A216516 Odd numbers n such that 2^n-1 can be written in the form a^2+7*b^2.

3, 7, 13, 19, 21, 31, 49, 61, 67, 91, 97, 103, 109, 127, 133, 147, 199, 217, 229, 241, 271, 343, 349, 427, 457, 469, 499, 523, 589, 601, 607, 637, 673, 679, 721, 733, 751, 763, 823, 877, 889, 907, 931, 997, 1029

Offset

1,1

Comments

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.

Links

Table of n, a(n) for n=1..45.

Samuel S. Wagstaff, Jr. The Cunningham Project

Crossrefs

Sequence in context: A258117 A034017 A034021 * A364722 A310260 A310261

Adjacent sequences: A216513 A216514 A216515 * A216517 A216518 A216519

Keyword

nonn

Author

V. Raman, Sep 08 2012

Status

approved