"chiral" polyominoes

[Graeme McRae]

Dear seqfans,

A000988, the number of n-celled one-sided polyominoes, uses the word "chiral" in a misleading way.  Chiral means distinct from its mirror image.  True, it is reasonable to characterize free polyominoes (A000105) as non-chiral in that no free polyomino is considered distinct from its mirror image.  However not all one-sided polyominoes (A000988) are chiral.  In fact, a better sequence to describe chiral polyominoes is A030228, which are the polyominoes lacking bilateral symmetry.

I suggest removing the word "chiral" from the description of A000988, and adding the following comment to all three sequences, along with appropriate cross-references:

A000105(n) + A030228(n) = A000988(n) because the number of free polyominoes plus the number of polyominoes lacking bilateral symmetry equals the number of one-sided polyominoes.

Also, A030228 can be extended by two terms using this identity.

--Graeme McRae
Palmdale, CA
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