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

Terms

a(0) =1a(1) =5a(2) =17a(3) =81a(4) =257a(5) =1345a(6) =4097a(7) =20737a(8) =69633a(9) =328705a(10) =1048577a(11) =5574657a(12) =16777217a(13) =83902465a(14) =286261249a(15) =1359020033a(16) =4294967297a(17) =22565617665a(18) =68719476737a(19) =348967141377