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Least k such that k*(Mersenne_prime(n)^2) + 1 is prime.
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%I #13 Mar 01 2019 08:56:29

%S 2,4,16,12,22,28,30,22,138,12,102,180,426,1444,9502,6568,2212,5380,

%T 9942,3034,24222,31530,2962,7848,93280,3718

%N Least k such that k*(Mersenne_prime(n)^2) + 1 is prime.

%C Subsequence of A035092. - _Amiram Eldar_, Mar 01 2019

%e 2*((2^2-1)^2)+1=19 prime so a(1)=2.

%t a[n_] := Module[{k = 1, m = (2^MersennePrimeExponent[n] - 1)^2}, While[! PrimeQ[k*m + 1], k++]; k]; Array[a, 20] (* _Amiram Eldar_, Mar 01 2019 *)

%Y Cf. A000043, A000668, A035092.

%K more,nonn

%O 1,1

%A _Pierre CAMI_, Oct 08 2004

%E a(19)-a(22) from _Donovan Johnson_, Mar 23 2008

%E a(23)-a(26) from _Amiram Eldar_, Mar 01 2019