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 A383154 #13 Apr 18 2025 13:57:51 %S A383154 2,2,22,1620,882130,3465050546 %N A383154 The number of 2n-by-2n fers-wazir tours. %C A383154 The simplest fairy chess pieces, going back to 9th-century Persia, are the fers -- a (1,1) leaper -- and the wazir -- a (1,0) leaper. (A king combines the moves of a fers and a wazir.) A fers-wazir tour visits every cell of a board exactly once, making fers and wazir moves alternately, and returns to the starting cell. %C A383154 Such tours exist only when the number of rows is even and the number of columns is even. %D A383154 D. E. Knuth, Hamiltonian paths and cycles, Section 7.2.2.4 of The Art of Computer Programming (to appear). %H A383154 George Jelliss, <a href="https://www.mayhematics.com/t/1n.htm">Introducing Knight's Tours</a>, has a 9th century example of a fers-knight tour due to As-Suli. %e A383154 For n=2 the a(2) = 2 solutions are transposes of each other: %e A383154 . %e A383154 0-f 4-3 0 e-d b %e A383154 X X |X X| %e A383154 e 1-2 5 f 1 a c %e A383154 | | | | %e A383154 d a-9 6 4 2 9 7 %e A383154 X X |X X| %e A383154 b-c 7-8 3 5-6 8 %Y A383154 Diagonal of A383153. %Y A383154 Cf. A140519. %K A383154 nonn,more %O A383154 1,1 %A A383154 _Don Knuth_, Apr 18 2025