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 A103139 #30 May 09 2021 02:26:17
%S A103139 1,2,6,9,15,22,28,39,49,59,73
%N A103139 Woolbright sequence: the maximum number of kings on an n X n chessboard such that every single king is attacking a number of other kings that is smaller or equal to the number of empty spaces around it.
%C A103139 Lower bounds for terms following 59 are as follows: 73, 86, 102, 117, 136, 153, 173, 195, 216, 239, 266, 289, 318, 345, 375, 405, 438, 471, 504, 540, 576, 614, 654, 693, 735, 777, ...
%D A103139 Bernardo Recamán, The Bogotá Puzzles, Dover Publications, 2020, p. 19.
%H A103139 J. E. Dunbar, D. G. Hoffman, R. C. Laskar and L. R. Markus, <a href="https://doi.org/10.1016/S0012-365X(99)00131-4">Alpha-domination</a>, Discrete Mathematics, 211 (2000), pp. 11-26.
%H A103139 T. Howard, E. J. Ionascu, and D. Woolbright, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL12/Ionascu/ionascu.html">Introduction to the Prisoners and Guards Game</a>, JIS 12 (2009) 09.1.3.
%H A103139 Eugen J. Ionascu, Dan Pritikin and Stephen E. Wright, <a href="https://arxiv.org/abs/math/0608140">k-Dependence and Domination in Kings Graphs</a>, arXiv:math/0608140 [math.OC], 2006.
%H A103139 Eugen J. Ionascu, Dan Pritikin and Stephen E. Wright, <a href="https://www.jstor.org/stable/27642610">k-Dependence and Domination in Kings Graphs</a>, Amer. Math. Monthly, 115 (2008), 820-836.
%F A103139 a(n) = n^2 - gamma_{1/2}(n) = approx floor(3*(n^2+1)/5). (I assume this is a lower bound? - _N. J. A. Sloane_)
%e A103139 a(3)=6. Indeed, on a 3 X 3 chessboard one can arrange six kings on two side columns to satisfy the requirement. It is not possible to arrange seven kings since the center has to be empty and then at least one of the squares in the middle of the sides must have a king on it, which requires at least three empty spaces around, and that is impossible.
%K A103139 nonn,more
%O A103139 1,2
%A A103139 _Eugen J. Ionascu_, Mar 17 2005
%E A103139 One more term [from the Ionascu et al. paper] from _Vladeta Jovovic_, Sep 17 2008