Riordan array (1/(1-3*x), (1-3*x-sqrt(1-6*x+5*x^2))/(2*x)).

Terms

a(0) =1a(1) =3a(2) =1a(3) =9a(4) =6a(5) =1a(6) =27a(7) =28a(8) =9a(9) =1a(10) =81a(11) =120a(12) =56a(13) =12a(14) =1a(15) =243a(16) =497a(17) =300a(18) =93a(19) =15a(20) =1a(21) =729a(22) =2034a(23) =1490a(24) =594a(25) =139a(26) =18a(27) =1a(28) =2187a(29) =8321