Math 361, Spring 2022, Assignment 3

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Revision as of 00:59, 13 February 2022 by Jingwen.feng001 (talk | contribs) (Definitions:)


Read:

  1. Section 19.

Carefully define the following terms, then give one example and one non-example of each:

  1. Zero-divisor.
  2. Integral domain.
  3. Field.
  4. Subring.
  5. Zero (a.k.a. trivial) subring.
  6. Improper subring.
  7. Subring generated by a subset.
  8. Prime subring (of a unital ring).

Carefully state the following theorems (you do not need to prove them):

  1. Zero-product property (of integral domains).
  2. Cancellation law (in integral domains).
  3. Theorem relating fields to integral domains.
  4. Theorem characterizing the units and zero-divisors of Zn.
  5. Theorem characterizing when Zn is a field, and when it is an integral domain.

Solve the following problems:

  1. 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).
  2. Describe the prime subrings of Q, of R, and of C.
  3. Describe the prime subring of Z.
  4. Describe the prime subring of Zn.
  5. Working in the field Z3, solve the equation x3=x.
  6. Working in the field Z5, solve the equation x5=x.
  7. Working in the field Z7, solve the equation x7=x.
  8. 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 gG=e for every gG. 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.)
  9. 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?
  10. 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.
--------------------End of assignment--------------------

Questions:

Solutions:

Definitions:

  1. Zero-divisor: Let R be a ring, and aR. 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 $ab = 0$.

  1. Integral domain.
  2. Field.
  3. Subring.
  4. Zero (a.k.a. trivial) subring.
  5. Improper subring.
  6. Subring generated by a subset.
  7. Prime subring (of a unital ring).

Theorems:

Problems: