%I #11 Jul 21 2015 00:44:16
%S 97,1417,2593,107167,128137,262993,468247,629821,879937,894127,
%T 1100347,1260847,1620307,1644967,1897417,2890717,3122773,3186397,
%U 3651667,4169197,6176467,7072477,7344187,8237707,8974717,9254647,13032547,15540037,15673027
%N Numbers n such that phi(n) = phi(2*n+1).
%H Donovan Johnson, <a href="/A092272/b092272.txt">Table of n, a(n) for n = 1..200</a>
%e phi(97) = 97-1 = 96 = 2*4*12 = phi(3*5*13) = phi(195) = phi(2*97+1)
%p with (numtheory): for i from 1 to 1000000 do if phi(i)=phi(2*i+1) then print(i) fi; od;
%t Do[ If[ EulerPhi[n] == EulerPhi[2n + 1], Print[n]], {n, 1, 15540036}] (* _Robert G. Wilson v_, Feb 20 2004 *)
%o (PARI) is(n)=my(t=eulerphi(2*n+1));t<n && t==eulerphi(n) \\ _Charles R Greathouse IV_, May 19 2013
%Y Cf. A000010, A005384.
%K nonn
%O 1,1
%A Pooya Farshim (P.Farshim.00(AT)cantab.net), Feb 17 2004
%E More terms from _Labos Elemer_ and _Robert G. Wilson v_, Feb 19 2004