# Problem

Let $$f_n:[0,1] \to \mathbb{R}$$ be Lebesgue measurable functions. Suppose they converge point-wise to a monotone function $$f:[0,1] \to \mathbb{R}$$. Prove that $$f$$ is Riemann integrable.

# Attempt

My first thought is to use Lebesgue-vitali theorem, but I don't have anything about boundedness or continuity. I'm wondering if maybe I need to do it directly showing the upper and lower sum difference converges.

Thanks!

• Can you use the Lebesgue theorem that states that monotone functions on closed, bounded interval are differentiable almost everywhere, and hence continuous almost everywhere, which proves Riemann integrability? – Sawyer R. Jan 2 '18 at 20:08
• That would have worked yes. – RhythmInk Jan 2 '18 at 20:10
• The problem is odd because the hypotheses are unnecessary. Any monotone function is Riemann integrable. – AlohaSine Jan 2 '18 at 20:12
• Agreed with mathstudent, the sequence is irrelevant. – Sawyer R. Jan 2 '18 at 20:13

## 2 Answers

Since $f$ is monotonic and the domain is a closed and bounded interval, $f$ must be bounded. And the set of points at which it is discontinuous is countable, and therefore its Lebesgue measure is $0$. So, $f$ is Riemann-integrable.

• Why is that set countable? – RhythmInk Jan 2 '18 at 20:10
• @az89 Because a monotone function can only have jump discontinuities. And it is easy to see that (since it is bounded), for each natural $n$ there can be only be finitely many points $a\in[0,1]$ such that the jump (that is $\lim_{x\to a^+}f(x)-\lim_{x\to a^-}f(x)$) is geater than or equal to $\frac1n$. – José Carlos Santos Jan 2 '18 at 20:13
• @SawyerR. Are you sure that it is false for monotone functions? – José Carlos Santos Jan 2 '18 at 20:13
• The set of discontinuities of a monotone function from an interval to $\mathbb{R}$ is definitely countable. – Ian Jan 2 '18 at 20:14
• I was definitely wrong, I recall the proof now. I was getting that fact confused with riemann integrable iff set of discontinuities has measure zero. – Sawyer R. Jan 2 '18 at 20:15

Here's an elementary proof that every monotone function on $$[0,1]$$ is Riemann integrable. Suppose $$f$$ is nondecreasing on $$[0,1].$$ Consider the uniform partition $$P_n$$ of $$[0,1]$$ with $$n$$ subintervals. Then

$$U(f,P_n) = \sum_{k=1}^{n}f\left(\frac{k}{n}\right)\frac{1}{n},\,\,\, L(f,P_n) = \sum_{k=1}^{n}f\left(\frac{(k-1)}{n}\right)\frac{1}{n}.$$

Thus

$$U(f,P_n)-L(f,P_n) = f(1)\frac{1}{n}-f(0)\frac{1}{n}=\frac{f(1)-f(0)}{n}.$$

Therefore $$U(f,P_n)-L(f,P_n)\to 0$$ as $$n\to \infty,$$ hence $$f$$ is Riemann integrable on $$[0,1].$$