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A098917 Least k such that k*Mersenne-prime(n)*Mersenne-prime(n+1) + 1 is prime. 0
2, 6, 4, 16, 36, 6, 42, 24, 172, 18, 52, 54, 6, 130, 2488, 1344, 12420, 5596, 364, 178, 3382, 10516, 44328 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..23.

EXAMPLE

2*(2^2-1)*(2^3-1) + 1 = 43 is prime, so a(1) = 2.

2*(2^3-1)*(2^5-1) + 1 = 435 is composite, 4*(2^3-1)*(2^5-1) + 1 = 867 is composite, 6*(2^3-1)*(2^5-1) + 1 = 1303 is prime, so a(2) = 6.

MATHEMATICA

f[n_] := Module[{k = 1}, While[! PrimeQ[k*n + 1], k++]; k]; f /@ Times @@@ Partition[2^MersennePrimeExponent[Range[15]] - 1, 2, 1] (* Amiram Eldar, Jul 24 2021 *)

CROSSREFS

Cf. A000668 (Mersenne primes).

Sequence in context: A226718 A181159 A127320 * A054786 A269372 A282902

Adjacent sequences:  A098914 A098915 A098916 * A098918 A098919 A098920

KEYWORD

nonn,more

AUTHOR

Pierre CAMI, Oct 17 2004

EXTENSIONS

a(15)-a(23) from Amiram Eldar, Jul 24 2021

STATUS

approved

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Last modified January 18 11:57 EST 2022. Contains 350455 sequences. (Running on oeis4.)