Math 417: Abstract Algebra Homework 5
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Homework 5
Math 417: Abstract Algebra
(Exercises are taken from Algebra: Abstract and Concrete, Edition 2.6 by Frederick M. Goodman.)
1. Exercise §2.4.8 Let ϕ : G → H be a homomorphism of G onto H (that is, ϕ is surjective). If A is a normal subgroup of G, show that ϕ(A) is a normal subgroup of H.
2. Exercise §2.5.8 Suppose N is a subgroup of a group G and [G : N] = 2. Show that N is normal in G. (Hint: use the fact that a subgroup is normal iff every left coset is also a right coset.)
3. Let D be the symmetry group of the disk, as described in class and in Goodman 2 .3. Show that there is a function φ : D → D such that φ(Tθ) = T2θ and φ(jθ) = j2θ (this means: show that φ is well-defined), and that this function φ is a homomorphism of groups. Also describe the kernel of φ .
The following exercise sets up an example which will appear in future problem sets. Here A will be a commutative ring with identity (examples: Z, Q, R, C, Zn.) I’ll write
C(A) :={(x, g) | x, g ∈ A,x2 + g2 = 1} .
For instance, C(R) is the unit circle in R2 .
4. Given (x1, g1 ) , (x2, g2 ) ∈ C(A), define
(x1, g1 ) ⊕ (x2, g2 ) := (x1x2 − g1g2,x1g2 + g1x2) .
Show that this always takes values in C(A), and that (C(A), ⊕) is an abelian group.
5. Show that φ(t) := (cost,sint) defines a homomorphism φ : (R, +) → (C(R), ⊕), Show that this homomorphism is surjective and determine its kernel.
2023-08-18