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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =7a(5) =18a(6) =61a(7) =188a(8) =677a(9) =2370a(10) =9141a(11) =35212a(12) =144905a(13) =602662a(14) =2631333a(15) =11671264a(16) =53772593a(17) =252184082a(18) =1220090757a(19) =6011588316a(20) =30413403457a(21) =156656758094a(22) =825751634149