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A216528 Even numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2. 0
2, 14, 38, 98 (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..4.

Samuel S. Wagstaff, Jr. The Cunningham Project

CROSSREFS

Sequence in context: A009368 A092344 A281711 * A329735 A290124 A192349

Adjacent sequences:  A216525 A216526 A216527 * A216529 A216530 A216531

KEYWORD

nonn

AUTHOR

V. Raman, Sep 08 2012

STATUS

approved

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Last modified May 7 06:49 EDT 2021. Contains 343636 sequences. (Running on oeis4.)