# Riemann integrable, function

• Apr 15th 2009, 07:27 PM
diondex223
Riemann integrable, function
Suppose that $f:[a,b] \rightarrow \mathbb{R}$ is a Riemann integrable on $[a,b]$ and $f(x) \geq 0$ for all $x \in [a,b]$. Prove that $\sqrt{f}$ is Riemann integrable on $[a,b]$.
Relevant Theorems & Definitions
Definition - Riemann integrable - if upper integral of $f(x)dx$= lower integral of $f(x)dx$.
Theorem - Riemann integrable iff $\exists A \in \mathbb{R}$ such that $\forall \epsilon >0 \exists$ a partition $P$ of $[a,b]$ such that $\forall$ marked refinements $Q$ of $P$, $|S(Q, f) - A | \leq \epsilon$, where $A=\int^a_b f(x)dx$.
Theorem - Let $f: [a,b] \rightarrow \mathbb{R}$ be a continuous function on $[a,b]$. Then $f$ is Riemann integrable on $[a,b]$.
etc.
Theorem - Riemann integrable iff for each $\epsilon > 0 \exists$ a partition $P$ of $[a,b]$ such that $U(P,f)-L(P,f) \leq \epsilon$.

I haven't gotten very far doing this problem. I need help with this problem. Thank you.
• Apr 23rd 2009, 09:09 AM
putnam120
This can be more "easily" solved if you work with the following.
Suppose $f$ is Riemann integrable, and $g$ continuous, on $[a,b]$. Then $g\circ f$ is Riemann integrable on $[a,b]$.

Hint to proof: First consider a partition $P$ such that $U(P.f)-L(P.f)<\delta^2$. Then make two sets as follows; $A:=\{i:M_i-m_i<\delta\}$ and $B:=A^c$. You choose $\delta$ needed for the definition of continuity for $g$. Also since $f$ is Riemann integrable, it is bounded on this interval (you will need this).