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 A056880 #16 Oct 10 2022 20:11:59 %S A056880 0,1,1,1,1,2,3,4,4,6,10,11,16,18,33,34,56,56,110,106,193,177,372,335, %T A056880 662,571,1269,1084,2282,1875,4365,3568,7914,6257,15141,11935,27615, %U A056880 21170,52911,40486,96980,72490,186144,138981,342638,250814,658884,482029,1217327,875697,2345272,1686803 %N A056880 Number of n-celled polyominoes without holes, symmetric about axis 2. %C A056880 In other words, free polyominoes, without holes, with both vertical and horizontal symmetries but no other. - _John Mason_, Oct 10 2022 %H A056880 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A056880 Cf. A000104, A056877 (with holes allowed). %K A056880 nonn %O A056880 1,6 %A A056880 _N. J. A. Sloane_, Sep 03 2000 %E A056880 a(29)-a(52) from _John Mason_, Oct 10 2022