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A360421
a(n) = the number of X-frame polyominoes with n cells, reduced for symmetry.
2
0, 0, 0, 0, 1, 2, 7, 20, 49, 109, 216, 414, 723, 1228, 1958, 3065, 4580, 6734
OFFSET
1,6
COMMENTS
An X-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 LLRLLRLLRLLR.
EXAMPLE
a(5) = 1 because of:
O
OOO
O
The a(7) = 7 polyominoes are:
O O O O O O O
OOOOO OOOOO OOOOO OOOOO OOOO OOOO OOOO
O O O O O O OO
O O
CROSSREFS
KEYWORD
nonn,more
AUTHOR
John Mason, Feb 06 2023
STATUS
approved