a(n) = n*x^n + (n-1)*x^(n-1) + . . . + x + 1 for x=3.

Terms

a(0) =1a(1) =4a(2) =22a(3) =103a(4) =427a(5) =1642a(6) =6016a(7) =21325a(8) =73813a(9) =250960a(10) =841450a(11) =2790067a(12) =9167359a(13) =29893558a(14) =96855124a(15) =312088729a(16) =1000836265a(17) =3196219036a(18) =10169787838a(19) =32252755711a(20) =101988443731a(21) =321655860994