a(n) = Pell(n+2) - 2^n.

Terms

a(0) =1a(1) =3a(2) =8a(3) =21a(4) =54a(5) =137a(6) =344a(7) =857a(8) =2122a(9) =5229a(10) =12836a(11) =31413a(12) =76686a(13) =186833a(14) =454448a(15) =1103921a(16) =2678674a(17) =6494037a(18) =15732284a(19) =38089677a(20) =92173782a(21) =222961529a(22) =539145416a(23) =1303349513a(24) =3150038746a(25) =7611815613a(26) =18390447188a(27) =44426264421a(28) =107310084894