I'm looking at Frieze groups. Wikipedia went to the point to depict how the basic pattern looks like.
I'm trying to understand the next claim about $p2mg$ group:
(TRVG) Vertical reflection lines, Glide reflections, Translations and
180° Rotations: The translations here arise from the glide
reflections, so this group is generated by a glide reflection and
either a rotation or a vertical reflection.
This image, I suppose, come to explain how the basic pattern of $p2mg$ looks like.
From the quote above, it seems that I can apply generators to the basic pattern and get (maybe a translated) basic pattern back.
In class, we denoted the basic glide reflection $\gamma : (x, y) \mapsto (x + \frac{1}{2}, -y)$. (Basic pattern is centered at $(0, 0)$ and have sizes of $1/2 \times 1/2$).
So using this definition I can see how glide reflection acts on the pattern, and it is mapped to itself. So far so good.
I then, can easily see why a vertical reflection $v : (x, y) \mapsto (-x, y)$ also maps the basic pattern to itself.
What am I failing to see is how the rotation ($180^{\circ}$) $r: (x, y)
\mapsto (-x, -y)$ acts on the pattern.
It seems, that rotation acts on the left and right halves of the basic pattern independently.
For comparison I paste here the images for basic pattern of $p2$ (rotation + translations)
and of $p11g$ (Glide reflections only)
In both I can see how generators acts on the pattern (with translation just mapping to the (one of the) next pattern(s)).
So, my question is
How can we see that either used by Wikipedia indeed describes
the same group of isometries?
Cause it seems, that Glide Reflection + Vertical Reflection is not the same thing as Glide Reflection + Rotation.
Best Answer
I will consider the following pattern, with the origin marked by a star
*
:The negative numbers $-4$, $-3$, $-2$, $-1$ are not shown, just place them correspondingly in the
o
places, using a reflection w.r.t. the origin $0=*$.Wikipedia claims on https://en.wikipedia.org/wiki/Frieze_group "The translations here arise from the glide reflections, so this group is generated by a glide reflection and either a rotation or a vertical reflection." And indeed: