These numbers of the form 2^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..24.
Samuel S. Wagstaff, Jr. The Cunningham Project
Sequence in context: A129966 A243128 A306771 * A110172 A261274 A014493
Adjacent sequences: A216518 A216519 A216520 * A216522 A216523 A216524
V. Raman, Sep 08 2012