a(n) = Product_{k=1..floor((n-1)/2)} (1 + 4*cos(2*Pi*k/n)^2).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =1a(5) =5a(6) =4a(7) =13a(8) =9a(9) =34a(10) =25a(11) =89a(12) =64a(13) =233a(14) =169a(15) =610a(16) =441a(17) =1597a(18) =1156a(19) =4181a(20) =3025a(21) =10946a(22) =7921a(23) =28657a(24) =20736a(25) =75025a(26) =54289a(27) =196418a(28) =142129a(29) =514229