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A020136
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Fermat pseudoprimes to base 4.
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14
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15, 85, 91, 341, 435, 451, 561, 645, 703, 1105, 1247, 1271, 1387, 1581, 1695, 1729, 1891, 1905, 2047, 2071, 2465, 2701, 2821, 3133, 3277, 3367, 3683, 4033, 4369, 4371, 4681, 4795, 4859, 5461, 5551, 6601, 6643, 7957, 8321, 8481, 8695, 8911, 9061, 9131
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OFFSET
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1,1
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COMMENTS
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Primes q and 2q-1 are a Cunningham chain of the second kind. - Walter Nissen, Sep 07 2009
Composite numbers n such that 4^(n-1) == 1 (mod n). - Michel Lagneau, Feb 18 2012
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LINKS
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MATHEMATICA
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Select[Range[9200], ! PrimeQ[ # ] && PowerMod[4, # - 1, # ] == 1 &] (* Farideh Firoozbakht, Sep 12 2006 *)
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PROG
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(PARI) isok(n) = (Mod(4, n)^(n-1)==1) && !isprime(n) && (n>1); \\ Michel Marcus, Apr 27 2018
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CROSSREFS
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Contains A001567 (Fermat pseudoprimes to base 2) as a subsequence.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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