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 A359521 #7 Jan 05 2023 11:04:34 %S A359521 4,0,26,0,194,17,1086,152,6140,1006,32713,6335,172379,38800 %N A359521 Number of free mature snake polyominoes of n cells, where mature means that the snake cannot grow in either direction. %e A359521 a(15) = 4 because of: %e A359521 OOO OOO OOO OOO %e A359521 O O O O O O O O %e A359521 O OO O OO O OO OO OO %e A359521 O O OO O OOO O OO O %e A359521 OOOOO OOOO OOO OOO %Y A359521 Cf. A002013. %K A359521 nonn,more %O A359521 15,1 %A A359521 _John Mason_, Jan 05 2023