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

Terms

a(0) =1a(1) =1a(2) =1a(3) =5a(4) =6a(5) =29a(6) =35a(7) =169a(8) =204a(9) =985a(10) =1189a(11) =5741a(12) =6930a(13) =33461a(14) =40391a(15) =195025a(16) =235416a(17) =1136689a(18) =1372105a(19) =6625109a(20) =7997214a(21) =38613965a(22) =46611179a(23) =225058681a(24) =271669860a(25) =1311738121a(26) =1583407981a(27) =7645370045a(28) =9228778026a(29) =44560482149