O.g.f.: Sum_{n>=0} n! * x^n * (1+x)^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =10a(4) =44a(5) =234a(6) =1470a(7) =10656a(8) =87624a(9) =806280a(10) =8211000a(11) =91707120a(12) =1114793280a(13) =14653936080a(14) =207138844080