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

2, 14, 38, 98

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