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 A002644 Numbers k such that (k^2 + k + 1)/21 is prime. (Formerly M5000 N1426) 1
 16, 25, 37, 46, 58, 88, 109, 130, 142, 151, 184, 193, 205, 247, 268, 298, 310, 319, 331, 340, 382, 394, 403, 415, 424, 457, 478, 487, 541, 550, 604, 613, 688, 697, 709, 730, 739, 751, 760, 793, 844, 865, 886, 907, 970, 1012, 1045, 1054, 1066, 1117, 1138 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES A. J. C. Cunningham, Binomial Factorisations, Vols. 1-9, Hodgson, London, 1923-1929; see Vol. 1, pp. 245-259. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 A. J. C. Cunningham, Binomial Factorisations, Vols. 1-9, Hodgson, London, 1923-1929. [Annotated scans of a few pages from Volumes 1 and 2] MATHEMATICA Select[Range[1200], PrimeQ[(#^2 + # + 1)/21] &] (* Vincenzo Librandi, Sep 25 2012 *) PROG (PARI) is(n)=isprime((n^2+n+1)/21) \\ Charles R Greathouse IV, Jun 06 2017 CROSSREFS Sequence in context: A348235 A218439 A192689 * A118489 A110922 A065408 Adjacent sequences: A002641 A002642 A002643 * A002645 A002646 A002647 KEYWORD nonn,easy AUTHOR N. J. A. Sloane EXTENSIONS More terms from Larry Reeves (larryr(AT)acm.org), Sep 21 2000 STATUS approved

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Last modified April 23 20:27 EDT 2024. Contains 371916 sequences. (Running on oeis4.)