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A216521 Numbers n such that 2^n+1 can be written in the form a^2+7*b^2. 0
3, 15, 21, 33, 39, 57, 105, 129, 147, 159, 183, 231, 237, 273, 303, 399, 501, 633, 735, 903, 1029, 1041, 1083, 1113 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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.

LINKS

Table of n, a(n) for n=1..24.

Samuel S. Wagstaff, Jr. The Cunningham Project

CROSSREFS

Sequence in context: A129966 A243128 A306771 * A110172 A261274 A014493

Adjacent sequences:  A216518 A216519 A216520 * A216522 A216523 A216524

KEYWORD

nonn

AUTHOR

V. Raman, Sep 08 2012

STATUS

approved

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Last modified July 24 04:25 EDT 2021. Contains 346273 sequences. (Running on oeis4.)