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 A232324 #6 Nov 27 2013 21:46:10
%S A232324 0,0,2,3,3,9,4,6,6,1,6,22,7,9,0,12,9,15,10,0,7,1,12,0,15,15,18,14,15,
%T A232324 33,16,24,33,1,6,29,19,21,52,10,21,39,22,66,21,1,24,60,28,66,30,6,27,
%U A232324 45,28,36,53,1,30,150,31,33,40,48,45,51,34,78,15,37,36
%N A232324 n(n+1)/2 modulo sigma(n).
%C A232324 Also antisigma(n) modulo sigma(n). Antisigma(n) = A024816(n) = the sum of the nondivisors of n that are between 1 and n, sigma(n) = A000203(n) = the sum of the divisors of n.
%C A232324 a(n) = 0 for numbers from A076617, a(n) = 1 for numbers from A232540, a(n) = n for numbers from A232538.
%H A232324 Jaroslav Krizek, <a href="/A232324/b232324.txt">Table of n, a(n) for n = 1..10000</a>
%F A232324 a(n) = n(n+1)/2 mod A000203(n).
%e A232324 For n=10, a(10) = antisigma(10) mod sigma(10) = 37 mod 18 = 1.
%t A232324 Table[Mod[n (n + 1)/2, DivisorSigma[1, n]], {n, 100}] (* _T. D. Noe_, Nov 27 2013 *)
%Y A232324 Cf. A000203, A076617, A024816, A232538, A232540.
%K A232324 nonn
%O A232324 1,3
%A A232324 _Jaroslav Krizek_, Nov 25 2013