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A230201 Numbers k such that sigma(phi(k)) < k. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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

MAPLE

for n do if sigma(phi(n))<n then print{n} fi od:

MATHEMATICA

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

PROG

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

CROSSREFS

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

Sequence in context: A209211 A064676 A147819 * A059534 A137804 A246414

Adjacent sequences:  A230198 A230199 A230200 * A230202 A230203 A230204

KEYWORD

nonn

AUTHOR

Vladimir Letsko, Oct 11 2013

STATUS

approved

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Last modified September 23 14:40 EDT 2021. Contains 347618 sequences. (Running on oeis4.)