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 A213699 #6 Jul 22 2025 23:03:27 %S A213699 70,7827,208072,2322599,16537086,84059234,342943680,1169405354, %T A213699 3500184570,9372540629,23022951048,52423261153,112270651514, %U A213699 227577256564,440658095360,818409534692,1466948929086,2544850623111,4291394833480 %N A213699 Half the number of 4X4 0..n symmetric arrays with no 2X2 subblock summing to 2n. %C A213699 Row 3 of A213697 %H A213699 R. H. Hardin, <a href="/A213699/b213699.txt">Table of n, a(n) for n = 1..144</a> %F A213699 Empirical: a(n) = 3*a(n-1) +5*a(n-2) -23*a(n-3) -4*a(n-4) +76*a(n-5) -28*a(n-6) -140*a(n-7) +98*a(n-8) +154*a(n-9) -154*a(n-10) -98*a(n-11) +140*a(n-12) +28*a(n-13) -76*a(n-14) +4*a(n-15) +23*a(n-16) -5*a(n-17) -3*a(n-18) +a(n-19) %e A213699 Some solutions for n=4 %e A213699 ..0..3..4..0....2..3..1..3....3..0..4..4....2..3..2..2....0..1..2..2 %e A213699 ..3..3..2..3....3..4..3..3....0..2..4..3....3..2..0..0....1..4..2..4 %e A213699 ..4..2..2..2....1..3..1..0....4..4..0..4....2..0..3..1....2..2..1..0 %e A213699 ..0..3..2..3....3..3..0..3....4..3..4..3....2..0..1..0....2..4..0..2 %K A213699 nonn %O A213699 1,1 %A A213699 _R. H. Hardin_ Jun 18 2012