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

Terms

a(0) =1a(1) =1a(2) =1a(3) =-4a(4) =-3a(5) =5a(6) =0a(7) =13a(8) =21a(9) =-68a(10) =-55a(11) =89a(12) =0a(13) =233a(14) =377a(15) =-1220a(16) =-987a(17) =1597a(18) =0a(19) =4181a(20) =6765a(21) =-21892a(22) =-17711a(23) =28657a(24) =0a(25) =75025a(26) =121393a(27) =-392836a(28) =-317811a(29) =514229