A070171 Numbers k such that sigma(phi(k)) = k-phi(k).

2, 18, 84, 15210, 15216, 66720, 2858880, 62905344, 358313760, 3633603510

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: A068605 A343514 A360301 * A357757 A172529 A201236

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