O.g.f.: A(x) = Sum_{n>=0} x^n*Product_{k=0..n} (1 + k*x)^2.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =26a(5) =91a(6) =349a(7) =1431a(8) =6253a(9) =28860a(10) =140064a(11) =711433a(12) =3768601a(13) =20754377a(14) =118513699a(15) =700096184a(16) =4269686754a(17) =26835429731a(18) =173541095213