A268126 Numbers n such that phi(n) = 4*phi(n-1).

1261, 13651, 17557, 18721, 24511, 42121, 113611, 244531, 266071, 712081, 749911, 795691, 992251, 1080721, 1286731, 1458271, 1849471, 2271061, 2457691, 3295381, 3370771, 3414841, 3714751, 4061971, 4736491, 5314051, 5827081, 6566911, 6935083, 7303981, 7864081

Offset: 1

Author

Jaroslav Krizek, Jan 26 2016

Keywords

• nonn

Comments

See A266276(n) = the smallest numbers k such that phi(k) = n * phi(k-1) for n >=1: 2, 3, 7, 1261, 11242771, ...

Examples

			1261 is in the sequence because phi(1261) = 1152 = 4*phi(1260) = 4*288.
		

Crossrefs

Cf. A000010, A171271 (numbers n such that phi(n) = 2*phi(n-1)), A266268 (numbers n such that phi(n) = 3*phi(n-1)), A266276.

Cf. A256937.

Programs

Magma

[n: n in [2..10^7] | EulerPhi(n) eq 4*EulerPhi(n-1)]

Mathematica

Select[Range@10000000, EulerPhi@# == 4 EulerPhi[# - 1] &] (* Vincenzo Librandi, Jan 27 2016 *)

PARI

isok(n) = (eulerphi(n) == 4*eulerphi(n-1)); \\ Michel Marcus, Jan 27 2016

Formula

a(n) = A172314(n) + 1. - Michel Marcus, Jan 27 2016