A216514 Numbers k such that 2^k-1 can be written in the form a^2 + 7*b^2.

3, 6, 7, 13, 19, 21, 31, 42, 49, 61, 67, 91, 97, 103, 109, 127, 133, 147, 199, 217, 229, 241, 271, 294, 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^k-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..48.

Samuel S. Wagstaff, Jr. The Cunningham Project

Crossrefs

Sequence in context: A248388 A354765 A107850 * A051218 A124130 A124132

Adjacent sequences: A216511 A216512 A216513 * A216515 A216516 A216517

Keyword

nonn

Author

V. Raman, Sep 08 2012

Status

approved