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) =8a(4) =9a(5) =55a(6) =64a(7) =377a(8) =441a(9) =2584a(10) =3025a(11) =17711a(12) =20736a(13) =121393a(14) =142129a(15) =832040a(16) =974169a(17) =5702887a(18) =6677056a(19) =39088169a(20) =45765225a(21) =267914296a(22) =313679521a(23) =1836311903a(24) =2149991424a(25) =12586269025a(26) =14736260449a(27) =86267571272a(28) =101003831721a(29) =591286729879