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 A346878 #30 Mar 17 2024 03:20:52
%S A346878 1,3,6,7,8,16,10,15,21,22,14,36,16,28,42,31,20,55,22,50,54,40,26,76,
%T A346878 43,46,66,64,32,108,34,63,78,58,74,123,40,64,90,106,44,140,46,92,144,
%U A346878 76,50,156,73,117,114,106,56,172,106,136,126,94,62,240,64,100,186,127
%N A346878 Sum of the divisors, except for the largest, of the n-th positive even number.
%C A346878 Sum of aliquot divisors (or aliquot parts) of the n-th positive even number.
%C A346878 a(n) has a symmetric representation.
%F A346878 a(n) = A001065(2*n).
%F A346878 a(n) = 1 + A346880(n).
%F A346878 Sum_{k=1..n} a(k) = (5*Pi^2/24 - 1) * n^2 + O(n*log(n)). - _Amiram Eldar_, Mar 17 2024
%e A346878 For n = 5 the 5th even number is 10 and the divisors of 10 are [1, 2, 5, 10] and the sum of the divisors of 10 except for the largest is 1 + 2 + 5 = 8, so a(5) = 8.
%t A346878 a[n_] := DivisorSigma[1, 2*n] - 2*n; Array[a, 100] (* _Amiram Eldar_, Aug 20 2021 *)
%o A346878 (Python)
%o A346878 from sympy import divisors
%o A346878 def a(n): return sum(divisors(2*n)[:-1])
%o A346878 print([a(n) for n in range(1, 65)]) # _Michael S. Branicky_, Aug 20 2021
%o A346878 (PARI) a(n) = sigma(2*n) - 2*n; \\ _Michel Marcus_, Aug 20 2021
%Y A346878 Bisection of A001065.
%Y A346878 Partial sums give A347154.
%Y A346878 Cf. A000203, A005843, A048050, A062731, A237593, A245092, A244049, A326124, A346870, A346877, A346880, A347153.
%K A346878 nonn,easy
%O A346878 1,2
%A A346878 _Omar E. Pol_, Aug 20 2021