Number of shapes of a Rubik's Snake with n wedges.

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =241a(5) =920a(6) =3384a(7) =12585a(8) =46471a(9) =172226a(10) =633138a(11) =2333757a(12) =8561679a(13) =31462176a(14) =115247629a(15) =422677188a(16) =1546186675a(17) =5661378449a(18) =20689242550a(19) =75663420126a(20) =276279455583