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%I #19 Sep 04 2022 21:03:55
%S 1,11,18,75,172,372,482,732,3276,7965,13068,46476,103488,252240,
%T 727956,1795605,6030528,8769600,15436092,56747712,224518995,297780084,
%U 317961000,855489144,2691137721,58016937600,127198874880,228517311456,411814778112
%N Numbers k such that phi(sigma(k)) + sigma(phi(k)) = 2k.
%C a(26) > 3*10^10. - _Donovan Johnson_, Mar 05 2014
%e phi(sigma(11)) + sigma(phi(11)) = phi(12) + sigma(10) = 4 + 18 = 2*11, so 11 is in the sequence.
%t f[n_] := EulerPhi[DivisorSigma[1, n]]+DivisorSigma[1, EulerPhi[n]]; Select[Range[1, 10^5], f[ # ]==2*#&]
%Y Cf. A000010, A000203.
%K more,nonn
%O 1,2
%A _Joseph L. Pe_, Jan 25 2002
%E Edited by _Dean Hickerson_, Jan 26 2002
%E a(20)-a(25) from _Donovan Johnson_, Sep 16 2009
%E a(26)-a(28) from _Kevin P. Thompson_, Apr 19 2022
%E a(29) from _Kevin P. Thompson_, Sep 04 2022
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