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 A216526 Prime numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2. 0
 2, 7, 13, 19, 23, 37, 47, 53, 59, 71, 73, 83, 101, 103, 127, 149, 151, 191, 211, 317 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These repunit numbers of the form (10^n-1)/9 have no prime factors congruent to 3 (mod 7) or 5 (mod 7) or 6 (mod 7) to an odd power. Also the power of 2 is not 1. LINKS Table of n, a(n) for n=1..20. Samuel S. Wagstaff, Jr. The Cunningham Project MATHEMATICA Select[Prime[Range[70]], Length[Solve[a^2+7b^2==(10^#-1)/9, {a, b}, Integers]] > 0&] (* Harvey P. Dale, Aug 19 2015 *) CROSSREFS Sequence in context: A156007 A067774 A063637 * A020623 A109346 A140550 Adjacent sequences: A216523 A216524 A216525 * A216527 A216528 A216529 KEYWORD nonn AUTHOR V. Raman, Sep 08 2012 STATUS approved

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Last modified August 12 03:07 EDT 2024. Contains 375085 sequences. (Running on oeis4.)