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 A213802 #8 Jul 22 2025 23:04:38 %S A213802 18,169,880,3249,9522,23753,52544,106009,198770,351233,590832,953625, %T A213802 1485778,2245489,3304704,4751377,6691410,9251257,12580080,16852705, %U A213802 22271986,29072121,37521216,47924969,60629426,76025041,94549616 %N A213802 Number of 4X4 0..n symmetric arrays with all rows summing to 2*n. %C A213802 Row 4 of A213800 %H A213802 R. H. Hardin, <a href="/A213802/b213802.txt">Table of n, a(n) for n = 1..173</a> %F A213802 Empirical: a(n) = 4*a(n-1) -3*a(n-2) -8*a(n-3) +14*a(n-4) -14*a(n-6) +8*a(n-7) +3*a(n-8) -4*a(n-9) +a(n-10). %F A213802 Empirical: G.f. -x*(18+97*x+258*x^2+380*x^3+266*x^4+86*x^5+22*x^6+4*x^7-4*x^8+x^9) / ( (1+x)^3*(x-1)^7 ). - _R. J. Mathar_, Jul 04 2012 %e A213802 Some solutions for n=4 %e A213802 ..2..3..2..1....3..1..1..3....0..1..3..4....2..4..0..2....1..3..1..3 %e A213802 ..3..2..0..3....1..4..1..2....1..0..4..3....4..2..2..0....3..4..0..1 %e A213802 ..2..0..3..3....1..1..3..3....3..4..1..0....0..2..3..3....1..0..3..4 %e A213802 ..1..3..3..1....3..2..3..0....4..3..0..1....2..0..3..3....3..1..4..0 %K A213802 nonn %O A213802 1,1 %A A213802 _R. H. Hardin_ Jun 20 2012