Number of 2-sided triangular strip polyedges with n cells.

Terms

a(0) =1a(1) =3a(2) =8a(3) =32a(4) =123a(5) =523a(6) =2201a(7) =9443a(8) =40341a(9) =172649a(10) =736926a(11) =3141607a(12) =13367012a(13) =56790498a(14) =240919918a(15) =1020753475a(16) =4319803799a(17) =18262494912a(18) =77134873774a(19) =325518862387