a(n) = phi(4^n-1)/(2*n).

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =60a(5) =144a(6) =756a(7) =2048a(8) =7776a(9) =24000a(10) =120032a(11) =276480a(12) =1719900a(13) =4741632a(14) =17820000a(15) =67108864a(16) =336849900a(17) =725594112a(18) =4822382628a(19) =11842560000a(20) =57802864896a(21) =200778006528a(22) =998132265920