a(n) = floor(2*(3/2)^n).

Terms

a(0) =2a(1) =3a(2) =4a(3) =6a(4) =10a(5) =15a(6) =22a(7) =34a(8) =51a(9) =76a(10) =115a(11) =172a(12) =259a(13) =389a(14) =583a(15) =875a(16) =1313a(17) =1970a(18) =2955a(19) =4433a(20) =6650a(21) =9975a(22) =14963a(23) =22445a(24) =33668a(25) =50502a(26) =75753a(27) =113630a(28) =170445a(29) =255668