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This is a previous year question from a competitive examination:


If $f:[0,\infty) \to [0, \infty)$ is a continuous function such that $\int_{0}^\infty f(x)dx < \infty$, then which of the following are true:

  1. $\{f_n\}$ is a bounded sequence.
  2. $f(n) \to 0 $ as $n \to \infty$.
  3. $\sum_{n=1}^\infty f(n)$ is convergent.

The answer given is None of the above.

I tried constructing some examples but none of them worked. Any ideas/suggestions are highly appreciated.

Kashif
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    This is asked way too often. (Change the first to ${f(n)}$ is a bounded sequence please.) Here is another duplicate to view while this one gets shut down: https://math.stackexchange.com/questions/4425712/integrability-of-f-on-bbb-r-does-not-necessarily-imply-the-convergence-of/4425776#4425776 – B. S. Thomson Apr 25 '22 at 16:14
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    Consider a function which is zero everywhere except with triangular “spikes” at integer values. By making these spikes increasingly narrow and tall, you can get a counterexample to all three of these. – Mark Saving Apr 25 '22 at 16:22

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