This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A069237 #21 Apr 19 2025 04:01:25 %S A069237 8,9,10,15,18,21,24,26,28,30,33,34,35,39,40,45,49,51,52,55,56,57,58, %T A069237 63,65,69,70,72,74,76,77,78,82,84,85,87,88,90,91,93,95,98,99,102,104, %U A069237 105,106,108,110,111,115,117,119,120,122,123,124,125,126,128,129,130,133 %N A069237 Composite numbers k such that tau(k) divides phi(k), where tau(k) is the number of divisors of k and phi(k) the Euler totient function. %C A069237 Includes A046388 and 2*A002144. - _Robert Israel_, Jan 05 2018 %H A069237 Robert Israel, <a href="/A069237/b069237.txt">Table of n, a(n) for n = 1..10000</a> %p A069237 filter:= n -> not isprime(n) and numtheory:-phi(n) mod numtheory:-tau(n)=0: %p A069237 select(filter, [$4..1000]); # _Robert Israel_, Jan 05 2018 %t A069237 nn=200;Rest[Select[Complement[Range[nn],Prime[Range[PrimePi[nn]]]],Divisible[ EulerPhi[#], DivisorSigma[0,#]]&]] (* _Harvey P. Dale_, Mar 31 2011 *) %o A069237 (PARI) isok(k) = if(k == 1 || isprime(k), 0, my(f = factor(k)); !(eulerphi(f) % numdiv(f))); \\ _Amiram Eldar_, Apr 19 2025 %Y A069237 Composite numbers in A020491. %Y A069237 Cf. A002144, A046388. %K A069237 easy,nonn %O A069237 1,1 %A A069237 _Benoit Cloitre_, Apr 13 2002