a(n) = Sum_{k=1..n} 3^(floor(n/k) - 1).

Terms

a(0) =1a(1) =4a(2) =11a(3) =32a(4) =87a(5) =258a(6) =745a(7) =2224a(8) =6605a(9) =19784a(10) =59151a(11) =177438a(12) =531733a(13) =1595104a(14) =4783811a(15) =14351228a(16) =43049043a(17) =129147030a(18) =387427357a(19) =1162281532a(20) =3486804959a(21) =10460413130a(22) =31381119537a(23) =94143358500a(24) =282429716209a(25) =847289143468