G.f.: Sum_{n>=0} [Sum_{k>=0} C(n+k-1,k)^2*x^k]^2 * x^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =57a(5) =295a(6) =1620a(7) =9261a(8) =54459a(9) =326952a(10) =1994085a(11) =12313323a(12) =76795452a(13) =482903473a(14) =3057550167a(15) =19472859840a(16) =124645558881a(17) =801367484155