# Open Methods Chapter 6 The Islamic University of Gaza

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Open Methods Chapter 6 The Islamic University of Gaza
Faculty of Engineering Civil Engineering Department Numerical Analysis ECIV 3306 Chapter 6 Open Methods

Open Methods Bracketing methods are based on assuming an interval of the function which brackets the root. The bracketing methods always converge to the root. Open methods are based on formulas that require only a single starting value of x or two starting values that do not necessarily bracket the root. These method sometimes diverge from the true root.

1. Simple Fixed-Point Iteration
Rearrange the function so that x is on the left side of the equation: Bracketing methods are “convergent”. Fixed-point methods may sometime “diverge”, depending on the stating point (initial guess) and how the function behaves.

Simple Fixed-Point Iteration
Examples: 1. f(x) = x 2-2x+3  x = g(x)=(x2+3)/2 f(x) = sin x  x = g(x)= sin x + x f(x) = e-x- x  x = g(x)= e-x

Simple Fixed-Point Iteration Convergence
x = g(x) can be expressed as a pair of equations: y1= x y2= g(x)…. (component equations) Plot them separately.

Simple Fixed-Point Iteration Convergence
Fixed-point iteration converges if : When the method converges, the error is roughly proportional to or less than the error of the previous step, therefore it is called “linearly convergent.”

Simple Fixed-Point Iteration-Convergence

Steps of Simple Fixed Pint Iteration
1. Rearrange the equation f(x) = 0 so that x is on the left hand side and g(x) is on the right hand side. e.g f(x) = x2-2x-1 = 0  x= (x2-1)/2 g(x) = (x2-1)/2 2. Set xi at an initial guess xo. 3. Evaluate g(xi) 4. Let xi+1 = g(xi) 5. Find a=(Xi+1 – xi)/Xi+1, and set xi at xi+1 6. Repeat steps 3 through 5 until |a|<= a

Example: Simple Fixed-Point Iteration
f(x) = e-x - x f(x) f(x)=e-x - x 1. f(x) is manipulated so that we get x=g(x) g(x) = e-x 2. Thus, the formula predicting the new value of x is: xi+1 = e-xi 3. Guess xo = 0 4. The iterations continues till the approx. error reaches a certain limiting value Root x f(x) f1(x) = x g(x) = e-x x

Example: Simple Fixed-Point Iteration
i xi g(xi) ea% et%

Example: Simple Fixed-Point Iteration
i xi g(xi) ea% et%

Ex 5.1

Flow Chart – Fixed Point
Start Input: xo , s, maxi i=0 a=1.1s 1

1 False Stop True while a< s & i >maxi
xn=0 x0=xn Print: xo, f(xo) ,a , i False True

2. The Newton-Raphson Method
Most widely used method. Based on Taylor series expansion: Solve for Newton-Raphson formula

The Newton-Raphson Method
A tangent to f(x) at the initial point xi is extended till it meets the x-axis at the improved estimate of the root xi+1. The iterations continues till the approx. error reaches a certain limiting value. f(x) Root x xi xi+1 Slope f /(xi) f(xi)

Example: The Newton Raphson Method
Use the Newton-Raphson method to find the root of e-x-x= 0  f(x) = e-x-x and f`(x)= -e-x-1; thus Iter. xi et% <10-8

Flow Chart – Newton Raphson
Start Input: xo , s, maxi i=0 a=1.1s 1

1 False Stop True while a >s & i <maxi
xn=0 x0=xn Print: xo, f(xo) ,a , i False True

Pitfalls of The Newton Raphson Method
Cases where Newton Raphson method diverges or exhibit poor convergence. a) Reflection point b) oscillating around a local optimum c) Near zero slop , and d) zero slop

Thus, the formula predicting the xi+1 is:
3. The Secant Method The derivative is sometimes difficult to evaluate by the computer program. It may be replaced by a backward finite divided difference Thus, the formula predicting the xi+1 is:

The Secant Method Requires two initial estimates of x , e.g, xo, x1. However, because f(x) is not required to change signs between estimates, it is not classified as a “bracketing” method. The scant method has the same properties as Newton’s method. Convergence is not guaranteed for all xo, x1, f(x).

Secant Method: Example
Use the Secant method to find the root of e-x-x=0; f(x) = e-x-x and xi-1=0, x0=1 to get x1 of the first iteration using: Iter xi-1 f(xi-1) xi f(xi) xi et%

Comparison of convergence of False Position and Secant Methods
Use two estimate xl and xu Use two estimate xi and xi-1 f(x) must changes signs between xl and xu f(x) is not required to change signs between xi and xi-1 Xr replaces whichever of the original values yielded a function value with the same sign as f(xr) Xi+1 replace xi Xi replace xi-1 Always converge May be diverge Slower convergence than Secant in case the secant converges. If converges, It does faster then False Position

Comparison of convergence of False Position and Secant Methods
Use the false-position and secant method to find the root of f(x)=lnx. Start computation with xl= xi-1=0.5, xu=xi = 5. False position method Secant method Iter xi xi xi+1 Iter xl xu xr

False Position and Secant Methods
Although the secant method may be divergent, when it converges it usually does so at a quicker rate than the false position method See the next figure xl xi-1 xu xi

Comparison of the true percent relative Errors Et for the methods to the determine the root of
f(x)=e-x-x

Flow Chart – Secant Method
Start Input: x-1 , x0,s, maxi i=0 a=1.1s 1

1 False Stop True while a >s & i < maxi
Xi-1=xi Xi=xi+1 Print: xi , f(xi) ,a , i False True

Modified Secant Method
Rather than using two initial values, an alternative approach is using a fractional perturbation of the independent variable to estimate  is a small perturbation fraction

Modified Secant Method: Example
Use the modified secant method to find the root of f(x) = e-x-x and, x0=1 and =0.01

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