a(n) = Product_{k=1..floor((n-1)/2)} (1 + 8*cos(k*Pi/n)^2) for n >= 0.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =5a(5) =11a(6) =21a(7) =43a(8) =85a(9) =171a(10) =341a(11) =683a(12) =1365a(13) =2731a(14) =5461a(15) =10923a(16) =21845a(17) =43691a(18) =87381a(19) =174763a(20) =349525a(21) =699051a(22) =1398101a(23) =2796203a(24) =5592405a(25) =11184811a(26) =22369621a(27) =44739243a(28) =89478485a(29) =178956971