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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =9a(5) =24a(6) =97a(7) =314a(8) =1285a(9) =4740a(10) =20161a(11) =81782a(12) =364037a(13) =1590768a(14) =7415361a(15) =34458418a(16) =167934917a(17) =822644860a(18) =4181343201a(19) =21456885262a(20) =113446435685a(21) =606954796712