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..4.
Samuel S. Wagstaff, Jr. The Cunningham Project
Sequence in context: A009368 A092344 A281711 * A329735 A290124 A192349
Adjacent sequences: A216525 A216526 A216527 * A216529 A216530 A216531
V. Raman, Sep 08 2012