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 A353015 #15 Apr 16 2022 09:39:55
%S A353015 1,1,4,27,257,3157,47385,839928,17168097,397517868,10283523826,
%T A353015 293950435376,9200707298514,312972062711525,11496160809184370,
%U A353015 453507273286873875,19122193920118268355,858239308206920523297,40851073484808464297979
%N A353015 a(n) = Sum_{k=0..floor(n/3)} (n-3*k)^n.
%F A353015 G.f.: Sum_{k>=0} (k * x)^k / (1 - (k * x)^3).
%t A353015 a[0] = 1; a[n_] := Sum[(n-3*k)^n, {k, 0, Floor[n/3]}]; Array[a, 20, 0] (* _Amiram Eldar_, Apr 16 2022 *)
%o A353015 (PARI) a(n) = sum(k=0, n\3, (n-3*k)^n);
%o A353015 (PARI) my(N=20, x='x+O('x^N)); Vec(sum(k=0, N, (k*x)^k/(1-(k*x)^3)))
%Y A353015 Cf. A352982, A353014, A353017.
%K A353015 nonn,easy
%O A353015 0,3
%A A353015 _Seiichi Manyama_, Apr 16 2022