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 A224661 #6 Jul 23 2025 05:16:54 %S A224661 5,20,98,180,412,806,1606,3070,5808,10798,19866,36184 %N A224661 Number of n X n 0..4 matrices with each 2X2 subblock idempotent. %C A224661 Column 4 of A224665 %F A224661 Empirical: a(n) = 6*a(n-1) -12*a(n-2) +5*a(n-3) +12*a(n-4) -12*a(n-5) -3*a(n-6) +6*a(n-7) -a(n-9) for n>10 %e A224661 Some solutions for n=3 %e A224661 ..1..0..2....1..0..3....0..0..0....0..0..0....1..0..0....1..1..0....0..0..0 %e A224661 ..1..0..1....1..0..1....0..0..0....0..0..0....0..0..0....0..0..0....0..0..0 %e A224661 ..4..0..1....2..0..1....2..1..1....0..0..0....0..0..1....0..0..1....4..1..1 %K A224661 nonn %O A224661 1,1 %A A224661 _R. H. Hardin_ Apr 14 2013