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

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =8a(5) =32a(6) =93a(7) =633a(8) =2524a(9) =22684a(10) =113405a(11) =1247405a(12) =7484406a(13) =97297206a(14) =681080407a(15) =10216206007a(16) =81729648008