a(n) = 1 + Sum_{k=0..n-3} binomial(n-2,k) * a(k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =8a(6) =20a(7) =57a(8) =171a(9) =548a(10) =1894a(11) =6998a(12) =27368a(13) =112653a(14) =486645a(15) =2201162a(16) =10397944a(17) =51161168a(18) =261571460a(19) =1386846249a(20) =7612315023a(21) =43190917004a(22) =252951090586