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

Terms

a(0) =1a(1) =4a(2) =7a(3) =18a(4) =21a(5) =66a(6) =71a(7) =196a(8) =305a(9) =648a(10) =1035a(11) =2526a(12) =4109a(13) =8774a(14) =16875a(15) =34288a(16) =65553a(17) =134860a(18) =262163a(19) =531506a(20) =1051237a(21) =2109594a(22) =4194327a(23) =8425348a(24) =16779257a(25) =33611984a(26) =67123631a(27) =134350206a(28) =268435485a(29) =537178750