a(n) = 4*3^n-1.

Terms

a(0) =3a(1) =11a(2) =35a(3) =107a(4) =323a(5) =971a(6) =2915a(7) =8747a(8) =26243a(9) =78731a(10) =236195a(11) =708587a(12) =2125763a(13) =6377291a(14) =19131875a(15) =57395627a(16) =172186883a(17) =516560651a(18) =1549681955a(19) =4649045867a(20) =13947137603a(21) =41841412811a(22) =125524238435a(23) =376572715307