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First Semester in Numerical Analysis with Julia, 2020a

First Semester in Numerical Analysis with Julia, 2020a

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CHAPTER 2. SOLUTIONS OF EQUATIONS: ROOT-FINDING 53<br />

Here is a more general analysis of this solution technique. Suppose we want to solve the<br />

equation g(x) =a, and that g is a l<strong>in</strong>ear map, that is, g(λx) =λg(x) for any constant λ.<br />

Then, the solution is x = ap/b where p is an <strong>in</strong>itial guess <strong>with</strong> g(p) =b. To see this, simply<br />

observe<br />

( ap<br />

)<br />

g = a g(p) =a.<br />

b b<br />

The general problem<br />

How can we solve equations that are far complicated than the ancient Egyptians solved? For<br />

example, how can we solve x 2 +5cosx =0? Stated differently, how can we f<strong>in</strong>d the root p<br />

such that f(p) =0, where f(x) =x 2 +5cosx? In this chapter we will learn some iterative<br />

methods to solve equations. An iterative method produces a sequence of numbers p 1 ,p 2 , ...<br />

such that lim n→∞ p n = p, and p is the root we seek. Of course, we cannot compute the exact<br />

limit, so we stop the iteration at some large N, and use p N as an approximation to p.<br />

The stopp<strong>in</strong>g criteria<br />

With any iterative method, a key question is how to decide when to stop the iteration. How<br />

well does p N approximate p?<br />

Let ɛ>0 be a small tolerance picked ahead of time. Here are some stopp<strong>in</strong>g criteria:<br />

stop when<br />

1. |p N − p N−1 |

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