This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A242958 #24 Feb 21 2024 10:54:52 %S A242958 11,22,44,55,110,220,440,880,1006003,2012006,4024012,11066033, %T A242958 22132066,44264132,55330165,88528264,110660330,221320660,442641320, %U A242958 885282640,1770565280,56224501667,112449003334,224898006668,393571511669,449796013336,618469518337,787143023338 %N A242958 Numbers n such that 3^phi(n) == 1 (mod n^2), where phi(n) is Euler's totient function. %C A242958 a(21) > 10^9. %C A242958 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 A242958 Giovanni Resta, <a href="/A242958/b242958.txt">Table of n, a(n) for n = 1..78</a> (terms < 10^15) %t A242958 Select[Range[1000], Mod[3^EulerPhi[#], #^2] == 1 &] (* _Alonso del Arte_, Jun 02 2014 *) %o A242958 (PARI) for(n=2, 10^9, if(Mod(3, n^2)^(eulerphi(n))==1, print1(n, ", "))) %Y A242958 Cf. A014127, A077816, A242959, A242960, A000010. %K A242958 nonn,hard %O A242958 1,1 %A A242958 _Felix Fröhlich_, May 27 2014 %E A242958 Terms a(21) and beyond from _Giovanni Resta_, Jan 27 2020