# Isometries of the Plane - Stabiliser of a Line and Further Discussions

I'm trying to determine the stabilizer of a line in a plane when acted upon by the group of isometries of the plane. Please note that I'm using the notation found in the Wikipedia article on Euclidean plane isometries.

I've identified (and hopefully exhausted) the following isometries that will 'fix' a line $l$ set-wise. For the point-wise case, clearly only the identity transformation will work.

1. The identity transformation
2. Rotation about a point on the line by $\pi$ radians, i.e. $R_{c,\pi}$ for any point $c$ on the line $l$. I think this can also be expressed as $R_{c,\pi}=T_cR_{\pi}T_{-c}$
3. Translation in a direction parallel to the line, i.e. $T_v$ where $v$ is a vector parallel to $l$
4. Reflection about $l$ itself, i.e. $F_{\alpha,u}$ for any $\alpha$ lying on $l$, $u$ perpendicular to $l$
5. Reflection about any line perpendicular to $l$, i.e. $F_{\beta, v}$ for any $\beta$ in the plane, with $v$ parallel to $l$

I can't think of any more, but if I've missed any out, I'd be very grateful to learn of them. This leads me into my questions:

How can I 'formally' express the response to the given question:

The group G of isometries of the plane acts on the set of lines in the plane. Determine the stabilizer of a line in the plane.

Is there a better way of responding than just listing the isometries, as I have done here? i.e. Can we notate the set of the required isomteries more neatly?

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In the case of the pointwise stabilizer, reflection in the line itself also works. – Ben Blum-Smith Feb 21 '12 at 21:54

Many thanks, do you know how could I express this as a generating set? Would it be something like $\langle F_{\alpha,u},F_{\beta_v} \rangle$? (Using notation from above) – Mathmo Feb 21 '12 at 22:11