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 A051261 #15 Apr 17 2022 22:54:29
%S A051261 0,4,6,1,3,7,11,2,5,10,17,23,21,9,28,26,24,8,34,12,35,13,32,29,44,19,
%T A051261 45,15,18,14,57,56,16,20,58,48,25,54,73,76,72,27,75,82,62,66,84,87,71,
%U A051261 78,95,31,74,94,97,103,22,40,39,30,86,33,41,91,46,42,117,119,122,37
%N A051261 Main diagonal of A018219, i.e., (n, n, a_n) is a winning position.
%H A051261 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%t A051261 mex[s_] := Min[Complement[Range[0, Max[{s, -1}] + 1], Flatten[s]]];
%t A051261 f[s_] := Join[s, s + Table[i, {i, Length[s]}]];
%t A051261 T[n_, k_] := T[n, k] = mex[{f[Table[T[n - i, k], {i, n}]], f[Table[T[n, k - i], {i, k}]], f[Table[T[n - i, k - i], {i, Min[n, k]}]]}];
%t A051261 a[n_] := T[n, n];
%t A051261 Table[a[n], {n, 0, 69}] (* _Jean-François Alcover_, Aug 19 2019, from A018219 Mma code *)
%Y A051261 Cf. A018219, Inverse is A077226.
%K A051261 nonn,nice
%O A051261 0,2
%A A051261 _Michael Kleber_
%E A051261 Edited and extended by _Christian G. Bower_, Oct 29 2002