a(n) = Sum_{d|n} d * 4^(n/d-1).

Terms

a(0) =1a(1) =6a(2) =19a(3) =76a(4) =261a(5) =1074a(6) =4103a(7) =16536a(8) =65593a(9) =262686a(10) =1048587a(11) =4196644a(12) =16777229a(13) =67117098a(14) =268436319a(15) =1073774896a(16) =4294967313a(17) =17180003478a(18) =68719476755a(19) =274878432636