells.

Terms

a(0) =1a(1) =3a(2) =4a(3) =13a(4) =25a(5) =72a(6) =166a(7) =450a(8) =1124a(9) =2973a(10) =7603a(11) =19865a(12) =51199a(13) =132982a(14) =343290a(15) =888309a(16) =2292325a(17) =5918742a(18) =15258937a(19) =39329418