a(n) = floor(n^n/2).

Terms

a(0) =0a(1) =0a(2) =2a(3) =13a(4) =128a(5) =1562a(6) =23328a(7) =411771a(8) =8388608a(9) =193710244a(10) =5000000000a(11) =142655835305