Search: id:A122786
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%I A122786
%S A122786 1,4,6,8,9,12,15,18,24,28,36,45,52,66,72,91,121,153,205,276,286,364,366,
%T A122786 369,396,435,511,532,561,616,671,697,703,726,804,946,949,1035,1036,1105,
%U A122786 1128,1288,1387,1541,1729,1737,1845,1854,1891,2196,2465,2501,2556,2665
%N A122786 Nonprimes n such that 9^n==9 (mod n).
%C A122786 Theorem: If both numbers q and 2q-1 are primes and n=q*(2q-1) then 9^n==9 
               (mod n) (n is in the sequence). So A005382*(2*A005382-1)= 6,15,91,
               703,1891,2701,12403,18721,... is the related subsequence. A020138 
               is a subsequence of this sequence.
%t A122786 Select[Range[4000], ! PrimeQ[ # ] && Mod[9^#, # ] == Mod[9, # ] &]
%Y A122786 Cf. A005382, A020138.
%Y A122786 Sequence in context: A130074 A067012 A157942 this_sequence A092630 A079142 
               A062002
%Y A122786 Adjacent sequences: A122783 A122784 A122785 this_sequence A122787 A122788 
               A122789
%K A122786 nonn
%O A122786 1,2
%A A122786 Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006

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