# MA213: Numerical Analysis

Unit 2: Polynomials and Polynomial Interpolation   Some say that 75% of applied mathematics is polynomials (the other 75% being linear algebra!).  We will therefore use this unit to review some things you already know about polynomials and (hopefully) introduce some new ideas as well.  Here, we are primarily interested in using polynomials to approximate unknown functions, more complicated functions, or just sets of points.  Finally, we will write a program to find a cubic polynomial that passes through two points with predefined slopes.

Unit2 Learning Outcomes
Upon successful completion of this unit, the student will be able to:
- Carefully define and understand a polynomial as a vector, its power as an approximation tool, and several ways of representing it.  - Approximate functions and data sets with polynomials with an understanding of the errors associated with this approximation.

2.1 Polynomial Functions   2.1.1 Definition of a Polynomial   - Reading: University of Arkansas: Mark Arnold’s “Polynomials” and “Polynomial Evaluation” Link: University of Arkansas: Mark Arnold’s “Polynomials” (PDF) and “Polynomial Evaluation” (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of the reading. Carefully read the pdf’s.  Refer to your linear algebra material, if needed, and determine the dimension of the vector space P_n of all polynomials of degree n or less.  Write down two distinct bases for P_n.  Why do you think Horner’s method is also called synthetic division?

This resource should take approximately 3 hours to complete.

2.1.2 Weierstrauss Approximation Theorem   - Reading: University of Arkansas: Mark Arnold’s “The Taylor Polynomial” Link: University of Arkansas: Mark Arnold’s “The Taylor Polynomial” (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of the reading. The last two readings refer to the Weierstrauss Approximation Theorem.  In your own words, describe what the theorem implies about the ability of polynomials to approximate continuous functions.  What do you think (i) lots of wiggles, (ii) a sharp corner, and/or (iii) a large interval of approximation, means to the degree of an approximating polynomial?

This resource should take approximately 1 hours to complete.

2.1.3 Vector Spaces of Polynomials   - Reading: University of Arkansas: Mark Arnold’s “Polynomials are Vectors” Link: University of Arkansas: Mark Arnold’s “Polynomials are Vectors" (PDF)

`````` Instructions: Click on the link above, then select the appropriate
for the inner product of two polynomials depends on an interval and
a weight function.  Therefore there are many popular types of
polynomial inner product, and we will choose an appropriate inner
product depending on our application.  An inner product in a vector
space defines a natural notion of length and angle.  Find a Calculus
III formula that relates an inner product, vector lengths, and an
angle.  How would you define the angle between two polynomials?  We
skipped some algebra when finding the best polynomial approximation
to a function f; fill in the gaps.

This resource should take approximately 2 hours to complete.

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2.1.4 Fundamental Theorem of Algebra   - Reading: University of Arkansas: Mark Arnold’s “Fundamental Theorem of Algebra Link: University of Arkansas: Mark Arnold’s “Fundamental Theorem of Algebra" (PDF)

`````` Instructions: Click on the link above, then select the appropriate
you see, the Fundamental Theorem of Algebra (FTA) is an existence
theorem.  It does not tell us how to factor a polynomial.  We will
discuss methods for factoring polynomials later.  The FTA does give
us yet another way to represent a polynomial; describe how it allows
us to represent a polynomial of degree n having real coefficients
using n+1 real numbers (some possibly repeated).   Find a formula
for the derivative of a polynomial p(x) in terms of its roots (hint:
use the product rule).

This resource should take approximately 2 hours to complete.

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2.2 Polynomial Interpolation   2.2.1 Taylor Polynomials   - Reading: University of Arkansas: Mark Arnold’s “The Taylor Polynomial”, “Rate of Convergence” and “A Taylor Polynomial Picture” University of Arkansas: Mark Arnold’s “The Taylor Polynomial”, “Rate of Convergence” and “A Taylor Polynomial Picture”

Link: University of Arkansas: Mark Arnold’s “The Taylor Polynomial” (PDF), “Rate of Convergence” (PDF) and “A Taylor Polynomial Picture” (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of each reading. In many ways the Taylor Polynomial is the central tool of mathematical physics, and hence applied mathematics, and engineering.  Find a calculus or physics text or website that introduces the differential equation modeling the simple pendulum.  It is fundamentally different than spring (or simple harmonic) motion.  We cannot solve the differential equation for the pendulum (the solution is impossible to write even as an integral of functions we know).  Show how the small angle approximation for the pendulum is a Taylor polynomial approximation.  You should memorize the formula for the Taylor polynomial P(x) of degree n about x_0 for an arbitrary function f(x), including the error term.  Show that P{(k)}(x_0) = f{(k)}(x_0) for k=0,1,…,n.  Does this property define P?

This resource should take approximately 4 hours to complete.

• Lecture: YouTube: University of South Florida: Autar Kaw’s “Taylor’s Theorem Revisited” Link: YouTube: University of South Florida: Autar Kaw’s “Taylor’s Theorem Revisited” (YouTube)

Instructions: Click on the link above, then watch the video lectures in the chapter listed above.  Here there are 4 lectures that have been split into 4 videos.

This resource should take approximately 1/2 hour to complete.

2.2.2 Lagrange Interpolation   - Reading: University of Arkansas: Mark Arnold’s “Lagrange Interpolation” and “FFT Tease” Link: University of Arkansas: Mark Arnold’s “Lagrange Interpolation" (PDF) and "FFT Tease" (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of the reading.  Carefully read the pdf.  Yes, we have yet another way to represent a polynomial.  Show that if the nodes are distinct, then there is exactly one polynomial of degree <= n passing through n+1 knots (hint: we know that there is at least one: the Lagrange interpolator P.  Assume there is another, say Q.  Think about the zeros if (P-Q)(x)…)  This existence and uniqueness means n+1 knots with distinct nodes represents a polynomial: the Lagrange interpolator.   The Vandermonde matrix is usually a hard matrix to work with, so the Lagrange (and other) pictures are applied more often.  But there is one place where the Vandermonde picture rules:  If the nodes are roots of unity (a set of n+1 points, including 1, which are equally spaced around the circle of radius 1 in the complex plane), then the Vandermonde picture is a discrete Fourier transform (DFT); we mention this because you may have heard of the FFT (one of the most important algorithms ever discovered).  The FFT is simply a fast way to solve this special Vandermonde system.

This resource should take approximately 2 hours to complete.

Instructions: Click on the link above, then watch the video lectures in the chapter listed above.  In this case there are 4 lectures that have been split into 6 videos.

This resource should take approximately 50 minutes to complete.

• Lecture: YouTube: University of South Florida: Duc Nguyen’s “Discrete Fourier Transform” Link: YouTube: University of South Florida: Duc Nguyen’s “Discrete Fourier Transform” (YouTube)

Instructions: Click on the link above, then watch the video lectures in the chapter listed above.  In this case there are 3 lectures that have been split into 7 videos.

This resource should take approximately 1.6 hours to complete.

2.2.3 Hermite Interpolation   - Reading: University of Arkansas: Mark Arnold’s “Hermite Interpolation” Link: University of Arkansas: Mark Arnold’s “Hermite Interpolation” (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of the reading. Carefully read the pdf.   The Hermite interpolator really shines when we know not only the knots, but also the slopes of the curve at each knot (for example, when solving an initial value problem).  What degree Hermite interpolator is required if there are two nodes?  Go ahead and work out the formula for the Hermite interpolator for (x_0,y_0), (x_0,y’_0), (x_1,y_1) and (x_1,y’_1).  This little polynomial is a real workhorse in computer aided design and manufacturing and differential equations!

This resource should take approximately 2 hours to complete.

2.2.4 General Polynomial Interpolation   - Reading: University of Arkansas: Mark Arnold’s “Kissing Polynomials” Link: University of Arkansas: Mark Arnold’s “Kissing Polynomials” (PDF)

Instructions: Click on the link above, then select the appropriate link to download a pdf of the reading. Carefully read the pdf.  It may be that we include this section for aesthetic (or theoretical) reasons.  Well, communicating an aesthetic is an important component of a mathematics course and we don’t want you underserved!   Explain how the Taylor polynomial, the Lagrange polynomial, and the Hermite polynomial are all special cases of the osculating polynomial (precisely what are the interpolation conditions for each?).

This resource should take approximately 1 hour to complete.

2.3 Programming Case Study: A 2-Node Hermite Interpolator   2.3.1 Principles of Good Programming   - Reading: University of Arkansas: Mark Arnold’s “Some Programming Principles” Link: University of Arkansas: Mark Arnold’s “Some Programming Principles” (PDF)

`````` Instructions: Click on the link above, then select the appropriate
generally agreed upon principles of good code.  How you balance
these is up to you, but they are all worth considering when writing
any program.

This resource should take approximately 1 hour to complete.

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2.3.2 The Interpolator   - Reading: University of Arkansas: Mark Arnold’s “Constructing a Hermite Cubic Interpolator” Link: University of Arkansas: Mark Arnold’s “Constructing a Hermite Cubic Interpolator” (PDF)

`````` Instructions: Click on the link above, then select the appropriate
2-node Hermite interpolator in both the Lagrange and Vandermonde
picture.  Write a program in Octave to compute either form of the
Hermite cubic h(x) (you choose). Input will be the nodes, x\_0 and
x\_1, the function values y\_0 and y\_1, and the slopes y’\_0 and
y’\_1, and output will be either the coefficients of h in the
standard ordered basis (Vandermonde picture), or the polynomials
H\_{10}, H\_{11}, hat{H)\_{10} and hat{H}\_{11} in standard form.
Use your program to plot h, and by all means: play with your
program!  What shapes can it make?  Can you break it?  What shapes
are sensitive to small changes, etc.

This resource should take approximately 4 hours to complete.

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2.4 Of Things Not Covered   - Reading: University of Arkansas: Mark Arnold’s “Interpolating Splines”, “Least Squares Approximation”, “Polynomial Least Squares Example”, and “Lagrange Interpolation Example” Link: University of Arkansas: Mark Arnold’s “Interpolating Splines” (PDF), “Least Squares Approximation” (PDF), “Polynomial Least Squares Example” (PDF), and “Lagrange Interpolation Example” (PDF)

Instructions: Click on the link above, then select the appropriate links to download the PDFs of the reading.  A drawback of working with polynomials is that they naturally oscillate, especially for high degree (a polynomial of degree n has n-1 critical points).  Furthermore, polynomials cannot form true corners, and require very high degree to approximate a corner well.  Polynomial spline functions are simply piecewise polynomial functions.  Polynomials can be pieced together smoothly or to form corners.

On the other hand, one might ask if interpolation is always the best approach to representing data.  For example, if one suspects that there may be errors in the knot values, then a useful alternative to interpolation is least squares approximation.

This resource should take approximately 5 hours to complete.

Instructions: Click on the link above, then watch the video lectures in the chapter listed above.  In this case there are 4 lectures that have been split into 6 videos.

This resource should take approximately 50 minutes to complete.

Unit 2 Assessment   - Assessment: The Saylor Foundation's "Unit 2 Assessment" Link: The Saylor Foundation's "Unit 2 Assessment" (PDF) and  “Guide to Responding” (PDF).

`````` Instructions: <span
style="font-family: arial, sans-serif; font-size: 12.727272033691406px; ">Carefully
answer the three questions below.  You may consider them essay
questions, but back up or explain with formulas and/or theorems as
needed.  When you are finished, you should check yourself by looking
at the response guide.  You may not have all of the points given in
the response guide, or you may have noted something that was not
included in the guide, and that may be just fine, but this is a time
to reflect on differences between your response and the guide
response and possibly review some material. </span>This assessment
should take about 30 minutes to complete.
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