These numbers of the form (10^n+1) 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..10.
Samuel S. Wagstaff, Jr. The Cunningham Project
Sequence in context: A191118 A298019 A169605 * A249375 A212492 A234310
Adjacent sequences: A216529 A216530 A216531 * A216533 A216534 A216535
V. Raman, Sep 08 2012