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A020136
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Pseudoprimes to base 4.
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2
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Primes q and 2q-1 are a Cunningham chain of the second kind. [From Walter Nissen (nissen(AT)gtcinternet.com), Sep 07 2009]
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LINKS
| Index entries for sequences related to pseudoprimes
Eric Weisstein's World of Mathematics, Fermat Pseudoprime
Chris Caldwell, Cunningham chain [From Walter Nissen (nissen(AT)gtcinternet.com), Sep 07 2009]
Chris Caldwell, et al., Top Twenty Cunningham Chains (2nd kind) [From Walter Nissen (nissen(AT)gtcinternet.com), Sep 07 2009]
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FORMULA
| Theorem: If q and 2q-1 are odd primes then n=q*(2q-1) is in the sequence. So for n>1 A005382(n)*(2*A005382(n)-1) is in the sequence - 15, 91, 703, 1891, 2701, 12403, 18721, ... is the related subsequence. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006
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MATHEMATICA
| Select[Range[9200], ! PrimeQ[ # ] && PowerMod[4, # - 1, # ] == 1 &] - Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2006
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CROSSREFS
| Cf. A005382, A122781.
Sequence in context: A108674 A050405 A206383 * A176033 A067401 A206169
Adjacent sequences: A020133 A020134 A020135 * A020137 A020138 A020139
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KEYWORD
| nonn
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AUTHOR
| David W. Wilson (davidwwilson(AT)comcast.net)
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