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Search: id:A122781
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| A122781 |
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Nonprimes n such that 4^n==4 (mod n). |
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+0
3
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| 1, 4, 6, 12, 15, 28, 66, 85, 91, 186, 276, 341, 435, 451, 532, 561, 645, 703, 946, 1068, 1105, 1247, 1271, 1387, 1581, 1695, 1729, 1891, 1905, 2044, 2046, 2047, 2071, 2465, 2701, 2821, 2926, 3133, 3277, 3367, 3683, 4033, 4369, 4371, 4681, 4795
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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. So A005382*(2*A005382-1)= 6,15,91,703,1891,2701, 12403,18721,... is the related subsequence.
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MATHEMATICA
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Select[Range[4800], ! PrimeQ[ # ] && Mod[4^#, # ] == Mod[4, # ] &]
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CROSSREFS
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Cf. A005382, A020136.
Adjacent sequences: A122778 A122779 A122780 this_sequence A122782 A122783 A122784
Sequence in context: A131863 A074870 A104236 this_sequence A153355 A024904 A139056
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KEYWORD
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easy,nonn
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006
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