These Mersenne 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..4.
Samuel S. Wagstaff, Jr. The Cunningham Project
Sequence in context: A057089 A110711 A156361 * A055272 A155196 A147838
Adjacent sequences: A216514 A216515 A216516 * A216518 A216519 A216520
V. Raman, Sep 08 2012