14-1100/Homework Assignment 4

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This assignment is due at class time on Thursday, November 20, 2011.

Solve the following questions

Problem 1. (Klein's 1983 course)

  1. Show that the ideal [math]\displaystyle{ I=\langle 3,\, x^3-x^2+2x-1\rangle }[/math] inside the ring [math]\displaystyle{ {\mathbb Z}[x] }[/math] is not principal.
  2. Is [math]\displaystyle{ {\mathbb Z}[x]/I }[/math] a domain?

Problem 2. Prove that a ring [math]\displaystyle{ R }[/math] is a PID iff it is a UFD in which [math]\displaystyle{ \gcd(a,b)\in\langle a, b\rangle }[/math] for every non-zero [math]\displaystyle{ a,b\in R }[/math].

Problem 3. (Lang) Show that the ring [math]\displaystyle{ {\mathbb Z}[i]=\{a+ib\colon a,b\in{\mathbb Z}\}\subset{\mathbb C} }[/math] is a PID and hence a UFD. What are the units of that ring?

Problem 4. (Dummit and Foote) In [math]\displaystyle{ {\mathbb Z}[i] }[/math], find the greatest common divisor of [math]\displaystyle{ 85 }[/math] and [math]\displaystyle{ 1+13i }[/math], and express it as a linear combination of these two elements.

Problem 5. (Klein's 1983 course) Show that [math]\displaystyle{ {\mathbb Z}[\sqrt{10}] }[/math] is not a UFD.

Problem 6. (Hard!) Show that the quotient ring [math]\displaystyle{ {\mathbb Q}[x,y]/\langle x^2+y^2-1\rangle }[/math] is not a UFD.