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A129521
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Numbers of the form p*q, p and q prime with q=2*p-1.
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22
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6, 15, 91, 703, 1891, 2701, 12403, 18721, 38503, 49141, 79003, 88831, 104653, 146611, 188191, 218791, 226801, 269011, 286903, 385003, 497503, 597871, 665281, 721801, 736291, 765703, 873181, 954271, 1056331, 1314631, 1373653, 1537381
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OFFSET
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1,1
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COMMENTS
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All terms are Fermat 4-pseudoprimes, i.e., satisfy 4^n == 4 (mod n). See A020136 and A122781.
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LINKS
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FORMULA
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MATHEMATICA
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p = Select[Prime[Range[155]], PrimeQ[2# - 1] &]; p (2p - 1) (* Robert G. Wilson v, Sep 11 2011 *)
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PROG
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(Magma) [2*n^2-n: n in [0..1000]|IsPrime(n) and IsPrime(2*n-1)]; // Vincenzo Librandi, Dec 27 2010
(PARI) forprime(p=2, 10000, q=2*p-1; if(isprime(q), print1(p*q, ", ")))
(Haskell)
a129521 n = p * (2 * p - 1) where p = a005382 n
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CROSSREFS
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KEYWORD
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nonn,changed
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AUTHOR
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STATUS
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approved
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