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Which of the following is a PID ?

  • A.\(\mathbf{Z}[x]\)
  • B.\(\mathbf{R}(x)\ [y]\)
  • C.\(\mathbf{C}[x, y]\)
  • D.\(\mathbf{Z}_{30}\)

Answer: B

The PID is \(\mathbf{R}(x)[y]\).

\(\mathbf{R}(x)\) is the field of rational functions in \(x\), and a polynomial ring in one variable over a field is a Euclidean domain, hence a principal ideal domain.

The others are not. In \(\mathbf{Z}[x]\) the ideal \((2, x)\) is not principal. In \(\mathbf{C}[x, y]\) the ideal \((x, y)\) is not principal. And \(\mathbf{Z}_{30}\) is not an integral domain at all, since \(2 \cdot 15 = 0\), so it cannot be a PID even though all its ideals are principal.

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