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 A209080 #20 Feb 28 2017 02:45:59 %S A209080 1,20,400,1702,8659,49401,287740,1934794,11569093,65443733,360231372, %T A209080 1872156836 %N A209080 Number of chess diagrams that can be obtained in exactly one way in n plies and cannot be obtained in fewer plies. This is also the number of dual-free shortest proof games in n plies. %C A209080 Chess diagrams are chess positions without regard to whether castling or en passant capturing is possible. They are counted in A019319. A shortest proof game (SPG) is a classic type of chess problem. Most published SPGs are dual-free (have only one solution), which is why we count diagrams that are obtained in exactly one way in the minimum number of plies. %H A209080 F. Labelle, <a href="http://wismuth.com/chess/statistics-positions.html#shortest">Statistics on chess positions</a> %H A209080 <a href="/index/Ch#chess">Index entries for sequences related to number of chess games</a> %Y A209080 Cf. A019319, A090051, A209433. %K A209080 nonn,hard,fini,more %O A209080 0,2 %A A209080 _François Labelle_, Mar 04 2012 %E A209080 a(11) from _François Labelle_, Feb 27 2017