%I A122781
%S A122781 1,4,6,12,15,28,66,85,91,186,276,341,435,451,532,561,645,703,946,1068,
%T A122781 1105,1247,1271,1387,1581,1695,1729,1891,1905,2044,2046,2047,2071,2465,
%U A122781 2701,2821,2926,3133,3277,3367,3683,4033,4369,4371,4681,4795
%N A122781 Nonprimes n such that 4^n==4 (mod n).
%C A122781 Theorem: If both numbers q and 2q-1 are primes then n=q*(2q-1) is in 
               the sequence. So A005382*(2*A005382-1)= 6,15,91,703,1891,2701, 12403,
               18721,... is the related subsequence.
%t A122781 Select[Range[4800], ! PrimeQ[ # ] && Mod[4^#, # ] == Mod[4, # ] &]
%Y A122781 Cf. A005382, A020136.
%Y A122781 Sequence in context: A131863 A074870 A104236 this_sequence A153355 A024904 
               A139056
%Y A122781 Adjacent sequences: A122778 A122779 A122780 this_sequence A122782 A122783 
               A122784
%K A122781 easy,nonn
%O A122781 1,2
%A A122781 Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006