%I A122780
%S A122780 1,6,66,91,121,286,561,671,703,726,949,1105,1541,1729,1891,2465,2665,
%T A122780 2701,2821,3281,3367,3751,4961,5551,6601,7107,7381,8205,8401,8646,8911,
%U A122780 10585,11011,12403,14383,15203,15457,15841,16471,16531,18721,19345
%N A122780 Nonprimes n such that 3^n==3 (mod n).
%C A122780 Theorem: If q!=3 and both numbers q and (2q-1) are primes then n=q*(2q-1) 
               is in the sequence. 6, 91, 703, 1891, 2701, 12403, 18721, 38503, 
               49141, ... is the related subsequence.
%e A122780 66 is composite and 3^66=66*468229611858069884271524875811+3 so 66 is 
               in the sequence.
%t A122780 Select[Range[30000], ! PrimeQ[ # ] && Mod[3^#, # ] == Mod[3, # ] &]
%Y A122780 Cf. A001567, A122014, A122781-9.
%Y A122780 Sequence in context: A137121 A110222 A119230 this_sequence A153514 A119144 
               A153087
%Y A122780 Adjacent sequences: A122777 A122778 A122779 this_sequence A122781 A122782 
               A122783
%K A122780 easy,nonn
%O A122780 1,2
%A A122780 Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 11 2006