cp's OEIS Frontend

A054021 Numbers n such that Chowla's function of n is divisible by the number of proper divisors of n.

%I A054021 #12 Oct 27 2023 22:00:46
%S A054021 2,3,4,5,7,8,11,12,13,14,17,18,19,23,24,26,27,29,31,32,35,37,38,40,41,
%T A054021 43,47,52,53,56,59,61,62,63,65,66,67,71,73,74,77,79,83,86,88,89,90,92,
%U A054021 95,97,101,103,104,107,109,110,112,113,119,122,125,127,128,131,134,136
%N A054021 Numbers n such that Chowla's function of n is divisible by the number of proper divisors of n.
%C A054021 Chowla's function (A048050) = sum of divisors of n except 1 and n.
%H A054021 Amiram Eldar, <a href="/A054021/b054021.txt">Table of n, a(n) for n = 1..10000</a>
%p A054021 with(numtheory):
%p A054021 [seq(`if`((sigma(i)-i-1) mod (tau(i)-1)=0,i,print( )),i=2..1000)];
%t A054021 Select[Range[2,150],Divisible[DivisorSigma[1,#]-1-#,DivisorSigma[ 0,#]-1]&] (* _Harvey P. Dale_, Aug 13 2018 *)
%Y A054021 Complement is A054020.
%Y A054021 Cf. A032741, A048050, A054015.
%K A054021 nonn
%O A054021 1,1
%A A054021 _Asher Auel_, Jan 19 2000