[seqfan] Re: Two-fold symmetry sequence

franktaw at netscape.net franktaw at netscape.net
Mon Dec 29 22:12:52 CET 2008

First of all, these are called "polyominoes".  "Shape" is a far
more general term; it would include, for example, squares
joined only at a corner.

 From your web site, it appears that you are looking only for
2-fold rotational symmetry.  2-fold symmetry includes also
reflective symmetries, such as are possessed by




amongst others.

The number of polyominoes with reflective symmetry is A030227.
I don't think your sequence is in the database, although I'm not
entirely certain.

Franklin T. Adams-Watters

-----Original Message-----
From: Frederick Schneider <frederick.william.schneider at gmail.com>

Sorry, that should be:


On Mon, Dec 29, 2008 at 2:28 PM, Frederick Schneider <
frederick.william.schneider at gmail.com> wrote:

> Hello,
> I tried to find but didn't see the number of distinct 2-fold symmetric
> shapes that can be made from n squares.
> Starting from n=1 :  1,1,1,3,3,6
> Here's an image explaining what I mean:
> http://www.bosola.org/grandpa/imgs/2-foldSymmetry.jpg
> Has anyone seen a similar sequence?  If not, I'll try to extend this 
> least a few terms out.
> Thanks,
> Fred


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