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A256937 Numbers n such that phi(n) = 4*phi(n+1).

%I A256937 #23 Sep 08 2022 08:46:12
%S A256937 629,1469,85139,100889,139859,154979,168149,304079,396899,838199,
%T A256937 1107413,1323449,1465463,2088839,2160899,2504879,2684879,2693249,
%U A256937 2800181,3404609,3512249,3576869,3885881,4241819,4500509,4620659,4822649,5530709,5805449
%N A256937 Numbers n such that phi(n) = 4*phi(n+1).
%H A256937 Mauro Fiorentini, <a href="/A256937/b256937.txt">Table of n, a(n) for n = 1..154</a> (all terms for n up to 10^9).
%e A256937 phi(629) = 576 = 4*phi(630).
%p A256937 A:= NULL:
%p A256937 y:= numtheory:-phi(1):
%p A256937 for n from 1 to 10^6 do
%p A256937 x:= numtheory:-phi(n+1);
%p A256937 if y = 4*x then A:= A, n fi;
%p A256937 y:= x;
%p A256937 od:
%p A256937 A;  # _Robert Israel_, Apr 15 2015
%t A256937 Select[Range@ 1000000, EulerPhi@ # == 4 EulerPhi[# + 1] &] (* _Michael De Vlieger_, Apr 13 2015 *)
%t A256937 Position[Partition[EulerPhi[Range[6*10^6]],2,1],_?(#[[1]]==4#[[2]]&),{1},Heads->False]//Flatten (* _Harvey P. Dale_, Sep 18 2016 *)
%o A256937 (PARI) s=[]; for(n=1, 1000000, if(eulerphi(n)==4*eulerphi(n+1), s=concat(s, n))); s \\ _Colin Barker_, Apr 13 2015
%o A256937 (Magma) [n: n in [1..10^7] | EulerPhi(n) eq 4*EulerPhi(n+1)]; // _Vincenzo Librandi_, Apr 14 2015
%o A256937 (Sage) [n for n in (1..1000000) if euler_phi(n) == 4*euler_phi(n+1)]; # _Bruno Berselli_, Apr 14 2015
%Y A256937 Cf. A171262, A256907.
%K A256937 nonn
%O A256937 1,1
%A A256937 _Mauro Fiorentini_, Apr 13 2015