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A216527 Odd numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2. 0
1, 7, 13, 19, 23, 37, 47, 49, 53, 59, 71, 73, 83, 91, 101, 103, 127, 133, 149, 151, 161, 191, 211, 259, 266, 317, 329 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

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..27.

Samuel S. Wagstaff, Jr. The Cunningham Project

CROSSREFS

Sequence in context: A058620 A038910 A035497 * A059335 A070419 A080199

Adjacent sequences:  A216524 A216525 A216526 * A216528 A216529 A216530

KEYWORD

nonn

AUTHOR

V. Raman, Sep 08 2012

STATUS

approved

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Last modified May 17 05:13 EDT 2021. Contains 343965 sequences. (Running on oeis4.)