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

Terms

a(0) =1a(1) =5a(2) =15a(3) =46a(4) =128a(5) =384a(6) =1114a(7) =3332a(8) =9903a(9) =29671a(10) =88721a(11) =266151a(12) =797593a(13) =2392649a(14) =7175709a(15) =21526834a(16) =64573556a(17) =193720536a(18) =581141026a(19) =1743422288a(20) =5230207428a(21) =15690619684a(22) =47071679294a(23) =141215037738a(24) =423644574301