a(n) = (n+1)^n - n!!.

Terms

a(0) =0a(1) =1a(2) =7a(3) =61a(4) =617a(5) =7761a(6) =117601a(7) =2097047a(8) =43046337a(9) =999999055a(10) =25937420761a(11) =743008360293