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

Terms

a(0) =1a(1) =4a(2) =7a(3) =20a(4) =21a(5) =88a(6) =71a(7) =296a(8) =373a(9) =1084a(10) =1035a(11) =5084a(12) =4109a(13) =16496a(14) =20787a(15) =67728a(16) =65553a(17) =286516a(18) =262163a(19) =1070180a(20) =1189937a(21) =4194568a(22) =4194327a(23) =17760824a(24) =16827241a(25) =67109228a(26) =72150655a(27) =269503660a(28) =268435485a(29) =1104603808