Chapter 17-21

advertisement
Chapter 17 Objectives
• Recognizing that Newton-Cotes integration
formulas are based on the strategy of replacing a
complicated function or tabulated data with a
polynomial that is easy to integrate.
• Knowing how to implement the following single
application Newton-Cotes formulas:
– Trapezoidal rule
– Simpson’s 1/3 rule
– Simpson’s 3/8 rule
• Knowing how to implement the following composite
Newton-Cotes formulas:
– Trapezoidal rule
– Simpson’s 3/8 rule
Objectives (cont)
• Recognizing that even-segment-odd-point
formulas like Simpson’s 1/3 rule achieve
higher than expected accuracy.
• Knowing how to use the trapezoidal rule to
integrate unequally spaced data.
• Understanding the difference between open
and closed integration formulas.
Integration
• Integration:
I
 f x dx
b
a
is the total value, or summation, of f(x) dx over the range
from a to b:

Newton-Cotes Formulas
• The Newton-Cotes formulas are the most
common numerical integration schemes.
• Generally, they are based on replacing a
complicated function or tabulated data with a
polynomial that is easy to integrate:
I
 f x dx   f x dx
b
b
a
a
n
where fn(x) is an nth order interpolating
polynomial.

Newton-Cotes Examples
• The integrating function
can be polynomials for
any order - for example,
(a) straight lines or (b)
parabolas.
• The integral can be
approximated in one step
or in a series of steps to
improve accuracy.
The Trapezoidal Rule
• The trapezoidal rule is
the first of the NewtonCotes closed integration
formulas; it uses a
straight-line
approximation for the
function:
I
 f x dx
b
a
n


f b  f a
I   a f (a) 
x  adx
ba


b
f a  f b
I  b  a
2

Error of the Trapezoidal Rule
• An estimate for the local
truncation error of a single
application of the trapezoidal rule
is:
1
3
Et   f b  a
12
where  is somewhere between
a and b.
• This formula indicates that the
error is dependent upon the
curvature of the actual function
as well as the distance between
the points.
• Error can thus be reduced by
breaking the curve into parts.
Composite Trapezoidal Rule
• Assuming n+1 data points are evenly
spaced, there will be n intervals over
which to integrate.
• The total integral can be calculated
by integrating each subinterval and
then adding them together:
I

xn
x0
fn x dx 

x1
x0
fn x dx 
x2
x1
fn x dx

f x0   f x1
f x1  f x2 
I  x1  x0 
 x2  x1

2
2
n1

h 
I  f x0   2 f xi   f xn 
2 

i 1
xn
x n 1
fn x dx
 xn  xn1
f xn1  f xn 
2
MATLAB Program
Simpson’s Rules
• One drawback of the trapezoidal rule is that the error is
related to the second derivative of the function.
• More complicated approximation formulas can improve the
accuracy for curves - these include using (a) 2nd and (b) 3rd
order polynomials.
• The formulas that result from taking the integrals under
these polynomials are called Simpson’s rules.
Simpson’s 1/3 Rule
• Simpson’s 1/3 rule corresponds to using
second-order polynomials. Using the
Lagrange form for a quadratic fit of three
points:
fn x 
x  x1  x  x2  f x  x  x0  x  x2  f x  x  x0  x  x1  f x
 
 
 
x0  x1  x0  x2  0 x1  x0  x1  x2  1 x2  x0  x2  x1  2
Integration over the three points simplifies to:
I   f x dx
x2
x0
I
n
h
f x0   4 f x1   f x2 

3
Error of Simpson’s 1/3 Rule
• An estimate for the local truncation error of a single
application of Simpson’s 1/3 rule is:
1
5
4 
Et  
f b  a
2880
where again  is somewhere between a and b.
• This formula indicates that the error is dependent upon the

fourth-derivative
of the actual function as well as the
distance between the points.
• Note that the error is dependent on the fifth power of the
step size (rather than the third for the trapezoidal rule).
• Error can thus be reduced by breaking the curve into parts.
Composite Simpson’s 1/3 Rule
• Simpson’s 1/3 rule can be used
on a set of subintervals in much
the same way the trapezoidal rule
was, except there must be an odd
number of points.
• Because of the heavy weighting of
the internal points, the formula is a
little more complicated than for the
trapezoidal rule:
I

xn
x0
fn x dx 

x2
x0
fn x dx  x fn x dx
x4
2
 x
xn
n2
fn x dx
h
h
f x0  4 f x1  f x2   f x2  4 f x3  f x4 

3
3


n1
n2
h 

I  f x0  4  f xi  2  f xi  f xn 
3
i1
j2


i, odd
j, even


I

h
f xn2  4 f xn1  f xn 

3
Simpson’s 3/8 Rule
• Simpson’s 3/8 rule corresponds
to using third-order polynomials
to fit four points. Integration over
the four points simplifies to:
I

x3
x0
fn x dx
3h
I   f x0   3 f x1   3 f x2   f x3 
8
• Simpson’s 3/8 rule is generally
used in concert with Simpson’s
1/3 rule when the number of
segments is odd.
Higher-Order Formulas
• Higher-order Newton-Cotes formulas may also be
used - in general, the higher the order of the
polynomial used, the higher the derivative of the
function in the error estimate and the higher the
power of the step size.
• As in Simpson’s 1/3 and 3/8 rule, the evensegment-odd-point formulas have truncation errors
that are the same order as formulas adding one
more point. For this reason, the even-segmentodd-point formulas are usually the methods of
preference.
Integration with Unequal Segments
• Previous formulas were simplified based on
equispaced data points - though this is not
always the case.
• The trapezoidal rule may be used with data
containing unequal segments:
I

xn
x0
fn x dx 

x1
x0
fn x  dx

x2
x1
fn x  dx


f x0   f x1 
f x1   f x2 
I  x1  x0 
 x2  x1 

2
2
xn
x n1
fn x dx
f xn1   f xn 
 xn  xn1 
2
Integration Code for Unequal
Segments
MATLAB Functions
• MATLAB has built-in functions to evaluate integrals
based on the trapezoidal rule
• z = trapz(y)
z = trapz(x, y)
produces the integral of y with respect to x. If x is
omitted, the program assumes h=1.
• z = cumtrapz(y)
z = cumtrapz(x, y)
produces the cumulative integral of y with respect
to x. If x is omitted, the program assumes h=1.
Multiple Integrals
• Multiple integrals can be
determined numerically by first
integrating in one dimension,
then a second, and so on for all
dimensions of the problem.
Chapter 18 Objectives
• Understanding how Richardson extrapolation
provides a means to create a more accurate
integral estimate by combining two less accurate
estimates.
• Understanding how Gauss quadrature provides
superior integral estimates by picking optimal
abscissas at which to evaluate the function.
• Knowing how to use MATLAB’s built-in functions
quad and quadl to integrate functions.
Richardson Extrapolation
• Richard extrapolation methods use two estimates
of an integral to compute a third, more accurate
approximation.
• If two O(h2) estimates I(h1) and I(h2) are calculated
for an integral using step sizes of h1 and h2,
respectively, an improved O(h4) estimate may be
formed using:
1
I  I(h2 )
(h1 / h2 ) 1
2
I(h2 )  I(h1 )
• For the special case where the interval is halved
(h2=h1/2), this becomes:

4
1
I  I(h2 )  I(h1 )
3
3
Richardson Extrapolation (cont)
• For the cases where there are two O(h4) estimates
and the interval is halved (hm=hl/2), an improved
O(h6) estimate may be formed using:
16
1
I  I m  Il
15
15
• For the cases where there are two O(h6) estimates
and the interval is halved (hm=hl/2), an improved
O(h8) estimate
may be formed using:

64
1
I
I m  Il
63
63
The Romberg Integration Algorithm
• Note that the weighting factors for the Richardson
extrapolation add up to 1 and that as accuracy
increases, the approximation using the smaller step
size is given greater weight.
• In general,
4 k1 I
I
I j,k 
j1,k1
k1
4
j,k1
1
where ij+1,k-1 and ij,k-1 are the more and less
accurate integrals, respectively, and ij,k is the new

approximation.
k is the level of integration and j is
used to determine which approximation is more
accurate.
Romberg Algorithm Iterations
• The chart below shows the process by which
lower level integrations are combined to
produce more accurate estimates:
MATLAB Code for Romberg
Gauss Quadrature
• Gauss quadrature describes
a class of techniques for
evaluating the area under a
straight line by joining any
two points on a curve rather
than simply choosing the
endpoints.
• The key is to choose the line
that balances the positive and
negative errors.
Gauss-Legendre Formulas
• The Gauss-Legendre formulas seem to optimize
estimates to integrals for functions over intervals
from -1 to 1.
• Integrals over other intervals require a change in
variables to set the limits from -1 to 1.
• The integral estimates are of the form:
I  c0 f x0  c1 f x1 
 cn1 f xn1 
where the ci and xi are calculated to ensure that the
method exactly integrates up to (2n-1)th order

polynomials
over the interval from -1 to 1.
Adaptive Quadrature
• Methods such as Simpson’s 1/3 rule has a
disadvantage in that it uses equally spaced
points - if a function has regions of abrupt
changes, small steps must be used over the
entire domain to achieve a certain accuracy.
• Adaptive quadrature methods for integrating
functions automatically adjust the step size
so that small steps are taken in regions of
sharp variations and larger steps are taken
where the function changes gradually.
Adaptive Quadrature in MATLAB
• MATLAB has two built-in functions for implementing
adaptive quadrature:
– quad: uses adaptive Simpson quadrature; possibly more
efficient for low accuracies or nonsmooth functions
– quadl: uses Lobatto quadrature; possibly more efficient
for high accuracies and smooth functions
• q = quad(fun, a, b, tol, trace, p1, p2, …)
– fun : function to be integrates
– a, b: integration bounds
– tol: desired absolute tolerance (default: 10-6)
– trace: flag to display details or not
– p1, p2, …: extra parameters for fun
– quadl has the same arguments
Chapter 19 Objectives
• Understanding the application of high-accuracy numerical
differentiation formulas for equispaced data.
• Knowing how to evaluate derivatives for unequally spaced
data.
• Understanding how Richardson extrapolation is applied for
numerical differentiation.
• Recognizing the sensitivity of numerical differentiation to
data error.
• Knowing how to evaluate derivatives in MATLAB with the
diff and gradient functions.
• Knowing how to generate contour plots and vector fields
with MATLAB.
Differentiation
• The mathematical definition of a derivative begins
with a difference approximation:
y f xi  x  f xi 

x
x
and as x is allowed to approach zero, the
difference becomes a derivative:


f xi  x  f xi 
dy
 lim
dx x0
x
High-Accuracy Differentiation
Formulas
• Taylor series expansion can be used to
generate high-accuracy formulas for
derivatives by using linear algebra to
combine the expansion around several
points.
• Three categories for the formula include
forward finite-difference, backward finitedifference, and centered finite-difference.
Forward Finite-Difference
Backward Finite-Difference
Centered Finite-Difference
Richardson Extrapolation
• As with integration, the Richardson extrapolation can be used to
combine two lower-accuracy estimates of the derivative to produce a
higher-accuracy estimate.
• For the cases where there are two O(h2) estimates and the interval is
halved (h2=h1/2), an improved O(h4) estimate may be formed using:
4
1
D  D(h2 )  D(h1 )
3
3
• For the cases where there are two O(h4) estimates and the interval is
halved (h2=h1/2), an improved O(h6) estimate may be formed using:

16
1
D  D(h2 )  D(h1 )
15
15
• For the cases where there are two O(h6) estimates and the interval is
halved (h2=h1/2), an improved O(h8) estimate may be formed using:

64
1
D
D(h2 )  D(h1 )
63
63
Unequally Spaced Data
• One way to calculated derivatives of
unequally spaced data is to determine a
polynomial fit and take its derivative at a
point.
• As an example, using a second-order
Lagrange polynomial to fit three points and
taking its derivative yields:
f x  f x0 
2x  x1  x2
2x  x0  x2
2x  x0  x1
 f x1 
 f x2 
x0  x1 x0  x2 
x1  x0 x1  x2 
x2  x0 x2  x1 
Derivatives and Integrals for Data
with Errors
• A shortcoming of numerical differentiation is that it tends to
amplify errors in data, whereas integration tends to smooth
data errors.
• One approach for taking derivatives of data with errors is to
fit a smooth, differentiable function to the data and take the
derivative of the function.
Numerical Differentiation with
MATLAB
• MATLAB has two built-in functions to help
take derivatives, diff and gradient:
• diff(x)
– Returns the difference between adjacent
elements in x
• diff(y)./diff(x)
– Returns the difference between adjacent values
in y divided by the corresponding difference in
adjacent values of x
Numerical Differentiation with
MATLAB
• fx = gradient(f, h)
Determines the derivative of the data in f at each
of the points. The program uses forward difference
for the first point, backward difference for the last
point, and centered difference for the interior points.
h is the spacing between points; if omitted h=1.
• The major advantage of gradient over diff is
gradient’s result is the same size as the original
data.
• Gradient can also be used to find partial derivatives
for matrices:
[fx, fy] = gradient(f, h)
Visualization
• MATLAB can generate contour plots of functions as
well as vector fields. Assuming x and y represent
a meshgrid of x and y values and z represents a
function of x and y,
– contour(x, y, z) can be used to generate a contour
plot
– [fx, fy]=gradient(z,h) can be used to generate
partial derivatives and
– quiver(x, y, fx, fy) can be used to generate
vector fields
Chapter 20 Objectives
• Understanding the meaning of local and global truncation
errors and their relationship to step size for one-step
methods for solving ODEs.
• Knowing how to implement the following Runge-Kutta (RK)
methods for a single ODE:
–
–
–
–
Euler
Heun
Midpoint
Fourth-Order RK
• Knowing how to iterate the corrector of Heun’s method.
• Knowing how to implement the following Runge-Kutta
methods for systems of ODEs:
– Euler
– Fourth-order RK
Ordinary Differential Equations
• Methods described here are for solving differential
equations of the form:
dy
 f t, y
dt
• The methods in this chapter are all one-step
methods and have the general format:

yi1  yi  h
where  is called an increment function, and is
used to extrapolate from an old value yi to a new
value yi+1. 
Euler’s Method
• The first derivative
provides a direct
estimate of the slope
at ti: dy
 f t i , yi 
dt ti
and the Euler method
uses that estimate as
 the increment
function:
  f ti , yi 
yi1  yi  f ti , yi h
Error Analysis for Euler’s Method
• The numerical solution of ODEs involves two types
of error:
– Truncation errors, caused by the nature of the techniques
employed
– Roundoff errors, caused by the limited numbers of
significant digits that can be retained
• The total, or global truncation error can be further
split into:
– local truncation error that results from an application
method in question over a single step, and
– propagated truncation error that results from the
approximations produced during previous steps.
Error Analysis for Euler’s Method
• The local truncation error for Euler’s method
is O(h2) and proportional to the derivative of
f(t,y) while the global truncation error is O(h).
• This means:
– The global error can be reduced by decreasing
the step size, and
– Euler’s method will provide error-free predictions
if the underlying function is linear.
• Euler’s method is conditionally stable,
depending on the size of h.
MATLAB Code for Euler’s Method
Heun’s Method
• One method to improve Euler’s method is to determine derivatives at the
beginning and predicted ending of the interval and average them:
• This process relies on making a prediction of the new value of y, then
correcting it based on the slope calculated at that new value.
• This predictor-corrector approach can be iterated to convergence:
Midpoint Method
• Another improvement to Euler’s method is
similar to Heun’s method, but predicts the
slope at the midpoint of an interval rather
than at the end:
• This method has a local truncation error of
O(h3) and global error of O(h2)
Runge-Kutta Methods
• Runge-Kutta (RK) methods achieve the accuracy of a Taylor
series approach without requiring the calculation of higher
derivatives.
• For RK methods, the increment function  can be generally
written as:
  a1k1  a2 k2   an kn
where the a’s are constants and the k’s are
k1  f ti , yi 
k2  
f ti  p1h, yi  q11k1h
k3  f ti  p2 h, yi  q21k1h  q22 k2 h
kn  f ti  pn1h, yi  qn1,1k1h  qn1,2 k2 h 
where the p’s and q’s are constants.
 qn1,n1kn1h
Classical Fourth-Order RungeKutta Method
• The most popular RK methods are fourthorder, and the most commonly used form is:
1
yi1  yi  k1  2k2  2k3  k4 h
6
where:
k1  f t i , yi 

 1
1 
k2  f t i  h, yi  k1h 
 2
2 
 1

1
k3  f t i  h, yi  k2 h 
 2

2
k4  f t i  h, yi  k3h
Systems of Equations
• Many practical problems require the solution of a
system of equations:
dy1
 f1 t, y1, y2 , , yn 
dt
dy2
 f2 t, y1, y2 , , yn 
dt
dyn
 fn t, y1, y2 , , yn 
dt
• The solution of such a system requires that n initial
conditions be known at the starting value of t.

Solution Methods
• Single-equation methods can be used to
solve systems of ODE’s as well; for example,
Euler’s method can be used on systems of
equations - the one-step method is applied
for every equation at each step before
proceeding to the next step.
• Fourth-order Runge-Kutta methods can also
be used, but care must be taken in
calculating the k’s.
MATLAB RK4 Code
Chapter 21 Objectives
• Understanding how the Runge-Kutta Fehlberg
methods use RK methods of different orders to
provide error estimates that are used to adjust step
size.
• Familiarizing yourself with the built-in MATLAB
function for solving ODEs.
• Learning how to adjust options for MATLAB’s ODE
solvers.
• Learning how to pass parameters to MATLAB’s
ODE solvers.
• Understanding what is meant by stiffness and its
implications for solving ODEs.
Adaptive Runge-Kutta Methods
• The solutions to some ODE
problems exhibit multiple time
scales - for some parts of the
solution the variable changes
slowly, while for others there are
abrupt changes.
• Constant step-size algorithms
would have to apply a small stepsize to the entire computation,
wasting many more calculations
on regions of gradual change.
• Adaptive algorithms, on the other
hand, can change step-size
depending on the region.
Approaches to Adaptive Methods
• There are two primary approaches to
incorporate adaptive step-size control:
– Step halving - perform the one-step algorithm
two different ways, once with a full step and once
with two half-steps, and compare the results.
– Embedded RK methods - perform two RK
iterations of different orders and compare the
results. This is the preferred method.
MATLAB Functions
• MATLAB’s ode23 function uses second- and thirdorder RK functions to solve the ODE and adjust
step sizes.
• MATLAB’s ode45 function uses fourth- and fifthorder RK functions to solve the ODE and adjust
step sizes. This is recommended as the first
function to use to solve a problem.
• MATLAB’s ode113 function is a multistep solver
useful for computationally intensive ODE functions.
Using ode Functions
• The functions are generally called in the same way;
ode45 is used as an example:
[t, y] = ode45(odefun, tspan, y0)
– y: solution array, where each column represents one of
the variables and each row corresponds to a time in the t
vector
– odefun: function returning a column vector of the righthand-sides of the ODEs
– tspan: time over which to solve the system
• If tspan has two entries, the results are reported for those times
as well as several intermediate times based on the steps taken
by the algorithm
• If tspan has more than two entries, the results are reported only
for those specific times
– y0: vector of initial values
Example - Predator-Prey
dy1
1.2y1  0.6y1y2
• Solve:
dt
•
dy2
 0.8y2  0.3y1y2
dt
with y1(0)=2 and y2(0)=1 for 20 seconds
predprey.m M-file:
function yp = predprey(t, y)

yp = [1.2*y(1)-0.6*y(1)*y(2);…
-0.8*y(2)+0.3*y(1)*y(2)];
• tspan = [0 20];
y0 = [2, 1];
[t, y] = ode45(@predprey, tspan, y0);
figure(1); plot(t,y); figure(2); plot(y(:,1),y(:,2));
ODE Solver Options
• Options to ODE solvers may be passed as
an optional fourth argument, and are
generally created using the odeset function:
options=odeset(‘par1’, ‘val1’, ‘par2’, ‘val2’,…)
• Commonly used parameters are:
– ‘RelTol’: adjusts relative tolerance
– ‘AbsTol’: adjusts absolute tolerance
– ‘InitialStep’: sets initial step size
– ‘MaxStep’: sets maximum step size (default:
one tenth of tspan interval)
Multistep Methods
• Multistep methods are based on
the insight that, once the
computation has begun,
valuable information from the
previous points exists.
• One example is the non-selfstarting Heun’s Method, which
has the following predictor and
corrector equations:
0
m
(a) Predictor yi1
 yi1
 f ti , yim 2h
m
j1
f
t
,
y

f
t
,
y



h
i
i
i1
i1
j
(b) Corrector yi1
 yim 
2
Stiffness
• A stiff system is one involving rapidly changing components
together with slowly changing ones.
• An example of a single stiff ODE is:
dy
 1000y  3000 2000et
dt
whose solution if y(0)=0 is:
y  3 0.998e1000t  2.002et


MATLAB Functions for Stiff
Systems
• MATLAB has a number of built-in functions
for solving stiff systems of ODEs, including
ode15s, ode23, ode23t, and ode23tb.
• The arguments for the stiff solvers are the
same as those for previous solvers.
Download
Related flashcards

Numerical software

79 cards

Create Flashcards