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%I #13 Mar 15 2020 07:54:00
%S 1,7,11891,130801,273493,1438811,3008423,6290339,15826921,33092653,
%T 69193729,144677797,174096131,364019183,761131019,1591455767,
%U 1915057441,3327589331,4004211013,8372441209,17506013437,21065631851,36603482641,44046321143,76534554613,92096853299
%N Numbers k such that sigma_2(k) = sigma_2(phi(k)).
%C The sequence is infinite since it contains all the numbers of the form 11^i*23^j*47 for i,j > 0. Up to 10^11 the only terms not of this form are 1 and 7. - _Giovanni Resta_, Mar 15 2020
%e 50 = 1^2 + 7^2 (sum of the squares of the divisors of 7) = 1^2 + 2^2 + 3^2 + 6^2 (sum of the squares of the divisors of 6 = phi(7)). So 7 is in the sequence.
%t Select[Range[10!],DivisorSigma[2,#]==DivisorSigma[2,EulerPhi[#]]&]
%o (PARI) isok(m) = sigma(m, 2) == sigma(eulerphi(m), 2); \\ _Michel Marcus_, Mar 15 2020
%Y Cf. A000010, A001157, A070418, A033631.
%K nonn
%O 1,2
%A _Ivan N. Ianakiev_, Mar 14 2020
%E More terms from _Giovanni Resta_, Mar 15 2020
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