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Numbers n such that sigma(phi(n))/sigma(n) = 2.
7

%I #12 Jun 24 2014 01:08:20

%S 13,71,89,203,305,319,667,1363,1421,1525,1711,1889,2407,2933,3103,

%T 4609,4615,4843,5191,6583,7123,7625,7627,9947,10063,10411,11107,13543,

%U 13891,14587,16327,17023,19693,20851,23075,24331,24721,25027,25723

%N Numbers n such that sigma(phi(n))/sigma(n) = 2.

%C Sequence is infinite. Contains subsequences like 5^i*61, 5^i*13*71, 7^i*29 ... (see also Farideh Firoozbakht's comment on A033631.) - vim(AT)gmx.li, Nov 03 2006

%H Donovan Johnson, <a href="/A067382/b067382.txt">Table of n, a(n) for n = 1..1000</a>

%t For[ n=1, True, n++, If[ DivisorSigma[ 1, EulerPhi[ n ] ]/DivisorSigma[ 1, n ]==2, Print[ n ] ] ]

%o (PARI) is(n)=sigma(eulerphi(n=factor(n)))/sigma(n)==2 \\ _Charles R Greathouse IV_, Nov 27 2013

%Y Cf. A033631, A066831, A067383, A067384, A067385, A197952.

%K nonn

%O 1,1

%A _Dean Hickerson_, Jan 20 2002