I've come across this problem as a part of another proof that I'm writing and I want to know if this is a right conclusion:
Let $X$ be a finite measure space and $\{f_n\}$ be a sequence of nonnegative integrable functions.
If I know: $$\lim_{n\rightarrow \infty} \int_X f_n - \int_X f \geq \delta > 0,$$ can I conclude that $\mu\{x: f_n \nrightarrow f\} > 0$ or in other words $f_n$ doesn't convege to $f\ a.e.$?
Answer
I assume you want to conclude that $\mu(\{x: f_n \not\to f\}) > 0$. The answer is NO.
Consider $$f_n = \begin{cases} n & x \in [0,1/n)\\ 0 & \text{otherwise} \end{cases}$$ and $$f = 0$$
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