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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =16a(6) =39a(7) =91a(8) =217a(9) =513a(10) =1216a(11) =2881a(12) =6825a(13) =16171a(14) =38311a(15) =90768a(16) =215047a(17) =509491a(18) =1207089a(19) =2859841a(20) =6775552a(21) =16052673a(22) =38032081a(23) =90105811a(24) =213479175a(25) =505776016a(26) =1198287271a(27) =2838988683a(28) =6726147337a(29) =15935624641