Number of CdI_2 polytypes that repeat after 2n layers.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =22a(5) =68a(6) =216a(7) =728a(8) =2513a(9) =8920a(10) =32116a(11) =117240a(12) =431550a(13) =1600734a(14) =5970704a(15) =22381184a(16) =84236890a(17) =318191804a(18) =1205692236a(19) =4581481776a(20) =17452915218a(21) =66637798834a(22) =254960616136a(23) =977346581800