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 A346880 #23 Mar 21 2024 08:36:13
%S A346880 0,2,5,6,7,15,9,14,20,21,13,35,15,27,41,30,19,54,21,49,53,39,25,75,42,
%T A346880 45,65,63,31,107,33,62,77,57,73,122,39,63,89,105,43,139,45,91,143,75,
%U A346880 49,155,72,116,113,105,55,171,105,135,125,93,61,239,63,99,185,126,121
%N A346880 Sum of the divisors, except the smallest and the largest, of the n-th positive even number.
%C A346880 a(n) has a symmetric representation.
%F A346880 a(n) = A048050(2*n).
%F A346880 Sum_{k=1..n} a(k) = (5*Pi^2/24 - 1) * n^2 + O(n*log(n)). - _Amiram Eldar_, Mar 21 2024
%e A346880 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 the smaller and the largest is 2 + 5 = 7, so a(5) = 7.
%t A346880 a[n_] := DivisorSigma[1, 2*n] - 2*n - 1; Array[a, 100] (* _Amiram Eldar_, Aug 19 2021 *)
%o A346880 (Python)
%o A346880 from sympy import divisors
%o A346880 def a(n): return sum(divisors(2*n)[1:-1])
%o A346880 print([a(n) for n in range(1, 66)]) # _Michael S. Branicky_, Aug 19 2021
%Y A346880 Bisection of A048050.
%Y A346880 Partial sums give A346870.
%Y A346880 Cf. A000203, A005843, A062731, A237593, A245092, A244049, A326124, A346879.
%K A346880 nonn,easy
%O A346880 1,2
%A A346880 _Omar E. Pol_, Aug 18 2021