G.f.: 3/(1 + 2*sqrt(1-12*x)).

Terms

a(0) =1a(1) =4a(2) =28a(3) =232a(4) =2092a(5) =19864a(6) =195352a(7) =1970896a(8) =20275660a(9) =211823800a(10) =2240795848a(11) =23951289520a(12) =258255469816