%I #10 Sep 08 2022 08:46:15

%S 1261,13651,17557,18721,24511,42121,113611,244531,266071,712081,

%T 749911,795691,992251,1080721,1286731,1458271,1849471,2271061,2457691,

%U 3295381,3370771,3414841,3714751,4061971,4736491,5314051,5827081,6566911,6935083,7303981,7864081

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

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

%F a(n) = A172314(n) + 1. - _Michel Marcus_, Jan 27 2016

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

%t Select[Range@10000000, EulerPhi@# == 4 EulerPhi[# - 1] &] (* _Vincenzo Librandi_, Jan 27 2016 *)

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

%o (PARI) isok(n) = (eulerphi(n) == 4*eulerphi(n-1)); \\ _Michel Marcus_, Jan 27 2016

%Y 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.

%Y Cf. A256937.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Jan 26 2016