A082747 a(n) is the least k such that k*Mrs(n)*Mrs(n+1)*Mrs(n+2) + 1 is prime, where Mrs(n) is the n-th Mersenne prime.

2, 4, 6, 4, 60, 280, 60, 210, 306, 154, 154, 538, 1272, 640, 4180, 6384, 12816, 2020, 10918, 9694, 45420, 47506, 11680, 1408

Offset

1,1

Links

Table of n, a(n) for n=1..24.

Example

2*(2^2-1)*(2^3-1)*(2^5-1) + 1 = 1303 is prime, so a(1) = 2.

Mathematica

f[n_] := Module[{k = 1}, While[! PrimeQ[k*n + 1], k++]; k]; f /@ Times @@@ Partition[2^MersennePrimeExponent[Range[15]] - 1, 3, 1] (* Amiram Eldar, Jul 18 2021 *)

Crossrefs

Cf. A000043, A000668.

Sequence in context: A353512 A257080 A099784 * A127275 A242796 A298527

Adjacent sequences: A082744 A082745 A082746 * A082748 A082749 A082750

Keyword

nonn,hard,more

Author

Pierre CAMI, Oct 22 2004

Extensions

a(11) inserted and a(17)-a(24) added by Amiram Eldar, Jul 18 2021

Status

approved