A070171 - OEIS

%I #25 Feb 18 2021 12:46:09

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

%N Numbers k such that sigma(phi(k)) = k-phi(k).

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

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

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

%o (Python)

%o from sympy import divisor_sigma as sigma, totient as phi

%o def aupto(limit):

%o   for k in range(1, limit):

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

%o aupto(2*10**4) # _Michael S. Branicky_, Feb 18 2021

%Y Cf. A051953, A062402, A070170.

%K more,nonn

%O 1,1

%A _Benoit Cloitre_ and _Labos Elemer_, May 06 2002

%E a(8)-a(9) from _Charles R Greathouse IV_, May 02 2011

%E a(10) from _Donovan Johnson_, May 03 2011