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..14.
Samuel S. Wagstaff, Jr. The Cunningham Project
Sequence in context: A068229 A323594 A071696 * A114564 A191118 A298019
Adjacent sequences: A216527 A216528 A216529 * A216531 A216532 A216533
V. Raman, Sep 08 2012