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%I #17 Feb 27 2017 01:00:51
%S 5,6,10,11,14,15,17,22,23,30,33,34,35,42,46,47,51,53,55,62,66,69,70,
%T 71,77,85,94,102,105,106,107,110,115,119,138,141,142,154,155,159,161,
%U 165,170,186,187,191,210,213,214,230,231,235,238,253,254,255,265,282,310
%N Numbers n such that sigma(n) = 3*phi(sigma(n)).
%C From _Robert Israel_, Feb 26 2017: (Start)
%C Numbers n such that sigma(n) is in A033845.
%C Contains A139257. (End)
%H Robert Israel, <a href="/A067612/b067612.txt">Table of n, a(n) for n = 1..10000</a>
%p filter:= proc(n) local s; s:= numtheory:-sigma(n); s mod 6 = 0 and s = 2^padic:-ordp(s,2)*3^padic:-ordp(s,3) end proc:
%p select(filter, [$1..10000]); # _Robert Israel_, Feb 26 2017
%t s3pQ[n_]:=Module[{s=DivisorSigma[1,n]},s==3*EulerPhi[s]]; Select[ Range[ 400],s3pQ] (* _Harvey P. Dale_, May 28 2015 *)
%o (PARI) for(n=1,500,if(sigma(n)==3*eulerphi(sigma(n)),print1(n,", "))) \\ _Derek Orr_, Feb 26 2017
%o (PARI) is(n)=my(s=sigma(n)); s%3==0 && s==3*eulerphi(s) \\ _Charles R Greathouse IV_, Feb 27 2017
%Y Cf. A033845, A139257.
%K easy,nonn
%O 1,1
%A _Benoit Cloitre_, Feb 22 2002
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