a(n) = Sum_{d|n} 3^(n-d).

Terms

a(0) =1a(1) =4a(2) =10a(3) =37a(4) =82a(5) =352a(6) =730a(7) =2998a(8) =7291a(9) =26488a(10) =59050a(11) =263170a(12) =531442a(13) =2127952a(14) =5373460a(15) =19669879a(16) =43046722a(17) =187086916a(18) =387420490a(19) =1607136634a(20) =3878987860a(21) =13947314752a(22) =31381059610a(23) =139902374692a(24) =285916320883