Floor((3^n-1)/n).

Terms

a(0) =2a(1) =4a(2) =8a(3) =20a(4) =48a(5) =121a(6) =312a(7) =820a(8) =2186a(9) =5904a(10) =16104a(11) =44286a(12) =122640a(13) =341640a(14) =956593a(15) =2690420a(16) =7596480a(17) =21523360a(18) =61171656a(19) =174339220a(20) =498112057a(21) =1426411800a(22) =4093181688a(23) =11767897353a(24) =33891544377a(25) =97764070320a(26) =282429536480a(27) =817028301962