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

Terms

a(0) =1a(1) =1a(2) =2a(3) =14a(4) =128a(5) =1563a(6) =23328a(7) =411772a(8) =8388608a(9) =193710245a(10) =5000000000a(11) =142655835306