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 A360420 #14 Feb 11 2023 02:26:25 %S A360420 0,0,0,1,2,6,10,19,27,43,58,85,105,143,175,226,266,334,386,475,534, %T A360420 641,717,854,933,1092,1187,1385,1482,1713,1820,2091,2203,2511,2637, %U A360420 2998,3110,3516,3647,4118,4226,4761,4865,5461,5576,6221,6319,7088,7138,7953 %N A360420 a(n) = the number of Z-frame polyominoes with n cells, reduced for symmetry. %C A360420 A Z-frame polyomino has a perimeter that forms a self-avoiding polygon such that as you traverse the perimeter counterclockwise you encounter turns in the order LLLRLLLR. %H A360420 Andrew Howroyd, <a href="/A360420/b360420.txt">Table of n, a(n) for n = 1..1000</a> %H A360420 Andrew Howroyd, <a href="/A360420/a360420.txt">Formula and PARI Program</a>, 2023. %H A360420 John Mason, <a href="/A360420/a360420.pdf">Examples</a> %F A360420 a(n) <= A028247(n). - _Andrew Howroyd_, Feb 08 2023 %e A360420 a(4)=1 because of: %e A360420 OO %e A360420 OO %o A360420 (PARI) seq(50) \\ See Links - _Andrew Howroyd_, Feb 08 2023 %Y A360420 Cf. A028247, A270060, A360419, A360421. %K A360420 nonn %O A360420 1,5 %A A360420 _John Mason_, Feb 06 2023 %E A360420 Terms a(19) and beyond from _Andrew Howroyd_, Feb 08 2023