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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =16a(5) =42a(6) =114a(7) =314a(8) =870a(9) =2426a(10) =6804a(11) =19168a(12) =54198a(13) =153730a(14) =437232a(15) =1246480a(16) =3560838a(17) =10190810a(18) =29212432a(19) =83860176a(20) =241051796a(21) =693709896a(22) =1998535892a(23) =5763312876a(24) =16635018146a(25) =48054500898a(26) =138923916700a(27) =401908892716