a(n+1) = 3*a(n) - n.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =23a(5) =64a(6) =186a(7) =551a(8) =1645a(9) =4926a(10) =14768a(11) =44293a(12) =132867a(13) =398588a(14) =1195750a(15) =3587235a(16) =10761689a(17) =32285050a(18) =96855132a(19) =290565377a(20) =871696111a(21) =2615088312a(22) =7845264914a(23) =23535794719a(24) =70607384133a(25) =211822152374a(26) =635466457096