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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 May 12 13:02 EDT 2021. Contains 343823 sequences. (Running on oeis4.)