a(n) = Product_{k=1..(n-1)/2} (5 + 4*cos(k*Pi/n)^2).

Terms

a(0) =1a(1) =1a(2) =1a(3) =6a(4) =7a(5) =41a(6) =48a(7) =281a(8) =329a(9) =1926a(10) =2255a(11) =13201a(12) =15456a(13) =90481a(14) =105937a(15) =620166a(16) =726103a(17) =4250681a(18) =4976784a(19) =29134601a(20) =34111385a(21) =199691526a(22) =233802911a(23) =1368706081a(24) =1602508992a(25) =9381251041a(26) =10983760033a(27) =64300051206a(28) =75283811239