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Numbers n such that phi(n) = phi(3n+1).
1

%I #12 Oct 15 2013 22:32:22

%S 75973,301513,2053393,3404053,4022573,5399123,6461473,8395613,9848813,

%T 11804173,14124133,15680573,17173333,20705533,23609083,28988353,

%U 30290563,32845633,35025943,36791813,39630803,42598663,43605943,44116333,53795083,54396533,54563693

%N Numbers n such that phi(n) = phi(3n+1).

%C Is it accidental that most terms have form 10k+3?

%H Donovan Johnson, <a href="/A091294/b091294.txt">Table of n, a(n) for n = 1..100</a>

%t Do[If[Equal[EulerPhi[n], EulerPhi[3*n+1]], Print[n]], {n, 1, 100000000}]

%o (PARI) is(n)=my(t=eulerphi(3*n+1)); t<n && t==eulerphi(n) \\ _Charles R Greathouse IV_, May 19 2013

%Y Cf. A000010, A092272.

%K nonn

%O 1,1

%A _Labos Elemer_, Feb 19 2004

%E a(20)-a(27) from _Donovan Johnson_, Jul 27 2011