a(n) = floor((1+2^n+3^n)/3).

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =32a(5) =92a(6) =264a(7) =772a(8) =2272a(9) =6732a(10) =20024a(11) =59732a(12) =178512a(13) =534172a(14) =1599784a(15) =4793892a(16) =14370752a(17) =43090412a(18) =129227544a(19) =387595252a(20) =1162610992a(21) =3487483452a(22) =10461751304a(23) =31383855812a(24) =94148771232a(25) =282440721292a(26) =847310979064