A090778 - OEIS

A090778

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

1, 2, 4, 6, 8, 10, 12, 16, 18, 20, 24, 32, 36, 40, 42, 48, 50, 54, 60, 64, 72, 80, 84, 96, 100, 108, 114, 120, 126, 128, 136, 144, 156, 160, 162, 168, 180, 192, 200, 216, 220, 228, 240, 250, 252, 256, 272, 288, 294, 300, 312, 320, 324, 336, 342, 360, 378, 384, 400

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OFFSET

1,2

COMMENTS

Also phi(k) divides k^2. - Enrique Pérez Herrero, Jul 04 2010

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

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

MATHEMATICA

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

CROSSREFS