Let f be a bounded function on [a, b]. If P and Q are partitions of [a, b], prove that L( f , P) ≤ U( f , Q).

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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 Let f be a bounded function on [a, b]. If P and Q are partitions of [a, b], prove that
L( f , P) ≤ U( f , Q). 

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Step 1

Given f be a bounded function on a,b. If P and Q are the partitions of a,b. Then

We have to prove that Lf,PUf,Q.

Let P=x0,x1,x2,.......,xn

Where, a=x0<x1<x2<.......<xn=b.

Since, f is bounded on a,b, hence, f is bounded on xr-1,xr where, r=1,2,3.....,n

Let M=supxxr-1,xrfx and m=infxxr-1,xrfx for r=1,2,3.....,n.

Then, Uf,P and Lf,P is defined as follows

Uf,P=r=1nMrxrxr1 and Lf,P=r=1nmrxrxr1.

We prove that 

infxxr1,xrfx=supxxr1,xrfx

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