A230201 Numbers k such that sigma(phi(k)) < k.

2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 30, 32, 34, 36, 40, 42, 44, 46, 48, 50, 54, 58, 60, 64, 66, 68, 70, 72, 78, 80, 84, 90, 92, 94, 96, 98, 100, 102, 106, 108, 110, 114, 118, 120, 126, 128, 130, 132, 136, 138, 140, 144, 150, 156, 160, 162, 166, 168

Offset: 1

Author

Vladimir Letsko, Oct 11 2013

Keywords

• nonn

Comments

All terms are even. However, sigma(phi(k)) may be equal to k for an odd number k if k+2 is a Fermat prime.

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A000203, A001229, A018784, A062402, A066694, A227011, A227927, A230203.

Programs

Maple

for n do if sigma(phi(n))
				

Mathematica

Select[Range[200], DivisorSigma[1, EulerPhi[#]] < # &] (* T. D. Noe, Oct 14 2013 *)

PARI

isok(n) = sigma(eulerphi(n)) < n; \\ Michel Marcus, Oct 12 2013