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

629, 1469, 85139, 100889, 139859, 154979, 168149, 304079, 396899, 838199, 1107413, 1323449, 1465463, 2088839, 2160899, 2504879, 2684879, 2693249, 2800181, 3404609, 3512249, 3576869, 3885881, 4241819, 4500509, 4620659, 4822649, 5530709, 5805449

Offset: 1

Author

Mauro Fiorentini, Apr 13 2015

Keywords

• nonn

Examples

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

Links

• Mauro Fiorentini, Table of n, a(n) for n = 1..154 (all terms for n up to 10^9).

Crossrefs

Cf. A171262, A256907.

Programs

Magma

[n: n in [1..10^7] | EulerPhi(n) eq 4*EulerPhi(n+1)]; // Vincenzo Librandi, Apr 14 2015

Maple

A:= NULL:
y:= numtheory:-phi(1):
for n from 1 to 10^6 do
x:= numtheory:-phi(n+1);
if y = 4*x then A:= A, n fi;
y:= x;
od:
A;  # Robert Israel, Apr 15 2015

Mathematica

Select[Range@ 1000000, EulerPhi@ # == 4 EulerPhi[# + 1] &] (* Michael De Vlieger, Apr 13 2015 *)
Position[Partition[EulerPhi[Range[6*10^6]],2,1],?(#[[1]]==4#[[2]]&),{1},Heads->False]//Flatten (* _Harvey P. Dale, Sep 18 2016 *)

PARI

s=[]; for(n=1, 1000000, if(eulerphi(n)==4*eulerphi(n+1), s=concat(s, n))); s \\ Colin Barker, Apr 13 2015

Sage

[n for n in (1..1000000) if euler_phi(n) == 4*euler_phi(n+1)]; # Bruno Berselli, Apr 14 2015