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 A224660 #7 Nov 10 2014 17:35:47 %S A224660 4,16,72,142,332,668,1356,2634,5046,9480,17594,32286,58730,106028, %T A224660 190204,339322,602416,1064888,1875104,3290166,5754586,10035236, %U A224660 17452462,30275242,52395822 %N A224660 Number of n X n 0..3 matrices with each 2 X 2 subblock idempotent. %C A224660 Column 3 of A224665. %F A224660 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 A224660 Some solutions for n=3: %e A224660 ..1..0..1....1..0..0....1..0..0....1..1..0....1..1..3....1..0..2....1..1..1 %e A224660 ..0..0..1....1..0..0....0..0..0....0..0..0....0..0..0....1..0..1....0..0..0 %e A224660 ..0..0..1....2..0..0....2..1..1....3..1..1....0..1..1....0..0..1....2..1..1 %K A224660 nonn,more %O A224660 1,1 %A A224660 _R. H. Hardin_, Apr 14 2013