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$\int_E \liminf _{n \rightarrow \infty} f_n \leq \int_E \liminf _{n \rightarrow \infty} f_n$

This problem appears in a textbook. Is this a typo?

Also, I don't know whether the sequence of functions converges, or whether it is dominated by a function. How can this be solved?

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  • $\begingroup$ Look up "Fatou's inequality". $\endgroup$ – E. Joseph Nov 20 '16 at 14:42
  • $\begingroup$ Yes, but don't I need the sequence to conv. ptwise a.e. to use Fatou's lemma? $\endgroup$ – Ninosław Brzostowiecki Nov 20 '16 at 14:50
  • $\begingroup$ No, the limit inf is always well defined. $\endgroup$ – E. Joseph Nov 20 '16 at 14:54
  • $\begingroup$ Both sides of the inequality are the same. $\endgroup$ – PhoemueX Nov 21 '16 at 21:49
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You can define $g_n=inf\{f_k:k\ge n\}$ then every $g_n$ is nonnegative, $\{g_n\}$ is an increasing sequence and $\lim_n g_n=\lim \inf f_n$ as n goes to infinity and then you can apply the monotone convergence theorem.

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