((-1)^n - 1 + 2*(n^floor((n + 1)/2)))/4.

Terms

a(0) =0a(1) =1a(2) =4a(3) =8a(4) =62a(5) =108a(6) =1200a(7) =2048a(8) =29524a(9) =50000a(10) =885780a(11) =1492992a(12) =31374258a(13) =52706752a(14) =1281445312a(15) =2147483648a(16) =59293938248a(17) =99179645184