# Math Help - Exam Prep Help

1. ## Exam Prep Help

For the dihedral group( $D_4 \mbox{or} \ D_8$) with $\sigma = \begin{pmatrix} 1&2&3&4\\2&3&4&1 \end{pmatrix} \ \mbox{and} \ \tau = \begin{pmatrix} 1&2&3&4\\2&1&4&3 \end{pmatrix} \mbox{and}$ $\ D_4 =${ ${1 , \sigma , \sigma ^{2}, \sigma ^{3}, \tau , \sigma \tau , \sigma ^{2} \tau , \sigma ^{3} \tau }$}

(3) Find $< \sigma^{2} , \tau > \ \mbox{and} \ <\sigma^{3} , \sigma \tau >$?

(4) What are the left cosets of $<\tau>, <\sigma^{2}>, \mbox{and} \ <\sigma^{2}, \tau>$?

(5) Prove if $<\tau> \ \mbox{and/or} \ <\sigma^{2}>$ is normal.

(6) Construct the Cayley table of $g/<\tau>$

I've tried really hard to work these questions out. I worked out the first two questions (from the attached image above) but I'm having trouble with the four posted here. I really need to be shown how to do these otherwise I wont get it. Ive been working on these for about 6 hours and Im not getting anywhere.

http://en.wikipedia.org/wiki/Examples_of_groups

$D_8 \mbox{ or } D_4$ is the example you are looking for

3. I've finished Q1's and 2 now, but I'm completely stuck on questions (3) - (6). My mid-term is tomorrow and I think I'll really need to know how to do this.
Thanks to anyone who can help.