a(n) = n! mod 4^n.

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =24a(5) =120a(6) =720a(7) =5040a(8) =40320a(9) =100736a(10) =483072a(11) =2168064a(12) =9239552a(13) =53005312a(14) =205203456a(15) =930568192a(16) =2004189184a(17) =12596379648a(18) =54936141824a(19) =81714020352a(20) =534768779264