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What is an example of a continuous function on a closed and unbounded set with no maximum? Is $f(x)=x^3$ a correct example?

  • 2
    $f(x)=x$ is perhaps a simpler example.2012-11-21

2 Answers 2

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Yes, your example $f:\mathbb R\to\mathbb R,f(x)=x^3$ is correct, since $\mathbb R$ is closed in $\mathbb R$ and unbounded and $f\to +\infty$ as $x\to +\infty$.

  • 0
    another good, concise answer2012-11-22
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Your example is correct. Simpler might be $f(x) = x$ and maybe more interesting $f(x) = x^{37} + x^{10} +1$, but not $f(x) = - x^2$ (why?)

Some other examples:

  • $f(x) = -e^{x}$: bounded above by $0$ but has no maximum.
  • $f(x) = -e^{-x^2}$: bounded above, assumes its minimum $-1$ at $0$ is bounded above by $0$ but has no maximum.
  • $f(x) = \arctan{x}$: bounded above and below by $\pm \frac{\pi}{2}$ but has neither maximum nor a minimum.

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