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Let \(f : [a, b] \to \mathbf{R}\) be a continuous function. Then which of the following statements is not true for the image set of \(f\), \(I_{m} f\) ?

  • A.\(I_{m} f\) is unbounded
  • B.\(I_{m} f\) has maximum
  • C.\(I_{m} f\) is an interval
  • D.\(I_{m} f\) has minimum

Answer: A

The statement that is not true is \(I_{m} f\) is unbounded.

[a, b] is compact and connected, and a continuous image of a compact connected set is compact and connected. In \(\mathbf{R}\) that means a closed bounded interval \([m, M]\), where \(m\) and \(M\) are the minimum and maximum of \(f\) (extreme value theorem).

So the image is an interval (intermediate value theorem), it has a maximum and a minimum, and it is bounded. Only the claim of unboundedness fails.

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