Translationlength by the coxeterelement of E9 along the coxeteraxis

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In summary, the conversation discusses the length of translation by the coxeter element of E9, which is the affine one point extension of E8. The participants also mention a graph and a table for highest weights in terms of a root system for classical and exceptional groups. Finally, one of them finds an answer stating that the translation length is √2.
  • #1
reinhard55
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I have searched on Google but i cannot find it.
Does anyone know how long is the translation by the coxerterelement of E9
(which is the affine one point extension of E8) along the coxeteraxis?
 
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  • #3
fresh_42 said:
I am not quite sure which number you are looking for. Unfortunately I have no books which especially deal with ##E_9##, but the closest I came (K.S. Brown, Buildings) let's me ask: Do you have any doubt that the matrix entry ##m_{89}## of the Coxeter matrix is not ##3\,?##

Maybe https://ncatlab.org/nlab/show/E9 or the links on https://ncatlab.org/nlab/show/Kac-Moody+group can help you.
Maybe it is more clear with a picture.
The picture shows the affine coxetergroup of A2.
The darkgray chamber makes a reflection and a translation by the coxeterelement of affine A2.

My question now is how long is the translationlength by the coxeterelement of E9 (affine E8)?

Coxeter-complex-of-typeAtype-typeA-2.png
 
  • #4
We would need the graph for ##E_9##, not the simple one of ##A_2##. But as the ninth root is basically analogue to the situation of ##A_n## I assume it is ##\sqrt{3}## as well.
 
  • #5
I thought that there already is a result for it but maybe not.I will try to calculate it on myself.
Thanks.
 
  • #6
reinhard55 said:
I thought that there already is a result for it but maybe not.I will try to calculate it on myself.
Thanks.
I found a table for highest weights in terms of a root system for the classical and exceptional groups, but as I said, nothing about ##E_9##. But per construction, it should look very similar to the differences between ##E_6 \to E_7## and ##E_7 \to E_8##. So once you have those, you should be able to see what changed. As said, the entry of the Coxeter matrix should be ##m_{89}=3##.
 
  • #7
I think it must be possible by the rootsystem of E9.
Thanks.
 
  • #8
fresh_42 said:
We would need the graph for ##E_9##, not the simple one of ##A_2##. But as the ninth root is basically analogue to the situation of ##A_n## I assume it is ##\sqrt{3}## as well.
As i found out it cannot be ##\sqrt{3}## because the lattice of E8 have only half integers and integers coordinates.
 
  • #9
I have found a paper about ##E_{10}##. Maybe it helps (##E_9## on page 6)
https://core.ac.uk/download/pdf/25173425.pdf
 
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  • #10
fresh_42 said:
I have found a paper about ##E_{10}##. Maybe it helps (##E_9## on page 6)
https://core.ac.uk/download/pdf/25173425.pdf
I think i have found the answer.The translation is √2.
 
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1. What is the coxeterelement of E9?

The coxeterelement of E9 is a mathematical concept used in the study of Coxeter groups, which are groups generated by reflections in a finite set of hyperplanes.

2. How is the translationlength calculated?

The translationlength of the coxeterelement of E9 along the coxeteraxis is calculated by measuring the distance between the starting and ending points of the translation, which is determined by the action of the coxeterelement on a given point.

3. What is the significance of the coxeteraxis in this context?

The coxeteraxis is a line in the space where the coxeterelement of E9 acts. It plays a crucial role in determining the translationlength of the coxeterelement, as the translationlength is measured along this axis.

4. How does the translationlength affect the coxeterelement of E9?

The translationlength affects the coxeterelement of E9 by determining the amount of translation or displacement that occurs when the element is applied to a point. A longer translationlength means a greater displacement along the coxeteraxis.

5. Can the translationlength be negative?

Yes, the translationlength can be negative in certain cases. This occurs when the coxeterelement of E9 acts on a point in the opposite direction of the coxeteraxis, resulting in a negative displacement along the axis.

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