a(n) = sigma(n*2^n).

Terms

a(0) =3a(1) =15a(2) =60a(3) =127a(4) =378a(5) =1020a(6) =2040a(7) =4095a(8) =13299a(9) =24570a(10) =49140a(11) =131068a(12) =229362a(13) =524280a(14) =1572840a(15) =2097151a(16) =4718574a(17) =13631475a(18) =20971500a(19) =50331642a(20) =134217696a(21) =201326580a(22) =402653160a(23) =1073741820a(24) =2080374753a(25) =3758096370a(26) =10737418200a(27) =17179869176