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%I #26 Sep 11 2020 22:34:51
%S 1,12,56,140,270,630,1672,4180,6426,18810,80104,93496,99484,102856,
%T 116116,140296,191862,200260,220616,223938,224536,233740,257140,
%U 350740,447678,449442,522522,551540,561340,702240,901170,1051830,1157130,1578330,2481930,2526030
%N Numbers k such that phi(k) = sigma(core(k)) where phi(k) is the Euler totient function, sigma(k) the sum of divisors of k and core(k) the squarefree part of k (the smallest integer such that k*core(k) is a square).
%C phi(k) = sigma(core(k)) | sigma(k) so this is a subsequence of A020492. - _David A. Corneth_, Sep 08 2020
%H David A. Corneth, <a href="/A069552/b069552.txt">Table of n, a(n) for n = 1..16559</a> (first 183 terms from Amiram Eldar, terms calculated from terms provided by Jud McCranie in A020492)
%e 140 is in the sequence as phi(140) = 48 = sigma(35) = sigma(core(140)). - _David A. Corneth_, Sep 08 2020
%t core[n_] := Times @@ Power @@@ ({#[[1]], Mod[ #[[2]], 2]} & /@ FactorInteger[n]); Select[Range[10^5], EulerPhi[#] == DivisorSigma[1, core[#]] &] (* _Amiram Eldar_, Jul 11 2019 after _Zak Seidov_ at A007913 *)
%o (PARI) for(n=1,10^6,if(eulerphi(n)==sigma(core(n)),print1(n,",")))
%Y Cf. A000010, A000203, A007913, A020492.
%K nonn,easy
%O 1,2
%A _Benoit Cloitre_, Apr 17 2002
%E More terms from _Amiram Eldar_, Jul 11 2019
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