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A122786
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Nonprimes n such that 9^n==9 (mod n).
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1
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1, 4, 6, 8, 9, 12, 15, 18, 24, 28, 36, 45, 52, 66, 72, 91, 121, 153, 205, 276, 286, 364, 366, 369, 396, 435, 511, 532, 561, 616, 671, 697, 703, 726, 804, 946, 949, 1035, 1036, 1105, 1128, 1288, 1387, 1541, 1729, 1737, 1845, 1854, 1891, 2196, 2465, 2501, 2556, 2665
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 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.
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MATHEMATICA
| Select[Range[4000], ! PrimeQ[ # ] && Mod[9^#, # ] == Mod[9, # ] &]
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CROSSREFS
| Cf. A005382, A020138.
Sequence in context: A130074 A067012 A157942 * A092630 A079142 A062002
Adjacent sequences: A122783 A122784 A122785 * A122787 A122788 A122789
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KEYWORD
| nonn
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AUTHOR
| Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006
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