%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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