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

%I

%S 629,1469,85139,100889,139859,154979,168149,304079,396899,838199,

%T 1107413,1323449,1465463,2088839,2160899,2504879,2684879,2693249,

%U 2800181,3404609,3512249,3576869,3885881,4241819,4500509,4620659,4822649,5530709,5805449

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

%H 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 phi(629) = 576 = 4*phi(630).

%p A:= NULL:

%p y:= numtheory:-phi(1):

%p for n from 1 to 10^6 do

%p x:= numtheory:-phi(n+1);

%p if y = 4*x then A:= A, n fi;

%p y:= x;

%p od:

%p A; # _Robert Israel_, Apr 15 2015

%t Select[Range@ 1000000, EulerPhi@ # == 4 EulerPhi[# + 1] &] (* _Michael De Vlieger_, Apr 13 2015 *)

%t Position[Partition[EulerPhi[Range[6*10^6]],2,1],_?(#[[1]]==4#[[2]]&),{1},Heads->False]//Flatten (* _Harvey P. Dale_, Sep 18 2016 *)

%o (PARI) s=[]; for(n=1, 1000000, if(eulerphi(n)==4*eulerphi(n+1), s=concat(s, n))); s \\ _Colin Barker_, Apr 13 2015

%o (MAGMA) [n: n in [1..10^7] | EulerPhi(n) eq 4*EulerPhi(n+1)]; // _Vincenzo Librandi_, Apr 14 2015

%o (Sage) [n for n in (1..1000000) if euler_phi(n) == 4*euler_phi(n+1)]; # _Bruno Berselli_, Apr 14 2015

%Y Cf. A171262, A256907.

%K nonn

%O 1,1

%A _Mauro Fiorentini_, Apr 13 2015

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Last modified May 28 21:37 EDT 2020. Contains 334690 sequences. (Running on oeis4.)