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A074875 Numbers n such that sigma(sigma(n)-n) = phi(n). 1

%I #9 Feb 11 2014 19:05:30

%S 2,21,51,369,3051,3783,5757,6477,6897,7929,15639,15925,20967,33003,

%T 50739,58797,73917,75627,118405,121975,131979,170973,219447,291369,

%U 310137,311675,347337,365679,396807,404559,437255,442599,550719,558429

%N Numbers n such that sigma(sigma(n)-n) = phi(n).

%H Donovan Johnson, <a href="/A074875/b074875.txt">Table of n, a(n) for n = 1..300</a>

%e sigma(sigma(21) - 21) = sigma(32 - 21) = sigma(11) = 12 = phi(21), so 21 is a term of the sequence.

%t Select[Range[10^5], DivisorSigma[1, DivisorSigma[1, # ] - # ] == EulerPhi[ # ] &]

%K nonn

%O 1,1

%A _Joseph L. Pe_, Sep 12 2002

%E a(19)-a(34) from _Donovan Johnson_, May 13 2010

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Last modified March 29 03:41 EDT 2024. Contains 371264 sequences. (Running on oeis4.)