%I #15 Jan 24 2020 07:15:12
%S 2,863,1726,3452,371953,743906,1487812,1747591,1859765,2975624,
%T 3495182,3719530,5242773,6990364,7439060,8737955,10485546,14878120,
%U 15993979,17475910,20971092,26213865,29756240,31987958,34951820,41942184,47981937,52427730,59512480
%N Numbers n such that 13^phi(n) == 1 (mod n^2), where phi(n) = A000010(n).
%C The subsequence of primes in this sequence is A128667.
%H Giovanni Resta, <a href="/A257660/b257660.txt">Table of n, a(n) for n = 1..140</a> (terms < 3*10^11)
%t Select[Range@ 1000000, Mod[13^EulerPhi[#], #^2] == 1 &] (* _Michael De Vlieger_, Jul 27 2015 *)
%o (PARI) for(n=2, 1e9, if(Mod(13, n^2)^(eulerphi(n))==1, print1(n, ", ")))
%Y Cf. A077816, A128667, A241977, A241978, A242958, A242959, A242960, A245529, A253016.
%K nonn
%O 1,1
%A _Felix Fröhlich_, Jul 26 2015