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 A337501 #9 Aug 30 2020 03:04:38 %S A337501 0,2,4,8,11,18,23,32,39,50 %N A337501 Minimum number of painted cells in an n X n grid to avoid unpainted trominoes. %e A337501 For n = 3, painting only 3 cells would leave an unpainted tromino, but painting the following 4 cells avoids all unpainted trominoes: %e A337501 . . X %e A337501 X X . %e A337501 . . X %K A337501 nonn,more %O A337501 1,2 %A A337501 _Rob Pratt_, Aug 29 2020