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

Terms

a(0) =3a(1) =2a(2) =3a(3) =8a(4) =18a(5) =45a(6) =115a(7) =298a(8) =787a(9) =2108a(10) =5714a(11) =15630a(12) =43089a(13) =119574a(14) =333695a(15) =935798a(16) =2635513a(17) =7450394a(18) =21132026a(19) =60116972a(20) =171481200a(21) =490329056a(22) =1405122072a(23) =4034707664a(24) =11606693280a(25) =33445603004a(26) =96526550442a(27) =278985273840a(28) =807416204292