G.f. satisfies: A(x) = (1 + x*(3-x)*A(x)) * (1 + x^2*A(x)).

Terms

a(0) =1a(1) =3a(2) =9a(3) =30a(4) =107a(5) =396a(6) =1503a(7) =5820a(8) =22907a(9) =91371a(10) =368523a(11) =1500354a(12) =6157669a(13) =25448907a(14) =105821865a(15) =442409844a(16) =1858482563a(17) =7840766601a(18) =33207750819a(19) =141137445258a(20) =601768494201