a(n) = 2^n * Product_{k=1..(n-1)/2} (2 + 3*cos(k*Pi/n)^2).

Terms

a(0) =1a(1) =2a(2) =4a(3) =22a(4) =56a(5) =290a(6) =748a(7) =3862a(8) =9968a(9) =51458a(10) =132820a(11) =685654a(12) =1769768a(13) =9136034a(14) =23581372a(15) =121733590a(16) =314211296a(17) =1622045954a(18) =4186725796a(19) =21613041046a(20) =55786259480a(21) =287984161058a(22) =743327100556