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 A257594 #42 Nov 22 2015 22:47:15 %S A257594 0,0,0,0,0,0,1,1,2,3,4,5,7,8,10 %N A257594 Consider the hexagonal lattice packing of circles; a(n) is the maximal number of circles that can be enclosed by a closed chain of n circles. %H A257594 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), 1-11. %H A257594 Craig Knecht, <a href="/A257594/a257594.png">Classification of spaces between the pennies</a> %H A257594 Craig Knecht, <a href="/A257594/a257594_1.png">Trying to relate Sloane's 1990 findings to intercircle volumes</a> %H A257594 R. J. Mathar, <a href="/A257594/a257594_2.pdf">Illustration of conjectured a(9) to a(24)</a> %H A257594 Kival Ngaokrajang, <a href="/A257594/a257594_1.pdf">Illustration of initial terms</a> %F A257594 Conjecture (derived from Euler's F+V=E+1 formula): a(n) = 1+(A069813(n)-n)/2 = A001399(n-6), which means g.f. is x^6 / ( (1+x)*(1+x+x^2)*(1-x)^3 ). - _R. J. Mathar_, Jul 14 2015 %e A257594 In the hexagonal lattice packing of pennies, one penny can be enclosed by 6 pennies, 2 pennies by eight pennies, 3 pennies by 9 pennies, 4 pennies by 10 pennies, 5 pennies by 11 pennies, and 7 pennies by 12 pennies. %Y A257594 Cf. A257481. %K A257594 nonn,more %O A257594 0,9 %A A257594 _N. J. A. Sloane_, May 18 2015 %E A257594 a(13) and a(14) from _R. J. Mathar_, Jul 10 2015