Automorphisms of the Dihedral Group D8

Dummit and Foote Section 4.4 Automorphisms, Exercise 3 reads as follows:

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Prove that under any automomorpism of , r has at most two possible images and s has at most 4 possible images. Deduce that |Aut( )| 8

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*Can anyone help with the above problem? Be grateful for help!*

The notation of D&F regarding the construction of is as follows:

Fix a square centred at the origin in an x,y plane and label the vertices from 1 to 4 in a clockweise manner. Let r be the rotation clockwise about the origin through radian. Let s be the reflection about the line of symmetry through vertex 1 and the origin.

Then

= {1, r, , , s, sr, s , s }

Peter

Re: Automorphisms of the Dihedral Group D8

Re: Automorphisms of the Dihedral Group D8

to fill in the gaps of the link Amer gave (which is a great site to look up various math proofs), note that .

if you really feel ambitious, try to prove that .

Re: Automorphisms of the Dihedral Group D8

Thanks for the link - looks really helpful!!!

Peter

Re: Automorphisms of the Dihedral Group D8

Quote:

Originally Posted by

**Bernhard** Thanks for the link - looks really helpful!!!

Peter

yea it is very it contains alot of problems from Dummit Footie Abstract Algebra