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%I #24 Feb 21 2024 10:54:52
%S 11,22,44,55,110,220,440,880,1006003,2012006,4024012,11066033,
%T 22132066,44264132,55330165,88528264,110660330,221320660,442641320,
%U 885282640,1770565280,56224501667,112449003334,224898006668,393571511669,449796013336,618469518337,787143023338
%N Numbers n such that 3^phi(n) == 1 (mod n^2), where phi(n) is Euler's totient function.
%C a(21) > 10^9.
%C All listed composite terms are multiples of the two known primes in this sequence, 11 and 1006003, the only known base 3 Wieferich primes.
%H Giovanni Resta, <a href="/A242958/b242958.txt">Table of n, a(n) for n = 1..78</a> (terms < 10^15)
%t Select[Range[1000], Mod[3^EulerPhi[#], #^2] == 1 &] (* _Alonso del Arte_, Jun 02 2014 *)
%o (PARI) for(n=2, 10^9, if(Mod(3, n^2)^(eulerphi(n))==1, print1(n, ", ")))
%Y Cf. A014127, A077816, A242959, A242960, A000010.
%K nonn,hard
%O 1,1
%A _Felix Fröhlich_, May 27 2014
%E Terms a(21) and beyond from _Giovanni Resta_, Jan 27 2020
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