Bairstow Method

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Lin-Bairstow MethodLin-Bairstow Method

Compiled by Compiled by Naser BagheriNaser BagheriStudent ID : 9016393Student ID : 9016393

Modulefor

The Lin-BairstowMethod

Quadratic Synthetic Division

Let the polynomial of degree n have coecients .Then has the familiarform

(1) .

Let be a xed quadratic term.Then can be expressed as

(2) ,

where is the remainder when is divided by.Here is a polynomial of degree and can be represented by

(3) .If we set and ,then

(4) ,where

(5)

and equation(4)can be written

(6) .

Lin-Bairstow Method http://math.fullerton.edu/mathews/n2003/bairstowmetho...

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The terms in(6) can be expanded so that is represented in powers of x.

(7)

The numbers are found by comparing the coecients of in equations(1)and(7).The coecients of and are computed recursively.

(8) Set , and

, and then

for .Proof Lin-BairstowMethod Lin-BairstowMethod

Example 1. Use quadratic synthetic division to divideby .

Solution 1.

Heuristics

In the days when"hand computations"were necessary,the quadratic synthetic divisiontableau(or table)was used.The coecients of the polynomial are entered on therst row in descending order,the second and third rows are reserved for the intermediatecomputation steps( and )and the bottom row contains the coecients, and .


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Example2.Use the"quadratic synthetic division tableau" to divideby .

Solution2.

Using vector coecients

As mentioned above,it is ecient to store the coecients of a polynomialof degree n in the vector .Notice that this is a shift of the

index for and the polynomial is written in the form .

Given the quadratic ,the quotient and remainder are

and .

The recursive formulas for computing the coecients of are

,and ,and then for .


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Example3. Use the vector form of quadratic synthetic division to divideby .

Solution3.

The Lin-BairstowMethod

We now build on the previous idea and develop the Lin-Bairstow's method for nding aquadratic factor of .Suppose that we start with the initial guess

(9)

and that can be expressed as

.

When u and v are small,the quadratic (9)is close to a factor of .We want to ndnew values so that

(10)

is closer to a factor of than the quadratic(9).Observe that u and v are functions of r and s,that is

,and.

The new values satisfy the relations

,and


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.The dierentials of the functions u and v are used to produce the approximations

and

The new values are to satisfy

,and.

When the quantities are small,we replace the above approximations withequations and obtain the linear system:

(11)

All we need to do is nd the values of the partial derivatives , ,and and then use Cramer's rule to compute .Let us announce that thevalues of the partial derivatives are

where the coecients are built upon the coecients given in(8) and arecalculated recursively using the formulas

(12)Set , and

, and then


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for .The formulas in(12)use the coecients in(8).Since

,and,and

the linear system in (11) can be written as

Cramer's rule can be used to solve this linear system.The required determinants are

, ,and .

and the new values are computed using the formulas

,and .ProofLin-BairstowMethodLin-BairstowMethod

The iterative process is continued until good approximations torandshave beenfound.If the initial guesses are chosen small,the iteration does not tend towander for a long time before converging.When ,the larger powers ofxcan beneglected in equation(1)and we have the approximation

.

Hence the initial guesses for could be and ,provided that.


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If hand calculations are done,then the quadratic synthetic division tableau can beextended to form an easy way to calculate the coecients .

Bairstow's method is a special case of Newton's method in two dimensions.

Algorithm(Lin-Bairstow Iteration).To nd a quadratic factor of given an initialapproximation .Computer ProgramsLin-BairstowMethodLin-BairstowMethodMathematica Subroutine(Lin-Bairstow Iteration).


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Example4.Given .Start with and and use theLin-Bairstowmethod to nd a quadratic factor of .Solution4.

Research Experience for UndergraduatesLin-BairstowMethodLin-BairstowMethodInternet hyperlinks to web sites and abibliography of articles.

Download this Mathematica Notebook Lin-Bairstow Method


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Example 1.Use quadratic synthetic division to divide by .Solution 1.First,construct the polynomial .

Second,given construct the polynomials .


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Third,verify that .


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We are done.

Aside.We can have Mathematica compute the quotient and remainder using the built inproceduresPolynomialQuotient andPolynomialRemainder.This is just for fun!


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Example2. Use the"quadratic synthetic division tableau" to divideby .

Solution2.Use the"quadratic synthetic division tableau"

Since we use .

Then simplify and get.

Thus we have

and


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This agrees with our previous computation in Example1.

We are done.

Aside. We can letMathematica verify the result.


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Example3. Use the vector form of quadratic synthetic division to divideby .

Solution3.First,construct the polynomial .

Second,given construct the polynomials .


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Third,verify that .


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Example4. Given .Start with and and use theLin-Bairstowmethod to nd a quadratic factor of .Solution4.Enter the coecients of the polynomial.

Enter the starting values and and call the subroutine Bairstow.

Verify that a quadratic factor has been found.


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We are done.

Aside.We can letMathematica nd the factors too.This is just for fun.

Aside. We can letMathematica nd the roots too.This is just for fun.


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/home/naser/Darss/Analize-adadi/bros/Bairsto.cppPage 1 of 3 Sun 20 Oct 2013 11:52:32 PM IRST 1 //********************************************** 2 // Programmer : Naser . Bagheri (9016393) 3 //********************************************** 4 #include 5 #include 6 using namespace std; 7 #define ESP 0.001 8 #define F(x) (x)*(x)*(x) + (x) + 10 9 #define a3 1 10 #define a2 0 11 #define a1 1 12 #define a0 10 13 //#define c3 0void main() 14 int main() 15 { 16 double u,v,u1,v1,u2,v2,b3,b2,p,b1,b0,c2,c1,c0,U,V; 17 int i=1; 18 float c3=0; 19 cout

/home/naser/Darss/Analize-adadi/bros/Bairsto.cppPage 2 of 3 Sun 20 Oct 2013 11:52:32 PM IRST 59 V=((-(b0*c1-c0*b1))/(p)); 60 u2=u+U; 61 v2=v+V; 62 cout

/home/naser/Darss/Analize-adadi/bros/Bairsto.cppPage 3 of 3 Sun 20 Oct 2013 11:52:32 PM IRST117 118 u = 2.002230119 v = -5.002025120 121 b0 = -0.001389122 b1 = 0.006900123 b2 = 2.002230124 b3 = 1.000000125 126 c0 = 6.121717127 c1 = 5.552230128 c2 = 1.000000129 c3 = 0.000000130 131 u = 2.000623132 v = -5.000003133 134 b0 = 0.001859135 b1 = 0.002490136 b2 = 2.000623137 b3 = 1.000000138 139 c0 = 11.224619140 c1 = 8.108540141 c2 = 1.000000142 c3 = 0.000000143 144 u = 2.000287145 v = -4.999767146 147 REAL ROOT = 2.000148 REAL ROOT = -5.000149 */150

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Wolfram Web Resources 13,192 entriesLast updated: Tue Oct 8 2013

Created, developed, andnurtured by Eric Weissteinat Wolfram Research

Applied Mathematics > Numerical Methods > Root-Finding >

Bairstow's MethodA procedure for finding the quadratic factors for the complex conjugate roots of a polynomial with real coefficients.

(1)

Now write the original polynomial as(2)(3)

(4)

(5)

(6)

(7)

(8)

Now use the two-dimensional Newton's method to find the simultaneous solutions.

REFERENCES:Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. Numerical Recipes in FORTRAN: The Art of ScientificComputing, 2nd ed. Cambridge, England: Cambridge University Press, pp. 277 and 283-284, 1989.

Referenced on Wolfram|Alpha: Bairstow's Method

CITE THIS AS:Weisstein, Eric W. "Bairstow's Method." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/BairstowsMethod.html

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Bairstow Method

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