# Reflections and Reflection Groups - Basic Geometry

1. Mar 30, 2012

### Math Amateur

I am reading Kane's book on Reflections and Reflection Groups and am having difficulties with some basic geometric notions - see attachment Kane paes 6-7.

On page 6 in section 1-1 Reflections and Reflection Groups (see attachment) we read:

" We can define reflections either with respect to hyperplanes or vectors. First of all, given a hyperplane $H \subset E$ through the origin, let L = the line through the origin that is orthogonal to H. So $E = H \oplus L$"

My question is why/how is $E = H \oplus L$???

Can anyone help?

(see my intuitive diagrams - my notion of $H \oplus L$ is a line going through a plane)

Maybe I need to read some basic differential geometry?

Peter

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• ###### Direct Sum of H and L.pdf
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Last edited: Mar 30, 2012
2. Mar 30, 2012

### DonAntonio

....

3. Mar 30, 2012

### HallsofIvy

If H is a "hyperplane" in space E through the origin, E having dimension n, then H is a subspace of E of dimension n-1. There exist a basis for H consisting of n- 1vectors which can be extended to a basis for E by adding one more vector perpendicular to all the n-1 basis vectors in H. The space spanned by that one vector is L. every vector in E can then be written as a linear combination of the basis vectors for H, and so is in H, and a vector in L. That is essentially what "E= H⊕L" means.

4. Mar 30, 2012

### Math Amateur

Thanks for the help!