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 A056881 #14 Oct 11 2022 06:59:21 %S A056881 0,0,1,0,2,2,6,5,21,18,70,61,239,206,828,711,2903,2485,10314,8791, %T A056881 37031,31451,134147,113566,489672,413419,1799084,1515282,6647009, %U A056881 5586834,24677902,20703840,92009606,77067674,344336294,288001040,1292929699,1079999410,4869184237,4062534430 %N A056881 Number of n-celled diagonally symmetric polyominoes without holes. %H A056881 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A056881 Cf. A000104, A006748 (with holes allowed). %K A056881 nonn %O A056881 1,5 %A A056881 _N. J. A. Sloane_, Sep 03 2000 %E A056881 a(29)-a(40) from _John Mason_, Oct 11 2022