a(n) = Sum_{k=1..n} (binomial(n, k) * 2^k) (mod 3^k).

Terms

a(0) =2a(1) =5a(2) =11a(3) =29a(4) =143a(5) =365a(6) =851a(7) =1793a(8) =3563a(9) =11573a(10) =26006a(11) =54497a(12) =292646a(13) =567125a(14) =1557050a(15) =4122959a(16) =8051231a(17) =27654668a(18) =65636279a(19) =108510746a(20) =550742636a(21) =1597852586a(22) =4027985846a(23) =6614624849a(24) =22893091367a(25) =63290283500a(26) =145223821835a(27) =290303956529