a(n) = floor(( (n+1)/2 )^n) - n!.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =15a(5) =123a(6) =1118a(7) =11344a(8) =127831a(9) =1590245a(10) =21700716a(11) =322880256a(12) =5209007463a(13) =90661989607