Midterm 1 Practice Problems

The questions below are intended to give you a rough idea of what you can expect on the midterm. The actual midterm will not be of the same length, nor will it have exactly the same types of questions.

Problem 1

Using the division algorithm, compute the quotient and remainder of division by \(f\) by \(g\).

  1. \(f=2x^4-3x^2-3,\ g=x-3\).
  2. \(f=x^3+4x^2-2x+1,\ g=2x^2-2\).

Problem 2

Compute the following greatest common divisors.

  1. \(\gcd(x^4-2x+1,x^2+2x-3)\).
  2. \(\gcd(x^3-3x-2, x^3-x^2-x+1)\).
  3. \(\gcd(x^6-1,x^4-1,x^3-1)\).

Problem 3

Reorder each of the following polynomials using the lex, grlex, and grevlex monomial orders.

  1. \(x^3-4x+x^4-3x^7\).
  2. \(x^2y + 4xyz^2 + 2x^2yz^2 + x^3yz + 4\).

Problem 4

Determine whether the polynomial \(f\) is an element of the ideal \(I\).

  1. \(f = x^3-8,\ I=\langle x-2 \rangle\).
  2. \(f = x^3-4x^2+x+6,\ I=\langle x^2-5x+6 \rangle\).
  3. \(f = x^2,\ I=\langle x^3-x, x^2+4x \rangle\).
  4. \(f = x,\ I=\langle x^3-x^2, x^4+4x^3 \rangle\).
  5. \(f = x^3-7x+6,\ I=\langle x^2-5x+6, x^2-4 \rangle\).
  6. \(f = x^3-3x^2-x+3,\ I=\langle x^2-5x+6, x^2-4 \rangle\).

Problem 5

Compute the remainder of \(f\) on division by the ordered set \(F\) using the division algorithm with the given monomial order.

  1. \(f=4x^3 - 2xy + y^2,\ F=(x^2-y,xy-y^3)\) using lex.
  2. \(f=3x^4-2xy^2+x^8,\ F=(x^3-3xy,y^2-2x)\) using grlex.

Problem 6

Find examples of each of the following. Justify your answers.

  1. Two polynomials \(f,g \in \mathbb{R}[x]\) such that \(f \ne g\), but \(\langle f \rangle = \langle g \rangle\).
  2. Two ideals \(I,J \subseteq \mathbb{R}[x]\) such that \(\mathbf{V}(I) = \mathbf{V}(J)\) but \(I \ne J\).
  3. An ideal \(I \subseteq \mathbb{C}[x]\) such that \(\mathbf{V}(I) = \{0\}\) but \(I \ne \langle x \rangle\).
  4. A polynomial \(f \in \mathbb{R}[x]\) such that \(\mathbf{V}(f)\) consists of exactly three distinct points.
  5. A polynomial \(f \in \mathbb{R}[x]\) such that \(\mathbf{V}(f)\) consists of exactly three distinct points, but \(f\) has total degree \(4\).
  6. An ideal \(I \subseteq \mathbb{R}[x,y,z]\) such that \(\mathbf{V}(I)\) is infinite.
  7. An ideal \(I \subseteq \mathbb{R}[x,y,z]\) such that \(\mathbf{V}(I)\) is finite.
  8. A sequence of polynomials \(f_1,\ldots,f_s\) in \(x,y\) such that \(\mathbf{V}(f_1, \ldots, f_s)\) is a non-empty finite set when the polynomials are considered as elements of \(\mathbb{R}[x,y]\), but \(\mathbf{V}(f_1, \ldots, f_s)\) is infinite when the polynomials are regarded as elements of \(\mathbb{C}[x,y]\).
  9. A polynomial \(f\) in \(\mathbb{Q}[x,y,z]\) such that \(\operatorname{LT}(f)\) is different for each of the monomial orders lex, grlex, and grevlex.