a(n) = 16*3^n + 2^(n+1) - 26 (n>=1).

Terms

a(0) =26a(1) =126a(2) =422a(3) =1302a(4) =3926a(5) =11766a(6) =35222a(7) =105462a(8) =315926a(9) =946806a(10) =2838422a(11) =8511222a(12) =25525526a(13) =76560246a(14) =229648022a(15) =688878582a(16) =2066504726a(17) =6199252086a(18) =18597232022a(19) =55790647542a(20) =167369845526a(21) =502105342326