A067382 Numbers n such that sigma(phi(n))/sigma(n) = 2.

13, 71, 89, 203, 305, 319, 667, 1363, 1421, 1525, 1711, 1889, 2407, 2933, 3103, 4609, 4615, 4843, 5191, 6583, 7123, 7625, 7627, 9947, 10063, 10411, 11107, 13543, 13891, 14587, 16327, 17023, 19693, 20851, 23075, 24331, 24721, 25027, 25723

Offset: 1

Author

Dean Hickerson, Jan 20 2002

Keywords

• nonn

Comments

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

Links

• Donovan Johnson, Table of n, a(n) for n = 1..1000

Crossrefs

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

Programs

Mathematica

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

PARI

is(n)=sigma(eulerphi(n=factor(n)))/sigma(n)==2 \\ Charles R Greathouse IV, Nov 27 2013