Expansion of g.f.: (1-2*x)*(1-4*x+x^2)/((1-x)*(1-3*x)*(1-4*x)).

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =70a(5) =252a(6) =926a(7) =3460a(8) =13110a(9) =50252a(10) =194446a(11) =758100a(12) =2973350a(13) =11716252a(14) =46333566a(15) =183739940a(16) =730176790a(17) =2906358252a(18) =11582386286a(19) =46200404980a(20) =184414199430a(21) =736494536252