2023T1 MATH2089 – Numerical Methods Tutorial Problems Week 2
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2023T1 MATH2089 – Numerical Methods Tutorial Problems (MATLAB*)
Week 2
*Python version for this tutorial is available as a Jupyter notebook from the course Moodle page. Topic: §1 Numerical Computing (continued), §2 Linear Systems
1. [M] The easiest way to deine a simple function in Matlab is to use an anonymous function.
(a) Write an anonymous function myf to evaluate f(x) = e −x2 .
(b) Create a vector x of 21 equally spaced points on [−3, 3] (Hint: linspace).
(c) Use your anonymous function to plot f on [ −3, 3]. Does it look correct?
(d) Repeat using 101 points.
(e) Zoom in (Magnifying glass with a + at the top of the igure window) around x = 0.
2. [M] The standard expressions for the solutions of the quadratic equation ax2 + bx + c = 0 are
−b +^b2 − 4ac (♠)
r2 = (♥)
r1 = 2c (♦)
r2 = (♣)
If a and c are much smaller in magnitude compared to b, then
. When b > 0, the numerically preferable expressions are (♦) and (♥).
. When b < 0, the numerically preferable expressions are (♠) and (♣).
(a) Write and test a Matlab function quadsolve to solve the quadratic in a numerically stable way. The function must be in the ile quadsolve.m. The speciications are
[r1, r2] = quadsolve(a, b, c)
Include comments at the beginning of your function giving the calling sequence and the purpose of the function.
(b) Write a Matlab M-ile to test your function quadsolve on a range of examples, including
i. x2 + 3x + 2 = 0
ii. 0 .01x2 + 2000x − 0 .001 = 0
iii . x2 − 1 = 0
iv . a = 1,b = 200,c = −0 .000015 (example in Wikipedia)
v. Any other cases you can think of?
3. [M] Let x = (5, −4, 0, −6)T . Calculate |x|1 , |x|2 and |x|∞ by hand and check your answers
4. [M] Let
A = l 0(3) −4(4) − 5(2) 」
5. [M] Consider the matrices
A = 「(l)1
3
−4
2」
26 ,
B = l−
「 1/16
−
5/32
」
1/32
(a) Verify that B = A−1 by showing that AB = I . What is |AB − I|1 ?
(b) Calculate |A|1 , |A−1|1 and κ 1 (A) by hand and check using Matlab.
(c) Calculate |A|∞ , |A−1|∞ and κ∞ (A) by hand and check using Matlab.
(d) You want to calculate the 2-norm condition number κ2 (A) = |A|2 |A−1|2 .
i. Use MATLAB’s cond to calculate κ2 (A).
ii. Use the Matlab function eig to calculate the eigenvalues of A and A−1 .
iii. How are the eigenvalues of A and A−1 related?
iv. If λ 1 ,λ2 ,λ3 denote the eigenvalues of A, calculate
max1≤i≤3 |λi | min1≤i≤3 |λi | .
Is this the same as κ2 (A)?
(e) The LU factorization produces a lower triangular matrix L and an upper triangular matrix U such that
PA = LU
where P is a permutation matrix reordering the rows of A (equations in a linear system).
i. Use the Matlab function lu to calculate the matrices P , L, and U .
ii. Calculate E = PA − LU and |E|2 .
(f) Use the LU factorization to solve the linear system Ax = b for b = (1, 2, 3)⊤ .
2023-04-29