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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 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: A176496 A091940 A068605 * 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 April 13 10:24 EDT 2021. Contains 342935 sequences. (Running on oeis4.)