My title - Departamento de Matemática da Universidade do Minho
My title - Departamento de Matemática da Universidade do Minho
My title - Departamento de Matemática da Universidade do Minho
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2 ITERATION/RECURSION 11<br />
If a is the greater part and b the less of a line of lenght a + b, Euclid’s requirement is<br />
a + b<br />
a<br />
There follows that the ratio φ = a/b satisfies 1 + 1/φ = φ. This division of an interval is used in<br />
Book IV of the Elements to construct a regular pentagon.<br />
= a b<br />
Exercícios.<br />
• Show that φ −1 is equal to φ − 1.<br />
Extreme and mean ratio, and regular pentagon.<br />
(from http://en.wikipedia.org/wiki/Gol<strong>de</strong>n_ratio)<br />
• Show that φ is irrational using its geometric <strong>de</strong>finition (see Euclid’s Elements, or [HW59]<br />
4.6.)<br />
2.4 Newton method to find roots of polynomials<br />
Roots of polynomials. Finding √ a means solving the polynomial equation z 2 − a = 0. What<br />
about finding roots of a generic polynomial p(x) ∈ R[x] <br />
Newton-Raphson iterative scheme. O “méto<strong>do</strong> <strong>de</strong> Newton” é um méto<strong>do</strong> proposto por<br />
Joseph Raphson em 1690 para aproximar raízes <strong>de</strong> um polinómio p(x) (o Newton só queria era<br />
resolver x 3 − 2x − 5 = 0). Consiste em “adivinhar” uma aproximação razoável x 0 <strong>de</strong> uma raiz, e<br />
<strong>de</strong>pois melhorar a conjectura usan<strong>do</strong> o zero <strong>da</strong> aproximação linear p(x 0 ) + p ′ (x 0 )(x − x 0 ).<br />
O méto<strong>do</strong>, portanto, consiste na recursão<br />
Search for a root of x 3 − 2x − 5 using Newton iterations.<br />
x n+1 = x n − p(x n)<br />
p ′ (x n ) .<br />
Se a sucessão converge, i.e. x n → x ∞ , e se p ′ (x ∞ ), então o limite x ∞ é uma raiz <strong>de</strong> p.