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%I A064440 #27 Sep 23 2022 05:39:34
%S A064440 6,21,31,79,121,265,271,379,541,631,1051,1351,1597,1711,2071,2401,
%T A064440 2551,2761,2881,2941,3811,4111,4471,4531,4621,5251,6091,8341,8371,
%U A064440 8401,8821,9871,11551,13651,16171,19531,21211,22681,23101,27931,30661,32551,33811,37591
%N A064440 Numbers k such that A048138(k) is a prime and sets a new record for such primes.
%H A064440 Amiram Eldar, <a href="/A064440/b064440.txt">Table of n, a(n) for n = 1..188</a>
%t A064440 seq[max_] := Module[{s = Table[0, {n, 1, max}], i, v = {}, m = 0}, Do[If[(i = DivisorSigma[1, n] - n) <= max, s[[i]]++], {n, 2, (max - 1)^2}]; Do[If[s[[i]] > m && PrimeQ[s[[i]]], m = s[[i]]; AppendTo[v, i]], {i, 1, max}]; v]; seq[2000] (* _Amiram Eldar_, Sep 23 2022 *)
%o A064440 (PARI) f(n) = my(s=0); for(x=1,n^2, if(sigma(x)-x==n,s++)); s;
%o A064440 a=0; for(n=1,10^5,x=f(n); if(isprime(x),b=x; if(b>a,a=b; print1(n, ", "))))
%Y A064440 Cf. A001065, A048138, A238895, A238896.
%K A064440 nonn
%O A064440 1,1
%A A064440 _Jason Earls_, Oct 01 2001
%E A064440 Corrected and extended by _Matthew Conroy_, Oct 02 2001
%E A064440 a(11)-a(44) from _Amiram Eldar_, Oct 01 2019

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