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%I #8 Jul 18 2021 04:42:36
%S 1,2,3,12,8,3,5,14,17,69,189,42,392,167,377,12,2007,434,744,705,1089,
%T 1109,7833,7328,1271,192,6770,2379
%N a(n) is the least k such that k*(k+1)*Mersenne-prime(n)+1 is prime.
%e 1*2*(2^2-1)+1 = 7 is prime, so a(1) = 1.
%e 2*3*(2^3-1)+1 = 43 is prime, so a(2) = 2.
%e 3*4*(2^5-1)+1 = 373 is prime, so a(3) = 3.
%t f[p_] := Module[{k = 1}, While[! PrimeQ[k*(k + 1)*p + 1], k++]; k]; f /@ (2^MersennePrimeExponent[Range[15]] - 1) (* _Amiram Eldar_, Jul 18 2021 *)
%Y Cf. A000043, A000668.
%K nonn,more
%O 1,2
%A _Pierre CAMI_, Mar 31 2005
%E a(14) inserted and a(24)-a(28) added by _Amiram Eldar_, Jul 18 2021
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