Quote (nerobellum @ Sep 15 2010 04:12pm)
For the purposes of this question, we define a Dihedral Group to be a group D = (x , y) where x and y are distinct involutions
1. Classify all abelian dihedral groups.
2. Let z = xy.
i. Show that zx = zy = z−1 and deduce that H = (z) is a normal subgroup of D.
ii. Prove that xH = yH, and deduce that G | H is cyclic of order 1 or 2.
1. x and y are distinct elements of order 2. Let z = xy which is clearly not an element of {1; x; y}.
Now G = {1; x; y; z} is closed under multiplication and z^2 = 1 so it is also closed under inversion.
Thus G is a group of size 4, and has the following multiplicative structure:
the product of any pair of dierent non-identity elements is the third non-identity element.