A276121 Smallest odd integer k such that k*2^prime(n)-1 is a prime number.

1, 1, 1, 1, 3, 1, 1, 1, 13, 7, 1, 21, 15, 3, 31, 147, 45, 1, 43, 73, 15, 69, 91, 1, 51, 81, 3, 1, 9, 85, 1, 55, 169, 225, 109, 145, 15, 103, 615, 69, 259, 69, 63, 45, 285, 471, 9, 255, 169, 489, 69, 273, 427, 43, 391, 169, 201, 21, 159, 181, 103, 15, 339

Offset

1,5

Comments

When k=1 the prime k*2^prime(n)-1 is a Mersenne prime.

Links

Pierre CAMI, Table of n, a(n) for n = 1..3000

Formula

a(n) = A126717(prime(n)). - Michel Marcus, Sep 07 2016

Example

1*2^7-1 = 127 prime so a(4) = 1 as prime(4)=7.
1*2^11-1 = 2047 composite, 3*2^11-1 = 6143 prime so a(5) = 3 as prime(5)=11.

Mathematica

Table[k = 1; While[! PrimeQ[k 2^Prime@ n - 1], k += 2]; k, {n, 63}] (* Michael De Vlieger, Aug 21 2016 *)

Prog

(PARI) a(n) = {my(k=1); while (!isprime(k*2^prime(n)-1), k+=2); k; } \\ Michel Marcus, Aug 21 2016

Crossrefs

Cf. A000043, A126717.

Sequence in context: A261959 A348988 A257565 * A262809 A331568 A010278

Adjacent sequences: A276118 A276119 A276120 * A276122 A276123 A276124

Keyword

nonn

Author

Pierre CAMI, Aug 21 2016

Status

approved