7^n mod 4^n.

Terms

a(0) =0a(1) =3a(2) =1a(3) =23a(4) =97a(5) =423a(6) =2961a(7) =4343a(8) =63169a(9) =245575a(10) =408305a(11) =1809559a(12) =84001a(13) =50919655a(14) =155110993a(15) =548906039a(16) =2768600449a(17) =15085235847a(18) =36877174193a(19) =258140219351a(20) =707469907681