1,1

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.

Table of n, a(n) for n=1..20.

Samuel S. Wagstaff, Jr. The Cunningham Project

Select[Prime[Range[70]], Length[Solve[a^2+7b^2==(10^#-1)/9, {a, b}, Integers]] > 0&] (* Harvey P. Dale, Aug 19 2015 *)

Sequence in context: A156007 A067774 A063637 * A020623 A109346 A140550

Adjacent sequences: A216523 A216524 A216525 * A216527 A216528 A216529

nonn

V. Raman, Sep 08 2012

approved