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Worksheet Problem Set 4

Instructions: You may type up or handwrite your work, but it must be neat, professional, and organized and it must be saved as a PDF file and uploaded to the appropriate Gradescope assignment. Use a scanner or scanning app to convert handwritten work on paper to PDF. I encourage you to type your work using the provided template.
All tasks below must have a complete solution that represents a good-faith attempt at being right to receive engagement credits. If your submission is complete and turned in on time, you will receive full engagement credits for the assignment. All other submissions will receive zero engagement credit. Read the guidelines at Grading Specifications carefully.
To abide by the class’ academic honesty policy, your work should represent your own understanding in your own words. If you work with other students, you must clearly indicate who you worked with in your submission. The same is true for using tools like generative AI although I strongly discourage you from using such tools since you need to build your own understanding here to do well on exams.

Problems Doable After Week 9.

1. Preimages of Ideals Under Surjections.

Suppose that \(R\) and \(S\) are commutative rings with identities, let \(\phi:R\to S\) be an onto ring homomorphism, and let \(I\) be an ideal of \(S\text{.}\)
(a)
If \(I\) is prime in \(S\text{,}\) show that \(\phi^{-1}(I)\) is prime in \(R\text{.}\)
(b)
If \(I\) is maximal in \(S\text{,}\) show that \(\phi^{-1}(I)\) is maximal in \(R\text{.}\)
Problem Specs/Notes: Be careful to show that all of the parts of the definition of prime and maximal ideals are satisfied. You do not need to show that the preimage of an ideal is an ideal, since we proved that already.

2. Ring Automorphisms of \(\Q[\sqrt{2}]\).

Let \(\Q[\sqrt{2}] =\{r+s\sqrt{2} \mid r,s\in \Q\}\text{.}\) Determine all ring automorphisms of \(\Q[\sqrt{2}]\text{.}\)
Problem Specs/Notes: It may be helpful to use a similar argument as in the exercises of Daily Prep 16 to start your solution.

3. Nonconstant Polynomials over \(\Z\) do not Generate Maximal Ideals.

Let \(f\) be a nonconstant polynomial in \(\Z[x]\text{.}\) Prove that \(\langle f\rangle\) is not maximal in \(\Z[x]\text{.}\)
Problem Specs/Notes: You may either carefully construct an ideal properly containing \(\ideal{f}\) which is not all of \(\Z[x]\text{,}\) justifying both claims, or you may use the First Isomorphism theorem and argue that \(\Z[x]/\ideal{f}\) is not a field.

4. Chinese Remainder Theorem for Rings.

If \(R\) is a commutative ring and \(I\) and \(J\) are two proper ideals with \(I+J=R\text{,}\) prove that \(R/(I\cap J)\) is isomorphic to \(R/I\times R/J\text{.}\)
Problem Specs/Notes: Be careful in doing your computation for surjectivity and notice that we are not assuming \(R\) has a \(1\) here.

Problems Doable After Week 10.

5. Rational Root Theorem.

Let
\begin{equation*} f(x)=a_nx^n+a_{n-1}x^{n-1}+\ldots+a_0 \in\Z[x] \end{equation*}
and \(a_n \neq 0\text{.}\) Prove that if \(r\) and \(s\) are relatively prime integers and \(f(r/s)=0\text{,}\) then \(r\mid a_0\) and \(s\mid a_n\text{.}\)
Problem Specs/Notes: You will want to use the other theorems we have proven about irreducibility to help you here.

6. Primes and Irreducibles Extended.

For a commutative ring with unity, we may define associates, irreducibles, and primes exactly as we did for integral domains. With these definitions, show the following.
(a)
Let \(n\) be a positive integer and \(p\) a prime that divides \(n\text{.}\) Prove that \(p\) is prime in \(\Z_n\text{.}\)
(b)
Let \(n\) be a positive integer and \(p\) be a prime that divides \(n\text{.}\) Prove that \(p\) is irreducible in \(\Z_n\) if and only if \(p^2\) divides \(n\text{.}\)
(c)
Prove or disprove that in a commutative ring with unity, prime implies irreducible.
Problem Specs/Notes: Do your computations mod \(n\) carefully for part (b).

7. Ascending Chain Condition.

An ideal \(A\) of a commutative ring \(R\) with identity 1 is said to be finitely generated if there exist elements \(a_1,a_2,\ldots, a_n\) of \(A\) such that \(A=\langle a_1,a_2,\ldots, a_n\rangle\text{.}\) An integral domain \(R\) is said to satisfy the ascending chain condition (ACC), if every strictly increasing chain of ideals \(I_1\subset I_2\subset \ldots\) must be finite in length. Show that an integral domain \(R\) satisfies the ascending chain condition if and only if every ideal of \(R\) is finitely generated.
Rings satisfying either of these equivalent conditions are called Noetherian.
Problem Specs/Notes: Proving the reverse direction may be easier to do by contrapositive.