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Let \(f : [0, \infty) \to \mathbb{R}\) be defined by

\[ f(x) = \int_{0}^{x} \sin^{2}(t^{2})\, dt. \]

Then the function \(f\) is

  • A.uniformly continuous on \([0, 1)\) but NOT on \((0, \infty)\)
  • B.uniformly continuous on \((0, \infty)\) but NOT on \([0, 1)\)
  • C.uniformly continuous on both \([0, 1)\) and \((0, \infty)\)
  • D.neither uniformly continuous on \([0, 1)\) nor uniformly continuous on \((0, \infty)\)

Answer: C

\(f\) is uniformly continuous on both \([0, 1)\) and \((0, \infty)\).

By the fundamental theorem of calculus, \(f'(x) = \sin^{2}(x^{2})\), so \(0 \le f'(x) \le 1\) for all \(x \ge 0\). By the mean value theorem,

\[ |f(x) - f(y)| \le |x - y| \]

so \(f\) is Lipschitz with constant 1 on the whole of \([0, \infty)\), and therefore uniformly continuous on every subset of it, including both \([0,1)\) and \((0, \infty)\).

The options denying uniform continuity on either set are ruled out by this bound; the growth of \(x^{2}\) inside the sine does not matter because the derivative stays bounded.

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