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 A059511 #9 Sep 10 2017 15:04:02
%S A059511 1,3,26,471,13953,627421,39873948,3406344237,376591685322,
%T A059511 52319828460127,8922977163384085,1832831375530080986,
%U A059511 446306812127457163805,127130280682997045232005,41881054710966221044222595,15800074329058136531647698779
%N A059511 Main diagonal of the array A059217.
%H A059511 G. C. Greubel, <a href="/A059511/b059511.txt">Table of n, a(n) for n = 1..240</a>
%t A059511 t[0, 0] = 1; t[0, _?EvenQ] = 1; t[_?OddQ, 0] = 1; t[n_, k_] /; OddQ[n + k](*up*):= t[n, k] = t[n + 1, k - 1] + Sum[t[n, j], {j, 0, k - 1}];
%t A059511 t[n_, k_] /; EvenQ[n + k](*down*):= t[n, k] = t[n - 1, k + 1] + Sum[t[j, k], {j, 0, n - 1}]; Table[t[n, n], {n,0,50}] (* _G. C. Greubel_, Sep 10 2017 *)
%K A059511 easy,nonn
%O A059511 1,2
%A A059511 _Floor van Lamoen_, Jan 21 2001
%E A059511 Terms a(11) onward added by _G. C. Greubel_, Sep 10 2017