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 A359203 #16 Aug 23 2023 08:42:25
%S A359203 1,7,28,127,406,1756,5104,20575,61237,230122,649540,2579932,6908734,
%T A359203 26044984,74578888,269985151,731794258,2799670555,7360989292,
%U A359203 27392181562,75948764752,268482753172,721764371008,2742292424188,7078172334031,25701091008418,71173405454680
%N A359203 a(n) = Sum_{d|n} (n/d) * 3^(n-d).
%F A359203 G.f.: Sum_{k>=1} x^k/(1 - (3 * x)^k)^2.
%t A359203 a[n_] := DivisorSum[n, 3^(n-#)*n/# &]; Array[a, 27] (* _Amiram Eldar_, Aug 23 2023 *)
%o A359203 (PARI) a(n) = sumdiv(n, d, n/d*3^(n-d));
%o A359203 (PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, x^k/(1-(3*x)^k)^2))
%Y A359203 Cf. A000203, A080267, A115607, A359204.
%Y A359203 Cf. A356539, A359112.
%K A359203 nonn,easy
%O A359203 1,2
%A A359203 _Seiichi Manyama_, Dec 20 2022