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 A337503 #14 Aug 30 2020 17:58:08 %S A337503 0,0,3,5,8,13,17,24,31,39 %N A337503 Minimum number of painted cells in an n X n grid to avoid unpainted pentominoes. %H A337503 Puzzling Stack Exchange, <a href="https://puzzling.stackexchange.com/questions/101485/least-amount-of-marked-symbols">Least number of marked symbols</a> %e A337503 For n = 3, painting only 2 cells would leave an unpainted pentomino, but painting the following 3 cells avoids all unpainted pentominoes: %e A337503 . . . %e A337503 . X X %e A337503 X . . %Y A337503 Cf. A337501, A337502. %K A337503 nonn,more %O A337503 1,3 %A A337503 _Rob Pratt_, Aug 29 2020