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 A126685 #10 Dec 08 2017 00:45:09 %S A126685 0,0,0,0,0,3,51,1106,3813,47300,216420,2057581,10276981,74358924 %N A126685 In chess, the number of checkmate dual-free proof games in n plies. %C A126685 Among all the proof game problems counted in A090051, this is the number of problems ending in checkmate. %H A126685 F. Labelle, <a href="http://www.cs.berkeley.edu/~flab/chess/statistics-positions.html#checkmate">Statistics on checkmate diagrams</a>. %H A126685 Retrograde Analysis Corner, <a href="http://www.janko.at/Retros/Records/ShortestMate/index.htm">Shortest Mates in Proof Games</a>. %e A126685 a(5)=3 because there are three checkmate proof games in 5 plies: %e A126685 1. e4 e5 2. Qh5 Ke7 3. Qxe5# %e A126685 1. e3 e5 2. Qh5 Ke7 3. Qxe5# %e A126685 1. e4 f5 2. exf5 g5 3. Qh5# %Y A126685 Cf. A090051, A102784. %K A126685 hard,nonn,fini %O A126685 0,6 %A A126685 _François Labelle_, Feb 13 2007 %E A126685 a(12)-a(13) from _François Labelle_, Dec 05 2017