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 A357647 #9 Oct 10 2022 20:12:23 %S A357647 0,0,0,0,0,0,0,1,0,0,0,2,2,0,0,7,7,0,0,22,24,0,0,71,82,0,0,239,280,0, %T A357647 0,817,970,0,0,2841,3403,0,0,10027,12064,0,0,35800,43193,0,0,129007, %U A357647 156011,0,0,468541,567664,0,0,1713174 %N A357647 a(n) is the number of free unholey polyominoes of n cells with 90-degree rotational symmetry and no other. %Y A357647 Cf. A000104, A144553. %K A357647 nonn %O A357647 1,12 %A A357647 _John Mason_, Oct 10 2022