G.f.: (4*x^2 + 2*x)/(4*x^3 - x^2 - 4*x + 1).

Terms

a(0) =0a(1) =2a(2) =12a(3) =50a(4) =204a(5) =818a(6) =3276a(7) =13106a(8) =52428a(9) =209714a(10) =838860a(11) =3355442a(12) =13421772a(13) =53687090a(14) =214748364a(15) =858993458a(16) =3435973836a(17) =13743895346a(18) =54975581388a(19) =219902325554a(20) =879609302220