A069237 - OEIS

%I #13 Jan 06 2018 11:34:44

%S 8,9,10,15,18,21,24,26,28,30,33,34,35,39,40,45,49,51,52,55,56,57,58,

%T 63,65,69,70,72,74,76,77,78,82,84,85,87,88,90,91,93,95,98,99,102,104,

%U 105,106,108,110,111,115,117,119,120,122,123,124,125,126,128,129,130,133

%N Composite n such that tau(n) divides phi(n), where tau(n) is the number of divisors of n and phi(n) the Euler totient function.

%C Includes A057388 and 2*A002144. - _Robert Israel_, Jan 05 2018

%H Robert Israel, <a href="/A069237/b069237.txt">Table of n, a(n) for n = 1..10000</a>

%p filter:= n -> not isprime(n) and numtheory:-phi(n) mod numtheory:-tau(n)=0:

%p select(filter, [$4..1000]); # _Robert Israel_, Jan 05 2018

%t nn=200;Rest[Select[Complement[Range[nn],Prime[Range[PrimePi[nn]]]],Divisible[ EulerPhi[#], DivisorSigma[0,#]]&]] (* _Harvey P. Dale_, Mar 31 2011 *)

%Y Composite numbers in A003601.

%Y Cf. A002144, A057388.

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, Apr 13 2002