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Numbers k such that A033831(k) is prime.
1

%I #11 Jun 18 2019 03:29:52

%S 8,10,12,14,15,16,18,20,21,22,26,27,28,30,32,33,34,35,38,39,42,44,45,

%T 46,48,50,51,52,55,56,57,58,62,63,64,65,68,69,72,74,75,76,77,80,81,82,

%U 85,86,87,90,91,92,93,94,95,98,99

%N Numbers k such that A033831(k) is prime.

%H Amiram Eldar, <a href="/A033872/b033872.txt">Table of n, a(n) for n = 1..10000</a>

%t f[n_] := DivisorSum[n, 1&, # > 2 && n/# < #-1 &]; Select[Range[1000], PrimeQ[f[#]] &] (* _Amiram Eldar_, Jun 11 2019 *)

%o (PARI) isok(n) = isprime(sumdiv(n, d, (d>=3) && (q=n/d) && (q>=1) && (q<=d-2))); \\ _Michel Marcus_, Jun 11 2019

%Y Cf. A033831.

%K easy,nonn

%O 1,1

%A _Naohiro Nomoto_