Search: id:A122784
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%I A122784
%S A122784 1,6,14,21,25,42,105,133,231,301,325,525,561,703,817,1105,1729,1825,
%T A122784 2101,2353,2465,2821,3277,3325,3486,3913,4011,4525,4825,5565,5719,5901,
%U A122784 6601,6697,7525,8321,8911,9331,10225,10325,10585,10621,11041,11521
%N A122784 Nonprimes n such that 7^n==7 (mod n).
%C A122784 Theorem: If both numbers q and 2q-1 are primes then n=q*(2q-1) is in 
               the sequence iff q=2 or mod(q,14) is in the set {1, 5, 13}. 6,703,
               18721,38503,88831,104653,146611,188191,... are such terms.
%t A122784 Select[Range[20000], ! PrimeQ[ # ] && Mod[7^#, # ] == Mod[7, # ] &]
%Y A122784 Cf. A005938.
%Y A122784 Sequence in context: A046712 A162823 A020171 this_sequence A063299 A110223 
               A125086
%Y A122784 Adjacent sequences: A122781 A122782 A122783 this_sequence A122785 A122786 
               A122787
%K A122784 nonn
%O A122784 1,2
%A A122784 Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006

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