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 A201812 #10 May 25 2018 08:56:37 %S A201812 19,61,151,313,571,949,1471,2161,3043,4141,5479,7081,8971,11173,13711, %T A201812 16609,19891,23581,27703,32281,37339,42901,48991,55633,62851,70669, %U A201812 79111,88201,97963,108421,119599,131521,144211,157693,171991,187129,203131 %N A201812 Number of arrays of 4 integers in -n..n with sum zero and equal numbers of elements greater than zero and less than zero. %C A201812 Row 4 of A201811. %H A201812 R. H. Hardin, <a href="/A201812/b201812.txt">Table of n, a(n) for n = 1..210</a> %F A201812 Empirical: a(n) = 4*n^3 + 14*n + 1. %F A201812 Conjectures from _Colin Barker_, May 25 2018: (Start) %F A201812 G.f.: x*(19 - 15*x + 21*x^2 - x^3) / (1 - x)^4. %F A201812 a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4) for n>4. %F A201812 (End) %e A201812 Some solutions for n=21: %e A201812 -6 15 -3 -8 -13 1 13 16 15 -15 3 -21 -6 8 -11 1 %e A201812 9 -12 8 9 9 21 -2 -18 13 9 -7 12 3 20 -18 18 %e A201812 4 -18 16 -17 -13 -14 -13 9 -20 21 -16 19 -11 -7 21 -15 %e A201812 -7 15 -21 16 17 -8 2 -7 -8 -15 20 -10 14 -21 8 -4 %Y A201812 Cf. A201811. %K A201812 nonn %O A201812 1,1 %A A201812 _R. H. Hardin_, Dec 05 2011