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) =8a(5) =21a(6) =78a(7) =247a(8) =950a(9) =3421a(10) =13872a(11) =54925a(12) =235262a(13) =1004213a(14) =4533144a(15) =20600553a(16) =97664002a(17) =468473957a(18) =2324089996a(19) =11693569137a(20) =60499459054a(21) =317757980069