a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 1,3,7,20.

Terms

a(0) =1a(1) =3a(2) =7a(3) =20a(4) =60a(5) =182a(6) =547a(7) =1641a(8) =4921a(9) =14762a(10) =44286a(11) =132860a(12) =398581a(13) =1195743a(14) =3587227a(15) =10761680a(16) =32285040a(17) =96855122a(18) =290565367a(19) =871696101a(20) =2615088301a(21) =7845264902a(22) =23535794706a(23) =70607384120a(24) =211822152361a(25) =635466457083