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Primes in A027612.
4

%I #10 Jul 15 2018 12:06:07

%S 5,13,223,4861,197698279,25472027467,6975593267347,218572480850557,

%T 1592457339642613,2955634782407818711841368777079578319,

%U 2950127241932882597818337002939124083061,232242878286351670588710938679161483012314573381293769

%N Primes in A027612.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HarmonicNumber.html">Harmonic Number</a>.

%F a(n) = A027612(A124879(n)).

%e A027612(n) begins {1, 5, 13, 77, 87, 223, 481, 4609, 4861, ...}.

%e Thus a(1) = 5, a(2) = 13, a(3) = 223, a(4) = 4861.

%t s=1;Do[s=s+1/(n+1);f=Numerator[(n+1)*(s-1)]; If[PrimeQ[f],Print[{n,f}]],{n,1,1942}]

%o (PARI) lista(nn) = {for (n=1, nn, if (isprime(p=numerator(sum(k=1, n, k/(n-k+1)))), print1(p, ", ")););} \\ _Michel Marcus_, Jul 14 2018

%Y A027612(n) are the numerators of second order harmonic numbers H(n, (2)).

%Y Corresponding numbers n such that A027612(n) is prime are listed in A124879.

%Y Cf. A001008, A002805, A067657, A056903, A027612, A124879, A124837, A124880, A124881.

%K nonn

%O 1,1

%A _Alexander Adamchuk_, Nov 11 2006

%E a(12) from, and crossrefs edited by _Michel Marcus_, Jul 14 2018