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 A203487 #7 Jul 22 2025 17:16:58 %S A203487 24,556,9540,173860,3065704,56438584,1004305424,18465834580, %T A203487 335571341040,6219202151476,114039550489064,2121345108007408, %U A203487 39325546973748384,735898709559550484,13729000925046178840,257888578347427756340 %N A203487 Number of nX4 0..2 arrays with row sums equal and column sums unequal to adjacent columns. %C A203487 Column 4 of A203491 %H A203487 R. H. Hardin, <a href="/A203487/b203487.txt">Table of n, a(n) for n = 1..31</a> %e A203487 Some solutions for n=5 %e A203487 ..0..0..1..2....1..1..2..2....1..1..1..1....2..2..2..0....2..1..1..0 %e A203487 ..2..0..0..1....2..1..2..1....1..0..1..2....0..2..2..2....2..2..0..0 %e A203487 ..2..0..0..1....2..1..2..1....2..1..0..1....0..2..2..2....0..1..2..1 %e A203487 ..1..0..0..2....1..1..2..2....1..0..2..1....0..2..2..2....0..1..2..1 %e A203487 ..2..0..0..1....1..1..2..2....1..0..1..2....1..1..2..2....2..0..2..0 %K A203487 nonn %O A203487 1,1 %A A203487 _R. H. Hardin_ Jan 02 2012