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First Semester in Numerical Analysis with Julia, 2020a

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CHAPTER 4. NUMERICAL QUADRATURE AND DIFFERENTIATION 174<br />

<strong>Numerical</strong> differentiation and roundoff error<br />

Arya and the mysterious black box<br />

College life is full of mysteries, and Arya<br />

faces one <strong>in</strong> an eng<strong>in</strong>eer<strong>in</strong>g class: a black<br />

box! What is a black box? It is a computer<br />

program, or some device, which produces<br />

an output when an <strong>in</strong>put is provided.<br />

We do not know the <strong>in</strong>ner work<strong>in</strong>gs of the<br />

system, hence comes the name black box.<br />

Let’s th<strong>in</strong>k of the black box as a function<br />

f, and represent the <strong>in</strong>put and output as<br />

x, f(x). Of course, we do not have a formula<br />

for f.<br />

What Arya’s eng<strong>in</strong>eer<strong>in</strong>g classmates want to do is compute the derivative <strong>in</strong>formation<br />

of the black box, that is, f ′ (x), whenx =2. (The <strong>in</strong>put to this black box can be any<br />

real number.) Students want to use the three-po<strong>in</strong>t midpo<strong>in</strong>t formula to estimate f ′ (2):<br />

f ′ (2) ≈ 1 [f(2 + h) − f(2 − h)] .<br />

2h<br />

They argue how to pick h <strong>in</strong> this formula. One of them says they should make h as<br />

small as possible, like 10 −8 . Arya is skeptical. She mutters to herself, "I know I slept<br />

through some of my numerical analysis lectures, but not all!"

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