Let $1\leq p < \infty$. Suppose that

- $\{f_k\} \subset L^p$ (the domain here does not necessarily have to be finite),
- $f_k \to f$ almost everywhere, and
- $\|f_k\|_{L^p} \to \|f\|_{L^p}$.
Why is it the case that $$\|f_k – f\|_{L^p} \to 0?$$

A statement in the other direction (i.e. $\|f_k – f\|_{L^p} \to 0 \Rightarrow \|f_k\|_{L^p} \to \|f\|_{L^p}$ ) follows pretty easily and is the one that I’ve seen most of the time. I’m not how to show the result above though.

**Answer**

This is a theorem by Riesz.

Observe that

$$|f_k – f|^p \leq 2^p (|f_k|^p + |f|^p),$$

Now we can apply Fatou’s lemma to

$$2^p (|f_k|^p + |f|^p) – |f_k – f|^p \geq 0.$$

If you look well enough you will notice that this implies that

$$\limsup_{k \to \infty} \int |f_k – f|^p \, d\mu = 0.$$

Hence you can conclude the same for the normal limit.

**Attribution***Source : Link , Question Author : user1736 , Answer Author : JT_NL*