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 A283123 #18 May 11 2024 06:26:36
%S A283123 13,39,40,91,78,120,104,195,121,234,156,280,182,312,240,403,234,363,
%T A283123 260,546,320,468,312,600,403,546,364,728,390,720,416,819,480,702,624,
%U A283123 847,494,780,560,1170,546,960,572,1092,726,936,624,1240,741,1209
%N A283123 a(n) = sigma(9*n).
%C A283123 In general, for k>=1, Sum_{j=1..n} sigma(j*k) ~ A069097(k) * Pi^2 * n^2 / (12*k). - _Vaclav Kotesovec_, May 11 2024
%H A283123 Seiichi Manyama, <a href="/A283123/b283123.txt">Table of n, a(n) for n = 1..10000</a>
%F A283123 a(n) = A000203(9*n).
%F A283123 Sum_{k=1..n} a(k) = (35*Pi^2/36) * n^2 + O(n*log(n)). - _Amiram Eldar_, Dec 16 2022
%t A283123 Table[DivisorSigma[1, 9 n], {n, 50}] (* _Michael De Vlieger_, Mar 01 2017 *)
%o A283123 (PARI) a(n) = sigma(9*n) \\ _Amiram Eldar_, Dec 16 2022
%Y A283123 Sigma(k*n): A000203 (k=1), A062731 (k=2), A144613 (k=3), A193553 (k=4), A283118 (k=5), A224613 (k=6), A283078 (k=7), A283122 (k=8), this sequence (k=9).
%Y A283123 Cf. A008591.
%K A283123 nonn
%O A283123 1,1
%A A283123 _Seiichi Manyama_, Mar 01 2017