Finding Roots of Polynomials with Complex Numbers
Date: 09/27/2001 at 00:33:24 From: Ed Subject: Find roots of polynomials with complex numbers Dr. Math, In one of your articles, I read that you can find the roots of 3rd- or higher-degree polynomials with complex numbers. You did not explain how to do it, since you assumed the student did not have much experience using complex variables. I would like to learn more about this topic. Please explain to me in detail how to find roots of such equations, along with examples and applications, history. Thanks in advance. Regards, Ed
Date: 09/27/2001 at 09:12:51 From: Doctor Rob Subject: Re: Find roots of polynomials with complex numbers Thanks for writing to Ask Dr. Math, Ed. The same methods that work for polynomials with real coefficients also work for those with complex coefficients. For such methods applied to cubic and quartic equations, see Cubic and Quartic Equations from our Frequently Asked Questions (FAQ): http://mathforum.org/dr.math/faq/faq.cubic.equations.html For higher-degree polynomials f(x), one can use Newton's method to find the roots numerically, even if they are complex. That involves starting with a guess x, and using the following recursive formula: x[n+1] = x[n] - f(x[n])/f'(x[n]), n = 0, 1, 2, ... Here f'(x) is the first derivatve of f(x) with respect to x: d f(x) = SUM a[i]*x^i, i=0 d f'(x) = SUM i*a[i]*x^(i-1). i=1 For example, suppose we had the polynomial f(x) = x^5 - (6*i)*x - 2, f'(x) = 5*x^4 - 6*i. Then suppose we start with the guess x = i. Then the recursive formula is x[n+1] = x[n] - (x[n]^5-6*i*x[n]-2)/(5*x[n]^4-6*i). The computation gives x = i, x = -0.2295 + 0.5246*i, x = 0.01873 + 0.3264*i, x = 0.0007003 + 0.33331*i, x = 0.000685769449 + 0.333207872*i, x = 0.0006857694535760589 + 0.3333262787490043*i. This gives you one root of f(x) = 0. Different initial guesses can give you different roots. Starting with x = 2 + i, you would get x = 1.457461395822838 + 0.5135279776944679*i, another root. Of course, once you have one root r, you can divide out x - r to reduce the degree of the polynomial, but giving a quotient that has approximate numerical coefficients. Newton's method was devised by Sir Isaac Newton. Sometimes it is called the Newton-Raphson method. Apparently Newton devised it first, about 1671, but it was published first by Joseph Raphson in 1691. - Doctor Rob, The Math Forum http://mathforum.org/dr.math/
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