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%I #7 Jul 24 2021 05:38:42
%S 2,2,2,24,2,50,6,26,14,306,86,846,104,1832,272,2222,2540,884,5474,
%T 1950,28358,13338,76740
%N Least k such that k*Mersenne-prime(n)*Mersenne-prime(n+1) - 1 is prime.
%e 2*(2^2-1)*(2^3-1) - 1 = 41 is prime, so a(1) = 2.
%e 2*(2^3-1)*(2^5-1) - 1 = 433 is prime, so a(2) = 2.
%t 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 *)
%Y Cf. A000043, A000668.
%K nonn,more
%O 1,1
%A _Pierre CAMI_, Oct 17 2004
%E a(15)-a(23) from _Amiram Eldar_, Jul 24 2021
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