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 A143870 #15 Feb 13 2016 10:09:01 %S A143870 1,2,11,91,1462,38483,1755113,140404442,19400886875,4764856837927, %T A143870 2050537030592506,1572046460892726633,2133798146501117397613, %U A143870 5167591018292995062973870,22288638410038000574365307819,171813750317653145879779979300275,2366759768251260378737273078723819964 %N A143870 Number of ways of placing kings with no more than 1 mutual attack on an n X n chessboard. %H A143870 R. H. Hardin, <a href="/A143870/b143870.txt">Table of n, a(n) for n = 0..26</a> %e A143870 Configurations of 0,1,2,3 or 4 kings on an 2 X 2 chessboard and the number of mutual attacks: %e A143870 .. .. .. K. .K .. K. KK .K .K K. K. .K KK KK KK %e A143870 .. K. .K .. .. KK K. .. .K K. .K KK KK .K K. KK %e A143870 0 0 0 0 0 1 1 1 1 1 1 3 3 3 3 6 %e A143870 From 16 configurations is 11 with no more than 1 mutual attack, a(2)=11. %Y A143870 Cf. A143875, A143881, A143886. %Y A143870 Cf. A143871, A143872, A143873, A143874. %Y A143870 Diagonal of A268781. %K A143870 nonn %O A143870 0,2 %A A143870 _R. H. Hardin_, Sep 04 2008 %E A143870 Example added by _Vaclav Kotesovec_, Oct 20 2014 %E A143870 Additional terms from _R. H. Hardin_, Feb 13 2016