GENG2140, S2, 2012 Week 7: Curve fitting

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1 GENG2140, S2, 2012 Week 7: Curve fitting Curve fitting is the process of constructing a curve, or mathematical function, f(x) that has the best fit to a series of data points Involves fitting lines and polynomial curves to data points. f(x) Polynomial Curve Connects exactly y = ax + b First order Straight line Two points y = ax + bx + c Second order Quadratic Three points y = ax + bx + cx + d Third order Cubic Four points Why get an approximate fit when we could just increase the degree of the polynomial equation and get an exact match? A divergent case (exact fit cannot be calculated, or it might take too long). Averaging out questionable data points in a sample, rather than distorting the curve to fit them exactly. Runge's phenomenon (oscillations at the edges of an interval which occurs when interpolating between equidistant points with high degree polynomials). Low-order polynomials tend to be smooth and high order polynomial curves tend to be "lumpy".

2 Methods 1. LAGRANGE INTERPOLATION: Curve made to pass through all the points exactly by a polynomial. Advantages: No need to solve linear equations Less susceptible to round-off errors Allows interpolation even when function values are expressed by symbols Disadvantages: Susceptible to Runge's phenomenon Changing the interpolation points requires recalculating the entire interpolant. 2. SPLINES: A series of unique polynomials are fitted between each pair of the data points by piecewise polynomials called a spline. Advantages: Spline interpolation is preferred over Lagrange interpolation high level of smoothness The interpolation error can be made small even when using low degree polynomials for the spline. avoids the problem of Runge's phenomenon. Disadvantages: Computationally intensive Rather slow

3 Linear spline: Draws straight lines between consecutive data Quadratic spline: Draws a second-order polynomial instead of a straight line going through consecutive data points. Cubic spline: A series of unique cubic polynomials are fitted between each of the data points. 100 random data points Clearly, no correlation can be established between the random 100 data points. A Lagrange interpolation would require a 99 th order polynomial to fit to these data points. However, cubic splines can interpolate all 100 points without the drastic behavior which a 99 th order polynomial would exhibit. Cubic spline curves through the random data

4 3. LEAST SQUARE METHOD Finds the parameter of a model that the best-fit the data when the residual is the minimum. A residual is defined as the difference between the actual value of the dependent variable and the value predicted by the model. Does not seek an exact fit, but fits to the trend of the data Apply when data is not reliable Least square fit

5 Week 8: Numerical methods for solving nonlinear algebraic equations (root finding) Nonlinear problems : In mathematics, a nonlinear system is one that does not satisfy the superposition principle, or one whose output is not directly proportional to its input. Most physical systems are inherently nonlinear in nature. Nonlinear equations are difficult to solve. It is often difficult to determine the existence or number solutions to nonlinear equations. Whereas for system of linear equations the number of solutions must be either zero, one or infinitely many, nonlinear equations can have any number of solutions. Determining existence and uniqueness of solutions is more complicated for nonlinear equations than for linear equations. For example, e x -x=0 has no solution, e x -x-1=0 has a repeated root. Solution of Non-Linear Equations: Two types Polynomial equations (algebraic equations) Transcendental equations (non-algebraic equations) Polynomial equations: Formed by equating a polynomial to zero, that is, given a function f(x), we seek value x for which f(x) = 0. Solution x is called a root of equation, or zero of function f. Thus, the problem is known as root or zero finding. f(x) = x 2-5x + 6 = 0 y=f(x) x (x -2) (x -3)=0 x = 2; x = 3 (zeros of the function) 2 3 x

6 Transcendental equations (non-algebraic equations) Made of transcendental functions, that is functions not expressible as a finite combination of the algebraic operations of addition, subtraction, multiplication, division, raising to a power, and extracting a root. Examples: log x, sin x, cos x, e x Such an equation cannot be solved for one factor in terms of another and are expressible in algebraic terms only as infinite series. Some methods of finding solutions to a transcendental equation use graphical or numerical methods - Graphical method may be time-consuming, numerical methods preferred Direct methods : 1. Give the exact value of the roots (in the absence of round off errors) in a finite number of steps. 2. These methods determine all the roots at the same time. Numerical (iterative) methods: 1. Based on the idea of successive approximations. Starting with one or more initial approximations to the root, we obtain a sequence of iterations which in the limit converges to the root. 2. These methods determine one or two roots at a time.

7 The problems we want to answer are: Does the problem have a solution? Is the solution unique? Is the iteration well-defined? Does the iteration converge to a limit? How quickly does the method converge? Solution of non-linear equations Find the value of x for which f(x) = 0 Heart of numerical analysis is iteration 1. Bisection method (binary search): Repeatedly bisects an interval and then selects a subinterval in which a root must lie for further processing. A convergence test is specified In order to decide when a sufficiently accurate solution has (hopefully) been found. y = f(x) f(x l ) f(x r ) x l f(x r ) x u x f(x u ) Make initial guess of x l and x r At each step the method divides the interval in two by computing the midpoint x r = (x l +x u )/2 f(x l ) f(x r ) < 0 ; Lower subinterval, make x u =x r f(x l ) f(x r ) > 0 ; Upper subinterval, make x l =x r f(x l ) f(x r ) =0 ; Root = x r

8 In each step, the interval is reduced by 50%. The process is continued until the interval (error), e a is sufficiently small. e x x x < tolerance To determine in advance the number of iterations that the bisection method would need to converge to a root to within a certain tolerance. x x 2 < tolerance Simple and robust but is slow. Therefore used to obtain a rough approximation to solution which is then used as a starting point for more rapidly converging methods, such as the Newton-Rhapson method. 2. Newton-Rhapson method Extremely powerful technique - in general the convergence is quadratic. Most widely used of all root finding methods. Method Make an initial guess which is reasonably close to the true root. Take the derivative of the function at a point Draw a tangent line from the point and extend it to the x-axis This x-intercept will typically be a better approximation to the function's root than the original guess. Iterate the Newton-Rhapson formula until the error falls below the tolerance. x = x ( ) ( ) i = 0,1,2. n The method will usually converge, provided this initial guess is close enough to the unknown zero.

9 Disadvantages Cases where it performs poorly multiple roots May result in slow convergence due to nature of certain functions An analytical expression for the derivative may not be easily obtainable In these situations, it may be appropriate to approximate the derivative by using the slope of a line through two nearby points on the function. Using this approximation would result in something like the secant method whose convergence is slower than that of Newton's method. 1) Multiple root at zero In these situations, it may be appropriate to approximate the derivative by using the slope of a line through two nearby points on the function. Using this approximation would result in something like the secant method whose convergence is slower than that of Newton's method.

10 3. Secant method Graphical interpretation is similar to the Newton - Rhapson method. Instead of a tangent line, we draw a secant line (a straight line which crosses the curve at two points). Derivative approximated by a backward finite divided difference. Secant formula: x = x f(x )(x x ) f(x ) f(x ) Since the calculation of x i+1 requires x i and x i-1, two initial points must be prescribed at the beginning; Advantage: Derivative of the function need not be calculated.

11 Week 10: One Dimensional Unconstrained Optimization Most of the mathematical models we have dealt with to this point have been descriptive models. That is, they have been derived to simulate the behavior of an engineering device or system. In contrast optimization typically deals with finding the best result, or optimum solution, of a problem Engineers must continuously design devices that perform tasks in an efficient fashion. In doing so, they are constrained by the limitations of the physical world. Further, they must keep costs down. Thus, they are always confronting optimization problems that balance performance and limitations. Some optimization examples. 1. Design aircraft for minimum weight and maximum strength 2. Optimal trajectories of space vehicle. 3. Design civil engineering structures for minimum cost. 4. Design pump and heat transfer equipment for maximum efficiency. 5. Shortest route of sales person visiting various cities during one sales trip.

12 Root location and optimization are related in the sense that both involve guessing and searching for a point on a function. f(x) f (x) = 0 f (x) < 0 X Maxima X f(x)=0 root root f (x) =0 f (x)>0 X Minima x One-dimensional optimization - minimum and maximum of a function f(x) of a single variable x Multi-dimensional optimization - minimum and maximum of a function of two or more variables f(x,y). Unconstrained optimization problem min x f(x) or max x f(x) Constrained optimization problem min x f(x) or max x f(x) subject to g(x) = 0 We will consider one-dimensional unconstrained optimization problem only

13 1. Golden section method Incremental search for max/min using Golden ratio The search interval in each iteration reduced by the GR = Step 1. Choose two initial guesses, x l, x u Step 2. Choose two interior points x 1, x 2 according to the GR Step 3. If f(x 1 ) > f(x 2 ) discard region left to x 2, set x l =x 2 If f(x 2 ) > f(x 1 ) discard region right to x 1, set x u =x 1 Step 4. Iterate until interval is less than tolerance Comparison of Bisection method and Golden section method Bisection method Method of incremental search Used to find roots of f(x) Golden Section method Method of incremental search Used to find max and min values of f(x) Find x for f(x)=0 Find x for f (x) =0 and f (x) > 0 for max point f (x) < 0 for min point Requires 2 points for the incremental search Looks for sign change of the function between the two points Interval section reduces by 0.5 in each iteration: x x 2 Requires min 3 points (upto 4 points) for incremental search Looks for higher/lower value of the function between the three points Interval section reduces by the Golden number in each iteration: 5 1 (x 2 x )

14 2. Quadratic (parabolic) interpolation A second order polynomial often provides a good approximation to the shape of f(x) near an optimum. Just as there is only one straight line connecting two points, there is only one quadratic or parabola connecting three points. Thus, if we have three points (x 0, x 1, x 2 ) that jointly bracket an optimum, we can fit a parabola to the points. Then we can differentiate it, set the results equal to zero, and solve for an estimate of the optimal x 3. It can be shown that : x = f(x )(x x ) + f(x )(x x ) + f(x )(x x ) 2 f(x )(x x ) + 2 f(x )(x x ) + 2 f(x )(x x ) where x o, x 1 and x 2 are the initial guesses, and x 3 is the optimal x obtained by the quadratic fitting. Advantages: Useful only when the function is quite smooth in the interval. Convergence is almost quadratic, (approximately 1.324). superior to that of other methods with only linear convergence (such as line search). Does not requiring the computation of derivatives, popular alternative to other methods that do require them (such as gradient descent and Newton's method).

15 Disadvantages: Convergence (even to a local extremum) is not guaranteed e.g. three points are collinear. (the resulting parabola is degenerate and thus does not provide a new candidate point). Can get hung up with just one end of the interval converging. Thus, the convergence can be slow. If function derivatives are available, Newton's method is applicable and exhibits quadratic convergence.

16 Week 10/11: Numerical Integration (Quadrature) In numerical analysis, numerical integration (numerical quadrature) is used for calculating the numerical value of a definite integral. The basic problem considered by numerical integration is to compute an approximate solution to a definite integral of a function f(x): A = f(x) dx which is the area of the curve (shaded) bounded by the limits a and b. Definite integral can be thought of as an infinite sum of rectangles of infinitesimal width.

17 There are several reasons for carrying out numerical integration. In real applications, some integrals may be too complex, and cannot be found exactly. Some integrals may require special functions which themselves are a challenge to compute, and are too slow. The integrand f(x) may be known only at certain points, such as obtained by sampling (observations at a certain number of points). In that case, we do not have a nice formula but only some data points. A formula for the integrand may be known, but it may be difficult or impossible to find an antiderivative e.g, f(x) = exp( x 2 ), the antiderivative of which (the error function, times a constant) cannot be written in elementary form.

18 Applications Chemical engineering: an analytic function is integrated numerically to determine the heat required to raise the temperature of a material Civil engineering: Numerical integration to determine the total wind force acting on the mast of a racing sailboat. The total force exerted on the mast can be expressed as z F = 200 ( 5 + z )e/ dz This non-linear equation is difficult to evaluate analytically. Therefore, it is convenient to apply numerical integration such as Simpson rule. Elect. engineering: Determination of the root mean square (rms) current of a electric circuit Mech. engineering: Calculation of work required to move a block

19 Methods Newton-Cotes formulas - approximates a complicated function tabulated at equally spaced intervals by an approximating polynomial. o Trapezoidal rule If the approximating polynomial is a first order (a straight line joining two end points) it is called the trapezoidal rule. o Simpson's rule If there is an additional point available in between, the three points can be connected by a parabola. If there are two additional points equally spaced in-between, the four points can be connected by a third order polynomial more accurate Gaussian quadrature points are not equally spaced.