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 A158276 #9 Jun 12 2024 16:34:13
%S A158276 2,3,5,6,7,8,10,11,12,13,14,15,17,18,19,21,22,23,24,26,27,28,29,30,31,
%T A158276 32,33,34,35,37,38,39,40,41,42,43,44,45,46,47,48,51,52,53,54,55,56,57,
%U A158276 58,59,60,61,62,63,65,66,67,68,69,70,71,72,73,74,75,76,77
%N A158276 Numbers k such that sigma_1(k) does not divide sigma_2(k).
%C A158276 Numbers k such that the antiharmonic mean of divisors of k is not an integer.
%C A158276 Antiharmonic mean of divisors of a number m = Product (p_i^e_i) is A001157(m)/A000203(m) = Product ((p_i^(e_i+1)+1)/(p_i+1)).
%C A158276 Numbers k such that A001157(k)/A000203(k) is not an integer.
%e A158276 a(12) = 15, sigma_2(15)/sigma_1(15)=260/24 = 65/6 (not integer).
%t A158276 Select[Range[100], Mod @@ DivisorSigma[{2, 1}, #] > 0 &] (* _Amiram Eldar_, Mar 22 2024 *)
%o A158276 (PARI) is(n) = {my(f = factor(n)); sigma(f, 2) % sigma(f);} \\ _Amiram Eldar_, Mar 22 2024
%Y A158276 Complement of A020487.
%Y A158276 Cf. A001157, A000203.
%K A158276 nonn
%O A158276 1,1
%A A158276 _Jaroslav Krizek_, Mar 15 2009
%E A158276 More terms from _Amiram Eldar_, Mar 22 2024