G.f.: exp( Sum_{n>=1} x^n/n * Sum_{k=0..n} binomial(n,k)^2 * x^k/(1-x)^k ).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =29a(6) =72a(7) =182a(8) =466a(9) =1207a(10) =3158a(11) =8334a(12) =22158a(13) =59299a(14) =159614a(15) =431838a(16) =1173710a(17) =3203244a(18) =8774780a(19) =24118522a(20) =66497316a(21) =183858411a(22) =509670494a(23) =1416231616a(24) =3944027402a(25) =11006186760a(26) =30772507128a(27) =86191006746a(28) =241815195292a(29) =679488418879