a(n) = 4*a(n-1) - a(n-2) + 3, with a(0) = -1, a(1) = 1.

Terms

a(0) =-1a(1) =1a(2) =8a(3) =34a(4) =131a(5) =493a(6) =1844a(7) =6886a(8) =25703a(9) =95929a(10) =358016a(11) =1336138a(12) =4986539a(13) =18610021a(14) =69453548a(15) =259204174a(16) =967363151a(17) =3610248433a(18) =13473630584a(19) =50284273906a(20) =187663465043a(21) =700369586269