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A067780 Numbers n such that sigma(n)/phi(n) is prime. 1

%I

%S 2,3,12,15,35,56,78,140,190,357,630,812,910,1045,1672,2214,2580,3339,

%T 4180,4522,4674,5049,5278,5544,8008,10659,11160,12441,16065,16120,

%U 16770,18630,18810,24244,24871,24969,25070,25758,27000,27170,28896

%N Numbers n such that sigma(n)/phi(n) is prime.

%H Vincenzo Librandi, <a href="/A067780/b067780.txt">Table of n, a(n) for n = 1..600</a>

%e sigma(12)/phi(12) = 28/4 = 7, a prime, so 12 is a term of the sequence.

%t Select[Range[10^5], PrimeQ[DivisorSigma[1, # ]/EulerPhi[ # ]] &]

%K nonn

%O 1,1

%A _Joseph L. Pe_, Feb 06 2002

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Last modified January 26 16:30 EST 2021. Contains 340442 sequences. (Running on oeis4.)