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%I #29 Mar 20 2022 06:43:50
%S 2,12,24,90,234,528,588,1456,4320,4680,9024,11466,26208,35640,55104,
%T 55552,90816,114660,121024,123648,185562,199584,297600,510720,674880,
%U 2142720,2190336,4112640,4316928,5322240,5659641,7600320,8714160,10281600,12999168,15268368
%N Integers n such that numerator and denominator of sigma(n)/n are both prime.
%H Michel Marcus, <a href="/A247086/b247086.txt">Table of n, a(n) for n = 1..69</a> (duplicated terms removed by Michel Marcus).
%e For n=2, sigma(n)/n = 3/2 with both numerator and denominator prime.
%t a247086Q[n_] := Block[{f = DivisorSigma[1, n]/n}, And[PrimeQ@ Numerator@ f, PrimeQ@ Denominator@ f]]; a247086[n_] := Select[Range@ n, a247086Q@ # &]; a247086[10^6] (* _Michael De Vlieger_, Jan 11 2015 *)
%o (PARI) isok(n) = my(ab = sigma(n)/n); isprime(numerator(ab)) && isprime(denominator(ab));
%Y Cf. A000203 (sigma(n)).
%Y Cf. A017665 (numerator of sigma(n)/n), A017666 (denominator of sigma(n)/n).
%K nonn
%O 1,1
%A _Michel Marcus_, Jan 10 2015
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