G.f.: Sum_{n>=0} n! * x^n * (1+x)^n / Product_{k=1..n} (1 + k*x).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =28a(6) =88a(7) =304a(8) =1144a(9) =4648a(10) =20248a(11) =94024a(12) =463144a(13) =2409928a(14) =13198888a(15) =75848584a(16) =456066664a(17) =2862257608a(18) =18708144808a(19) =127096142344a(20) =895846801384