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 A257481 #36 Feb 16 2025 08:33:25 %S A257481 0,0,0,0,0,0,1,1,1,2,2,3,3,4 %N A257481 Consider a hole-less cluster of n circles in the hexagonal lattice packing of circles; a(n) is the maximal number of circles that touch 6 circles. %H A257481 R. L. Graham and N. J. A. Sloane, <a href="http://neilsloane.com/doc/RLG/138.pdf">Penny-Packing and Two-Dimensional Codes</a>, Discrete and Comput. Geom. 5 (1990), <a href="http://dx.doi.org/10.1007/BF02187775">1-11</a>. %H A257481 Kival Ngaokrajang, <a href="/A257481/a257481.pdf">Illustration of initial terms</a> %H A257481 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexagonalGrid.html">Hexagonal grid</a> %H A257481 Wikipedia, <a href="http://en.wikipedia.org/wiki/Circle_packing">Circle packing</a> %e A257481 For a(7), one circle can be completely enclosed by six surrounding circles, so a(7)=1, a(n)=0 for n<7. %e A257481 For a(10), two circles can be completely enclosed by eight surrounding circles, so a(10)=2. %Y A257481 Cf. A182619, A257594, A069813. %K A257481 nonn,more %O A257481 1,10 %A A257481 _Peter Woodward_, Apr 26 2015 %E A257481 Edited by _N. J. A. Sloane_, May 18 2015