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URL: https://oeis.org/A276121

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A276121
Smallest odd integer k such that k*2^prime(n)-1 is a prime number.
1
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.
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
Sequence in context: A261959 A348988 A257565 * A262809 A331568 A010278
KEYWORD
nonn
AUTHOR
Pierre CAMI, Aug 21 2016
STATUS
approved