a(n) = 1 + Sum_{k=0..n-2} binomial(n-1,k) * a(k).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =11a(5) =34a(6) =122a(7) =487a(8) =2144a(9) =10276a(10) =53165a(11) =294760a(12) =1740950a(13) =10899841a(14) =72033470a(15) =500664496a(16) =3648211139a(17) =27792215302a(18) =220802394110