a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), starting with 1, 2, 6, 20.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =61a(5) =183a(6) =547a(7) =1640a(8) =4920a(9) =14762a(10) =44287a(11) =132861a(12) =398581a(13) =1195742a(14) =3587226a(15) =10761680a(16) =32285041a(17) =96855123a(18) =290565367a(19) =871696100a(20) =2615088300a(21) =7845264902a(22) =23535794707a(23) =70607384121a(24) =211822152361a(25) =635466457082