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# Math 212, Assignment 5

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Math 212, Assignment 5

All questions are equally weighted. They will be marked for correctness
and clarity of explanation.
1. Consider the set R
3 with componentwise addition, and with multiplication defined by
(x, y, z) · (x
0
, y0
, z0
) = (0, yy0
, 0).
Is R
3 a ring with these operations? If so, is it commutative? Does it have an
identity? If it has an identity, determine which elements are units.
2. Prove that the set
S =
a b
0 c

: a, b, c ∈ R

is a subring of M2(R) with its usual addition and multiplication. Is S a commutative ring? Does it have an identity? If so, find the identity and determine
which elements are units of S.
3. For each of the following, decide if the set S is a subring of the real numbers,
with its usual operations of addition and multiplication. If it is, give a proof,
and if not, explain why.
(a) S = {
n
4
: n ∈ Z}
(b) S = {a + b
√3
2 + c
√3
4 : a, b, c ∈ Z}.
4. TRUE or FALSE. (If true, give a proof and if false, give an explicit counterexample.)
If R is a field and S is a subring of R, then S is also a field.
5. Let R be a commutative ring with identity 1 6= 0, and suppose the cancellation
rule holds in R. That is, for all a, b, c ∈ R, with a 6= 0, if ab = ac then b = c.
Prove that if R is finite, then R is a field.
6. Prove that F = {a+b

5 : a, b ∈ Q}, with ordinary addition and multiplication,
is a field.
1
7. Let R = {i + j

5 : i, j ∈ Z}, and let Q be the field of quotients of R.
(a) Prove that if [a, b] is in Q, then there exists c in R and k ∈ Z, k 6= 0, such
that [a, b] = [c, k].
(b) Define an isomorphism from the field F of problem 6 to Q. You do not
need to prove it is an isomorphism, but use part (a) to prove it is surjective.
8. For each of the following subsets of the rational numbers, determine if the set
has (i) a maximum, (ii) an upper bound, (iii) a least upper bound. (This means
within the set of rational numbers.) In parts (i) and (iii) if the answer is yes,
name one. In part (ii) if the answer is yes, name three.
(a) {
1
2n
: n = 1, 2, 3, . . . }.
(b) {
a
b
| a, b ∈ Z, 0 < a < b}.
(c) {
n
3+5n
n2
: n = 1, 2, 3, . . . }.
(d) {
a
b
: a, b ∈ Z
+, a2 + 3ab − b
2 < 0}. (You may assume there is no rational
number r with r
2 = 13.)
Rules for group assignments. Make sure you follow the universal rules for group
Contract.
1. Each group member is expected to contribute to the best of their ability, and
assignment submissions should only include the names of group members who
meet this expectation.
2. Each group member should be able to explain the group’s solution to me and
answer any questions I may have about it. It is the whole group’s responsibility
to ensure that this standard is met.
3. The task of composing final solutions and writing them up in good copy must be
shared equally among all group members (after a collaborative problem-solving
process).
4. After good copy solutions are complete, they should be shared among all group
members to be double-checked and proofread. This should be done in advance
of the due date, to allow time for any necessary corrections. Corrections should
be completed by the person who wrote the original solution.
2 Math 212, Assignment 5
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