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Search: id:A122784
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| A122784 |
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Nonprimes n such that 7^n==7 (mod n). |
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2
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| 1, 6, 14, 21, 25, 42, 105, 133, 231, 301, 325, 525, 561, 703, 817, 1105, 1729, 1825, 2101, 2353, 2465, 2821, 3277, 3325, 3486, 3913, 4011, 4525, 4825, 5565, 5719, 5901, 6601, 6697, 7525, 8321, 8911, 9331, 10225, 10325, 10585, 10621, 11041, 11521
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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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.
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MATHEMATICA
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Select[Range[20000], ! PrimeQ[ # ] && Mod[7^#, # ] == Mod[7, # ] &]
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CROSSREFS
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Cf. A005938.
Adjacent sequences: A122781 A122782 A122783 this_sequence A122785 A122786 A122787
Sequence in context: A118129 A046712 A020171 this_sequence A063299 A110223 A125086
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KEYWORD
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nonn
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006
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