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A216525 Numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2. 0
1, 2, 7, 13, 14, 19, 23, 37, 38, 47, 49, 53, 59, 71, 73, 83, 91, 98, 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..31.

Samuel S. Wagstaff, Jr. The Cunningham Project

CROSSREFS

Sequence in context: A130710 A145300 A240029 * A232637 A333383 A018322

Adjacent sequences:  A216522 A216523 A216524 * A216526 A216527 A216528

KEYWORD

nonn

AUTHOR

V. Raman, Sep 08 2012

STATUS

approved

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Last modified July 9 14:08 EDT 2020. Contains 335543 sequences. (Running on oeis4.)