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A070171 Numbers k such that sigma(phi(k)) = k-phi(k). 1
2, 18, 84, 15210, 15216, 66720, 2858880, 62905344, 358313760, 3633603510 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Apart from the first term, all elements are composite.  So far all terms beyond the first are divisible by 6.

LINKS

Table of n, a(n) for n=1..10.

MATHEMATICA

Do[s=DivisorSigma[1, EulerPhi[n]]-(n-EulerPhi[n]); If[Equal[s, 0], Print[n]], {n, 1, 2000000}]

PROG

(PARI) for(n=2, 2000000, if(sigma(eulerphi(n))==n-eulerphi(n), print1(n, ", ")))

(Python)

from sympy import divisor_sigma as sigma, totient as phi

def aupto(limit):

  for k in range(1, limit):

    if sigma(phi(k), 1) == k - phi(k): print(k, end=", ")

aupto(2*10**4) # Michael S. Branicky, Feb 18 2021

CROSSREFS

Cf. A051953, A062402, A070170.

Sequence in context: A091940 A068605 A343514 * A172529 A201236 A206623

Adjacent sequences:  A070168 A070169 A070170 * A070172 A070173 A070174

KEYWORD

more,nonn

AUTHOR

Benoit Cloitre and Labos Elemer, May 06 2002

EXTENSIONS

a(8)-a(9) from Charles R Greathouse IV, May 02 2011

a(10) from Donovan Johnson, May 03 2011

STATUS

approved

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Last modified May 16 11:51 EDT 2021. Contains 343942 sequences. (Running on oeis4.)