a(0) = 1, a(1) = 6, a(2) = 24; for n>=3, a(n) = 4a(n-1) - a(n-2).

Terms

a(0) =1a(1) =6a(2) =24a(3) =90a(4) =336a(5) =1254a(6) =4680a(7) =17466a(8) =65184a(9) =243270a(10) =907896a(11) =3388314a(12) =12645360a(13) =47193126a(14) =176127144a(15) =657315450a(16) =2453134656a(17) =9155223174a(18) =34167758040a(19) =127515808986a(20) =475895477904