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 A054020 #13 Oct 27 2023 22:00:46 %S A054020 6,9,10,15,16,20,21,22,25,28,30,33,34,36,39,42,44,45,46,48,49,50,51, %T A054020 54,55,57,58,60,64,68,69,70,72,75,76,78,80,81,82,84,85,87,91,93,94,96, %U A054020 98,99,100,102,105,106,108,111,114,115,116,117,118,120,121,123,124,126 %N A054020 Chowla's function of n is not divisible by the number of proper divisors of n. %C A054020 Chowla's function (A048050) = sum of divisors of n except 1 and n. %H A054020 Amiram Eldar, <a href="/A054020/b054020.txt">Table of n, a(n) for n = 1..10000</a> %p A054020 with(numtheory): %p A054020 [seq(`if`((sigma(i)-i-1) mod (tau(i)-1) <> 0,i,print( )),i=2..500)]; %t A054020 Select[Range[2,150],!Divisible[DivisorSigma[1,#]-#-1,DivisorSigma[ 0,#]- 1]&] (* _Harvey P. Dale_, May 27 2014 *) %Y A054020 Complement is A054021. %Y A054020 Cf. A032741, A048050, A054015. %K A054020 nonn %O A054020 1,1 %A A054020 _Asher Auel_, Jan 19 2000