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a(n) = the number of Z-frame polyominoes with n cells, reduced for symmetry.
4

%I #14 Feb 11 2023 02:26:25

%S 0,0,0,1,2,6,10,19,27,43,58,85,105,143,175,226,266,334,386,475,534,

%T 641,717,854,933,1092,1187,1385,1482,1713,1820,2091,2203,2511,2637,

%U 2998,3110,3516,3647,4118,4226,4761,4865,5461,5576,6221,6319,7088,7138,7953

%N a(n) = the number of Z-frame polyominoes with n cells, reduced for symmetry.

%C 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 Andrew Howroyd, <a href="/A360420/b360420.txt">Table of n, a(n) for n = 1..1000</a>

%H Andrew Howroyd, <a href="/A360420/a360420.txt">Formula and PARI Program</a>, 2023.

%H John Mason, <a href="/A360420/a360420.pdf">Examples</a>

%F a(n) <= A028247(n). - _Andrew Howroyd_, Feb 08 2023

%e a(4)=1 because of:

%e OO

%e OO

%o (PARI) seq(50) \\ See Links - _Andrew Howroyd_, Feb 08 2023

%Y Cf. A028247, A270060, A360419, A360421.

%K nonn

%O 1,5

%A _John Mason_, Feb 06 2023

%E Terms a(19) and beyond from _Andrew Howroyd_, Feb 08 2023