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Which one of the following statements is TRUE?

  • A.The set of all \(2 \times 2\) real matrices under usual matrix addition and multiplication is a Unique Factorization Domain.
  • B.The set of all real valued functions under usual function addition and multiplication is a Unique Factorization Domain.
  • C.The set of all polynomials with coefficients in \(\mathbb{R}\) under usual polynomial addition and multiplication is a Unique Factorization Domain.
  • D.The set of Riemann integrable functions in [0,1] under usual function addition and multiplication is a Unique Factorization Domain.

Answer: C

The true statement is that the polynomials with real coefficients form a Unique Factorization Domain.

\(\mathbb{R}[x]\) is a polynomial ring over a field, hence a Euclidean domain (division with remainder by degree), hence a principal ideal domain and so a UFD. Every nonzero non-unit factors uniquely into irreducibles, which over \(\mathbb{R}\) are linear and irreducible quadratic factors.

The other rings are not even integral domains. \(2 \times 2\) real matrices are not commutative and have zero divisors. Real valued functions on a set with two or more points have zero divisors: a function vanishing on one half times one vanishing on the other is 0. The same construction works for Riemann integrable functions on [0,1].

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