a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n-1)*a(n-2) + 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =6a(5) =15a(6) =46a(7) =137a(8) =460a(9) =1557a(10) =5698a(11) =21269a(12) =83948a(13) =339177a(14) =1430502a(15) =6178981a(16) =27636512a(17) =126500209a(18) =596320914a(19) =2873324677a(20) =14203422044a(21) =71669915585a(22) =369941778510