A275245 - OEIS

%I #16 May 28 2024 01:06:07

%S 10,20,40,42,50,60,80,84,100,114,120,126,136,156,160,168,180,200,220,

%T 228,240,250,252,272,294,300,312,320,336,342,360,378,400,440,444,456,

%U 468,480,500,504,540,544,588,600,624,640,672,684,720,756,800,816

%N Numbers k such that phi(k) divides k^2 while phi(k) does not divide k.

%H Amiram Eldar, <a href="/A275245/b275245.txt">Table of n, a(n) for n = 1..10000</a>

%e 10 is a term because phi(10) = 4; 10 mod 4 = 2 and 10^2 mod 4 = 0.

%t Select[Range[10^3], Function[k, And[Divisible[#^2, k], ! Divisible[#, k]]]@ EulerPhi@ # &] (* _Michael De Vlieger_, Jul 21 2016 *)

%o (PARI) isok(n) = (n % eulerphi(n) != 0) && (n^2 % eulerphi(n) == 0)

%Y Cf. A000010, A007694, A090778.

%K nonn

%O 1,1

%A _Altug Alkan_, Jul 21 2016