a(n) = Sum_{d|n} 2^(d-1) * binomial(d, n/d).

Terms

a(0) =1a(1) =4a(2) =12a(3) =34a(4) =80a(5) =204a(6) =448a(7) =1072a(8) =2308a(9) =5280a(10) =11264a(11) =25088a(12) =53248a(13) =116032a(14) =245920a(15) =527880a(16) =1114112a(17) =2369152a(18) =4980736a(19) =10508880a(20) =22022336a(21) =46193664a(22) =96468992a(23) =201469408a(24) =419430416a(25) =872734720a(26) =1811960832a(27) =3758844096a(28) =7784628224a(29) =16107909312