CIRCLE WITH A DOT

Echocardiograms are so expensive that I really think they should offer a 30 percent discount if they fail to locate an alien chest-burster.

I know some people are keen on buying their eBooks from Bookshop dot org, so I’m happy to announce that almost* all my self-published DRM-free eBooks are now available there.

* Except for DICHRONAUTS, because the US rights are held by Night Shade so my own edition can only be sold outside North America.

This page lists all the eBooks:

eBooks — Greg Egan

Greg Egan eBooks

(www.gregegan.net)

@SvenGeier @tonwood

Yes, they are mirror images if you ignore the position of the white space within the square.

So if I was trying to construct, say, a set of unique wooden objects that could be rotated and flipped over, it would only contain one of that pair.

Similarly, the last two figures in the original post’s third-last row look like they are duplicates, but one has a column of white space on the left, the other has it on the right.

@tonwood

This is very cool!

Here are representatives of the 45 orbits of these figures under the group D_4 (the eight rotations and reflections that are isometries of a square), so no pair of these 45 figures are rotations/reflections of each other.