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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 (list; graph; refs; listen; history; text; internal format)
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 Juri-Stepan 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

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Last modified February 27 15:16 EST 2020. Contains 332307 sequences. (Running on oeis4.)