Math 817: Problem set 4
Instructions: Write up any four of the following problems (but
you should try all of them, and
in the end you should be sure you know how to do all of them).
Your write ups are due Friday, September 24, 2004.
Each problem is worth 10 points, 9 points for correctness
and 1 point for communication. (Your goal is not only
to give correct answers but to communicate
your ideas well. Make sure you use good English,
so proofread your solutions.
Once you finish a solution, you
should restructure awkward sentences, and strike out
anything that is not needed in your approach to the problem.)
- Let H and N be subgroups of a group G, with N normal in G.
Let I be the intersection of H and N. You may assume without proof
the fact that I is normal in H and N is normal in HN
(but of course you should be able to prove this fact
if you need to!). The Second Isomorphism Theorem states that:
H/I is isomorphic to HN/N. Prove the Second Isomorphism Theorem.
[Hint: Define an appropriate homomorphism F: H -> HN/N and apply
the First Isomorphism Theorem.]
- Let K and N be normal subgroups of a group G, with
N contained in K. Use the FFF to show that K/N is a normal
subgroup of G/N, and prove the Third Isomorphism Theorem
(AKA the Freshman's Dream), that
(G/N)/(K/N) is isomorphic to G/K. [Hint: Apply the FFF and the
First Isomorphism Theorem.]
- Let g: Z -> G and h: Z -> H be surjective
homomorphisms of groups, where Z denotes the integers.
Let |G| = m and |H| = n. Let F : Z -> GxH denote
the homomorphism whose components are g and h.
- (a) Find ker F and |Im(F)|. Justify your answers.
- (b) If gcd(m, n) = 1, show that Z/mnZ is
isomorphic to GxH. [Hint: apply (a), using the First isomorphism Theorem.]
- (c) Given positive integers m and n such that
gcd(m, n) = 1 and arbitrary integers a and b,
prove the Chinese Remainder Theorem (CRT), that
there is an integer c which is a simultaneous solution x = c
to x = a mod m and x = b mod n (I'm using "=" since the usual
triple equals is not really available in html), and that the set
of all solutions is precisely the set of integers d
such that mn | c - d. [Hint: Restate the CRT in terms of F.]
- Let m and n be relatively prime positive integers.
Given any integer i, let [i]m denote the
coset i+mZ in Z (thus [i]m
is just the congruence class of i modulo m).
- (a) You may assume that the map
g: Z/mnZ -> Z/mZ x Z/nZ
defined by g([i]mn) = ([i]m, [i]n)
is an isomorphism. What is the inverse? (I.e., give a formula for where
([i]m, [j]n) must map to, given
arbitrary integers i and j. You do not need to give proof, but be sure
your answer really is the inverse.)
- (b) You may assume that the map
h: Z/mZ x Z/nZ -> Z/mnZ
defined by h(([i]m, [j]n)) = [ni+mj]mn
is an isomorphism. What is the inverse? (I.e., give a formula for where
[i]mn must map to, given an
arbitrary integer i. You do not need to give proof, but be sure
your answer really is the inverse.)
- This is #8, p. 75: if G is a group with normal subgroups
of orders 3 and 5, prove that G has an element of order 15.
- (This is essentially #10.1, p. 76.)
Let G be the group of invertible real upper triangular 2x2 matrices.
For each of the following conditions, determine if it defines
a normal subgroup H of G (it is enough to just write "Yes" or "No"
without justification), and if so, determine G/H (it is enough
without justification to
define an explicit surjective homomorphism G -> K to some group K
such that the kernel is H; by the First Isomorphism Theorem, this
would show that G/H is isomorphic to K, just be sure that
your map really is an isomorphism with the required kernel!).
(a) a11 = 1 |
(b) a12 = 0 |
(c) a11 = a22 |
(d) a11 = a22 = 1 |