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 A057730 #98 Sep 08 2022 10:50:47 %S A057730 0,1,1,3,6,25,86,416,1988,10640,57987,328956,1900321,11204350, %T A057730 67042778,406780346 %N A057730 Number of polyominoes (A000105) with perimeter 2n. %C A057730 Does this include polyominoes with holes? - _Franklin T. Adams-Watters_, Sep 12 2006. Answer from _R. J. Mathar_: Yes! See the illustrations in the links (e.g. perimeter 16, area 7, No 81 or perimeter 16, area 8, No 174). %C A057730 All lines (sides of cells which are not common to a pair of cells) contribute to the perimeter, including the interior sides of cavities and holes. - _R. J. Mathar_, Feb 19 2021 %H A057730 Andrew Clarke, <a href="http://www.recmath.com/PolyPages/PolyPages/Isopolyos.html">Isoperimetrical Polyominoes</a> %H A057730 Andrew Clarke, <a href="/A057730/a057730.gif">Picture from Andrew Clarke's page showing the polyominoes of perimeters 4, 6, 8 and 10.</a> %H A057730 Gordon Hamilton, <a href="http://youtu.be/z0xUwJmUiFs">Grade 3 area and perimeter activity</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730.p10.txt">perimeter 10</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730.p12.txt">perimeter 12</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730_1.p14.txt">perimeter 14</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730_4.p16.txt">Illustrating perimeter 16, area <= 13</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730_4.p18.txt">Illustrating perimeter 18, area <= 13</a> %H A057730 R. J. Mathar, <a href="/A057730/a057730_1.p20.txt">Illustrating perimeter 20, area <= 13</a> %Y A057730 Cf. A000105, A002931, A057753, A266549 (same, but holes not allowed), column sums of A342243, A131487 (polyominoes by total number of edges). %K A057730 nonn,nice,more %O A057730 1,4 %A A057730 _N. J. A. Sloane_, Oct 29 2000 %E A057730 Additional comments from _Barry Cipra_, Jun 08 2004 %E A057730 Link updated by _William Rex Marshall_, Dec 16 2009 %E A057730 a(9)-a(10) added by _Luca Petrone_, Jan 08 2016 %E A057730 a(1)-a(9) confirmed by _Bert Dobbelaere_, Oct 19 2018 %E A057730 a(10)-a(12) corrected and extended by _John Mason_, Jul 26 2021 %E A057730 a(13)-a(16) added by _John Mason_, Sep 08 2022