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

Terms

a(0) =1a(1) =2a(2) =2a(3) =16a(4) =20a(5) =288a(6) =392a(7) =8192a(8) =11664a(9) =320000a(10) =468512a(11) =15925248a(12) =23762752a(13) =963780608a(14) =1458000000a(15) =68719476736a(16) =105046700288