A090778 - OEIS

%I #17 Sep 19 2019 11:02:17

%S 1,2,4,6,8,10,12,16,18,20,24,32,36,40,42,48,50,54,60,64,72,80,84,96,

%T 100,108,114,120,126,128,136,144,156,160,162,168,180,192,200,216,220,

%U 228,240,250,252,256,272,288,294,300,312,320,324,336,342,360,378,384,400

%N Numbers k such that phi(k) divides k*(k - phi(k)).

%C Also phi(k) divides k^2. - _Enrique Pérez Herrero_, Jul 04 2010

%H Amiram Eldar, <a href="/A090778/b090778.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) seems to be asymptotic to c*n^2 with c=0.11.....

%t pdnQ[n_]:=Module[{ephi=EulerPhi[n]},Divisible[n(n-ephi),ephi]]; Select[ Range[ 400],pdnQ] (* _Harvey P. Dale_, Dec 22 2015 *)

%K nonn

%O 1,2

%A _Benoit Cloitre_, Feb 08 2004