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

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =252a(5) =996a(6) =3936a(7) =15552a(8) =61452a(9) =242820a(10) =959472a(11) =3791232a(12) =14980572a(13) =59193828a(14) =233896896a(15) =924213888a(16) =3651913836a(17) =14430073860a(18) =57018604752a(19) =225301777344a(20) =890251367868