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 A056883 #18 Oct 11 2022 07:00:11 %S A056883 0,0,0,1,1,5,4,18,19,72,72,268,276,1002,1043,3733,3926,14000,14810, %T A056883 52658,55987,199020,212364,754826,807861,2872926,3082202,10966333, %U A056883 11789248,41970489,45198317,160996986,173640109,618839771,668316169,2383000591,2576516523,9191269314,9947949563,35502661446 %N A056883 Number of n-celled rotationally symmetric polyominoes without holes. %C A056883 In other words, free unholey polyominoes with 180-degree rotational symmetry and no other. - _John Mason_, Oct 11 2022 %H A056883 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A056883 Cf. A000104, A006747 (with holes allowed). %K A056883 nonn %O A056883 1,6 %A A056883 _N. J. A. Sloane_, Sep 03 2000 %E A056883 a(29)-a(40) from _John Mason_, Oct 11 2022