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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =2a(5) =4a(6) =6a(7) =16a(8) =18a(9) =40a(10) =62a(11) =160a(12) =210a(13) =448a(14) =660a(15) =2048a(16) =2570a(17) =5184a(18) =9198a(19) =24672a(20) =32508a(21) =76032a(22) =121574a(23) =344064a(24) =405000a(25) =1005888a(26) =1569780a(27) =4511520a(28) =6066336a(29) =12672000