

A238554


Smallest k such that k + 2^n and k*2^n + 1 are both prime.


4



1, 1, 1, 5, 1, 11, 3, 9, 1, 35, 15, 39, 3, 39, 63, 35, 1, 149, 3, 419, 7, 221, 25, 155, 73, 735, 69, 29, 193, 261, 3, 135, 81, 149, 85, 125, 117, 809, 303, 509, 27, 699, 325, 29, 27, 285, 639, 65, 61, 1911, 639, 165, 295, 1295, 163, 905, 175, 75, 1593, 249
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OFFSET

0,4


COMMENTS

If a(n) = 1, then the two primes are the same and they are Fermat primes.  Michel Marcus, Mar 01 2014


LINKS

Giovanni Resta, Table of n, a(n) for n = 0..500


EXAMPLE

5 is in this sequence because 5 + 2^3 = 13 and 5*2^3 + 1 = 41 are both prime.


PROG

(PARI) a(n) = {k = 1; while (!(isprime(k + 2^n) && isprime(k*2^n + 1)), k++); k; } \\ Michel Marcus, Mar 01 2014


CROSSREFS

Cf. A019434 (Fermat primes).
Sequence in context: A245211 A330774 A296307 * A067292 A131782 A242060
Adjacent sequences: A238551 A238552 A238553 * A238555 A238556 A238557


KEYWORD

nonn


AUTHOR

Ilya Lopatin and JuriStepan Gerasimov, Feb 28 2014


EXTENSIONS

a(15) corrected and a(24) from Michel Marcus, Mar 01 2014
Missing term and a(25)a(59) from Giovanni Resta, Mar 01 2014


STATUS

approved



