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%I #11 Aug 21 2017 19:14:49
%S 1,2,12,75960,17156160
%N a(n) is the smallest number k such that sigma(k) - phi(k) = n*k, or 0 if no such k exists.
%C a(5) and following terms > 10^13, if they exist. - _Giovanni Resta_, Aug 21 2017
%e a(2) = 12 because sigma(12) = 28, phi(12) = 4; sigma(12) - phi(12) = 2*12 and 12 is the smallest number with this property.
%o (PARI) a(n) = {my(k=1); while (sigma(k)-eulerphi(k)!=n*k, k++); k; }
%Y Cf. A000010, A000203, A055681.
%K nonn,more
%O 0,2
%A _Altug Alkan_, Aug 21 2017
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