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 A128504 #24 Oct 24 2024 15:31:43
%S A128504 1,3,3,-2,-9,-9,3,18,18,-4,-30,-30,5,45,45,-6,-63,-63,7,84,84,-8,-108,
%T A128504 -108,9,135,135,-10,-165,-165,11,198,198,-12,-234,-234,13,273,273,-14,
%U A128504 -315,-315,15,360,360,-16,-408,-408,17,459,459
%N A128504 Row sums of array A128503 (second convolution of Chebyshev's S(n,x) = U(n,x/2) polynomials).
%C A128504 Second convolution of A010892.
%C A128504 Convolution of A099254 with A010892.
%C A128504 a(n) equals the coefficient of x^2 of the characteristic polynomial of the (n+2)X(n+2) tridiagonal matrix with 1's along the main diagonal, the superdiagonal, and the subdiagonal (see Mathematica code below). [_John M. Campbell_, Jul 10 2011]
%H A128504 Tewodros Amdeberhan, George E. Andrews, and Roberto Tauraso, <a href="https://arxiv.org/abs/2409.20400">Further study on MacMahon-type sums of divisors</a>, arXiv:2409.20400 [math.NT], 2024. See p. 18.
%F A128504 a(n) = Sum_{m=0..floor(n/2)} A128503(n,m).
%F A128504 G.f.: 1/(1-x+x^2)^3.
%F A128504 a(n) = (floor(n/3)+1)*(floor(n/3)-floor((n-1)/3)+(3/2)*(floor(n/3)+2)*(3*floor((n+1)/3)-n))*(-1)^n. - _Tani Akinari_, Jul 03 2013
%t A128504 Table[Coefficient[CharacteristicPolynomial[Array[KroneckerDelta[#1, #2] + KroneckerDelta[#1, #2 - 1] + KroneckerDelta[#1, #2 + 1] &, {n + 2, n + 2}], x], x^2], {n, 0, 70}] (* _John M. Campbell_, Jul 10 2011 *)
%o A128504 (PARI) Vec(1/(1-x+x^2)^3+O(x^66)) \\ _Joerg Arndt_, Jul 02 2013
%Y A128504 Cf. A010892, A099254, A128503.
%K A128504 sign,easy
%O A128504 0,2
%A A128504 _Wolfdieter Lang_ Apr 04 2007