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 A123491 #7 Oct 14 2017 21:27:16
%S A123491 1,2,5,10,22,48,112,274,715,1982,5837,18180,59644,205296,739032,
%T A123491 2775180,10846965,44039754,185391469,807776198,3637193474,16900721824,
%U A123491 80939650552,399061251246,2023408865983,10540656630118
%N A123491 Diagonal sums of number triangle A123490.
%H A123491 G. C. Greubel, <a href="/A123491/b123491.txt">Table of n, a(n) for n = 0..675</a>
%F A123491 a(n) = Sum_{k=0..floor(n/2)} ((k+2)^(n-2k)+k)/(k+1).
%t A123491 Table[Sum[((k + 2)^(n - 2 k) + k)/(k + 1), {k, 0, Floor[n/2]}], {n, 0, 50}] (* _G. C. Greubel_, Oct 14 2017 *)
%o A123491 (PARI) for(n=0,25, print1(sum(k=0,floor(n/2), ((k + 2)^(n - 2 k) + k)/(k + 1)), ", ")) \\ _G. C. Greubel_, Oct 14 2017
%Y A123491 Cf. A123490.
%K A123491 easy,nonn
%O A123491 0,2
%A A123491 _Paul Barry_, Oct 01 2006