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A238694 Smallest k such that 2^n - k and k*2^n - 1 are both prime or 0 if no such k exists. 4
0, 1, 1, 3, 1, 3, 1, 5, 25, 5, 31, 5, 1, 15, 49, 17, 1, 5, 1, 17, 9, 33, 69, 89, 61, 111, 199, 309, 75, 297, 1, 5, 49, 131, 31, 17, 31, 131, 165, 437, 55, 33, 309, 495, 361, 437, 999, 89, 139, 195, 129, 183, 685, 315, 915, 189, 585, 1035, 931, 93, 1, 57, 165 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

If a(n)=1, then the two primes are same and they are Mersenne primes (A000668).

LINKS

Table of n, a(n) for n=1..63.

EXAMPLE

a(9) = 25 because 2^9 - 25  = 487 and 25*2^9 - 1 = 12799 are both prime.

MAPLE

a:= proc(n) local k, p; p:= 2^n;

      for k while not (isprime(p-k) and isprime(k*p-1))

        do if k>=p then return 0 fi od; k

    end:

seq(a(n), n=1..70);  # Alois P. Heinz, Mar 03 2014

CROSSREFS

Cf. A238554.

Sequence in context: A283461 A146907 A322865 * A320221 A236939 A236936

Adjacent sequences:  A238691 A238692 A238693 * A238695 A238696 A238697

KEYWORD

nonn

AUTHOR

Ilya Lopatin and Juri-Stepan Gerasimov, Mar 03 2014

EXTENSIONS

More terms from Alois P. Heinz, Mar 03 2014

STATUS

approved

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Last modified August 14 19:09 EDT 2020. Contains 336483 sequences. (Running on oeis4.)