Math 361, Spring 2022, Assignment 3
From cartan.math.umb.edu
Read:
- Section 19.
Carefully define the following terms, then give one example and one non-example of each:
- Zero-divisor.
- Integral domain.
- Field.
- Subring.
- Zero (a.k.a. trivial) subring.
- Improper subring.
- Subring generated by a subset.
- Prime subring (of a unital ring).
Carefully state the following theorems (you do not need to prove them):
- Zero-product property (of integral domains).
- Cancellation law (in integral domains).
- Theorem relating fields to integral domains.
- Theorem characterizing the units and zero-divisors of Zn.
- Theorem characterizing when Zn is a field, and when it is an integral domain.
Solve the following problems:
- Section 19, problems 1, 2, 3, 4, and 14 (hint for 14: to show that this matrix is a left zero-divisor, find another matrix whose image in contained in the kernel of this one; to show that it is a right zero-divisor, find another matrix whose kernel contains the image of this one).
- Describe the prime subrings of Q, of R, and of C.
- Describe the prime subring of Z.
- Describe the prime subring of Zn.
- Working in the field Z3, solve the equation x3=x.
- Working in the field Z5, solve the equation x5=x.
- Working in the field Z7, solve the equation x7=x.
- By now you probably have a conjecture about Z11. Do not try to prove this. Instead, prove the conjecture for Zp where p is an arbitrary prime. (Hint: the conjecture is obviously true if x=0. Otherwise x is an element of the group of units of Zp (why?). But as we have seen, Lagrange's Theorem implies that in any group G we have g∣G∣=e for every g∈G. This gives rise to a certain identity for non-zero elements of Zp. Multiplying both sides of this identity by x will prove the conjecture.)
- Show by a simple counterexample (e.g. in Z6) that the result above is not generally true in Zn when n is composite. Exactly which part of your proof above breaks in the composite case?
- Try to correctly generalize the conjecture to the composite case (i.e. formulate and prove a statement which encompasses the prime case but is also true in the composite case). In doing this you will be following in the footsteps of Leonhard Euler; this result (like many others) is known as Euler's Theorem, and it is in fact the mathematical basis of RSA encryption.
Questions:
Solutions:
Definitions:
- Zero-divisor: Let R be a ring, and a∈R. We say that a is a \textbf{left zero-divisor} (may not be commutative) if
\begin{enumerate} \item $a \neq 0$, and \item $\exists b \in R, $ with $b\neq 0$ but $ab = 0$.
- Integral domain.
- Field.
- Subring.
- Zero (a.k.a. trivial) subring.
- Improper subring.
- Subring generated by a subset.
- Prime subring (of a unital ring).