2023-02-15 Projections, rotations, and reflections

Last time

• Algebra of linear transformations

• Polynomial evaluation and fitting

Today

• Orthogonality

• Projections

• Rotations

• Reflections

• Oh my!

using LinearAlgebra
using Plots
using Polynomials
default(linewidth=4, legendfontsize=12)

function vander(x, k=nothing)
    if isnothing(k)
        k = length(x)
    end
    m = length(x)
    V = ones(m, k)
    for j in 2:k
        V[:, j] = V[:, j-1] .* x
    end
    V
end

vander (generic function with 2 methods)

Matrices as linear transformations

Linear algebra is the study of linear transformations on vectors, which represent points in a finite dimensional space. The matrix-vector product $y=Ax$ is a linear combination of the columns of $A$. The familiar definition,

$$y_i=\sum _j{A}_{i,j}{x}_j$$

can also be viewed as

$$\begin{array}{r}y=[A_{:,1}|A_{:,2}|\cdots ]\left[\begin{array}{c}{x}_1\\ x_2\\ ⋮\end{array}\right]=[A_{:,1}]x_1+[A_{:,2}]x_2+\cdots .\end{array}$$

Inner products and orthogonality

The inner product

$$x^{T}y=\sum _i{x}_i{y}_i$$

of vectors (or columns of a matrix) tell us about their magnitude and about the angle. The norm is induced by the inner product,

$$\Vert x\Vert =\sqrt{x^{T}x}$$

and the angle $\theta$ is defined by

$$\cos \theta =\frac{x^{T}y}{\Vert x\Vert \Vert y\Vert }.$$

Inner products are bilinear, which means that they satisfy some convenient algebraic properties

$$\begin{array}{r}\begin{array}{rl}(x+y{)}^{T}z& =x^{T}z+y^{T}z\\ x^T(y+z)& =x^{T}y+x^{T}z\\ (\alpha x{)}^T(\beta y)& =\alpha \beta x^{T}y\end{array}.\end{array}$$

Examples with inner products

x = [0, 1]
y = [1, 2]
@show x' * y
@show y' * x;

x' * y = 2
y' * x = 2

ϕ = pi/6
y = [cos(ϕ), sin(ϕ)]
@show x' * y
cos_θ = x'*y / (norm(x) * norm(y))
@show cos_θ
@show cos(ϕ-pi/2);
acosd(cos_θ)

x' * y = 0.49999999999999994
cos_θ = 0.49999999999999994
cos(ϕ - pi / 2) = 0.4999999999999999

60.00000000000001

Polynomials can be orthogonal too!

x = LinRange(-1, 1, 50)
A = vander(x, 4)
M = A * [.5 0 0 0; # 0.5
         0  1 0 0;  # x
         0  0 1 0]' # x^2
scatter(x, M, legend=:none)
plot!(x, 0*x, label=:none, color=:black)

• Which inner product will be zero?

  • Which functions are even and odd?

Polynomial inner products

M[:,1]' * M[:,2]

-2.220446049250313e-16

acosd(M[:,1]' * M[:,3] / (norm(M[:,1]) * norm(M[:,3])))

41.79321606483336

M[:,2]' * M[:,3]

-4.440892098500626e-16

Geometry of Linear Algebra

We’d like to develop intuition about transformations on vectors. We’ll start with a peanut-shaped cluster of points and see how they transform.

default(aspect_ratio=:equal)

function peanut()
    theta = LinRange(0, 2*pi, 50)
    r = 1 .+ .4*sin.(3*theta) + .6*sin.(2*theta)
    x = r .* cos.(theta)
    y = r .* sin.(theta)
    x, y
end
x, y = peanut()
scatter(x, y)

We’ll group all these points into a $2\times n$ matrix $X$. Note that multiplication by any matrix $A$ is applied to each column separately, i.e.,

$$A\underset{X}{\underbrace{[{\mathbf{x}}_1|{\mathbf{x}}_2|\cdots ]}}=\underset{AX}{\underbrace{[A{\mathbf{x}}_1|A{\mathbf{x}}_2|\cdots ]}}$$

X = [x y]'
size(X)

(2, 50)

Inner products and projections

Consider the operation $f(x)=v(v^{T}x)$ for some vector $v$. We’ll apply this same operation to every point in the original cluster $X$.

function tplot(X, Y)
    "Plot a transformation from X to Y"
    scatter(X[1,:], X[2,:], label="X")
    scatter!(Y[1,:], Y[2,:], label="Y")
end

v = [1, 0]
tplot(X, v * (v' * X))

Are the parentheses important? Why or why not?

$$v(v^{T}X)\stackrel{?}{=}v{v}^{T}X$$

Questions

v = [1, 1] / sqrt(2)

# What happens if v is not normalized?
# Discuss with your group and make a prediction before uncommenting and running the line below.
tplot(X,  v * v' * X)

# Vector transformations are "nice" if norm(v) = 1

theta = pi * .7
v = [cos(theta), sin(theta)]
@show norm(v)
tplot(X, v * v' * X)

norm(v) = 0.9999999999999999

A related projector

Compare to the operation $I-v{v}^T$. Note that $(I-v{v}^T)X=X-v{v}^{T}X$.

tplot(X, X - v * v' * X)

# This also works since I is the automatically-sized identity.
#tplot(X, (I - v * v') * X)

Discuss

Discuss with your group what the vector $v$ looks like in these diagrams.

• What is the null space of each transformation?

• What is the range of each transformation?

• For each figure, what happens if you apply the transformation $v{v}^T$ to the red points?

• What about $I-v{v}^T$?

Givens Rotation

We can rotate the input using a $2\times 2$ matrix, parametrized by $\theta$.

function givens(theta)
    s = sin(theta)
    c = cos(theta)
    A = [c -s; s c]
    A
end

givens(0)

2×2 Matrix{Float64}:
 1.0  -0.0
 0.0   1.0

givens(1*π)

2×2 Matrix{Float64}:
 -1.0          -1.22465e-16
  1.22465e-16  -1.0

tplot(X, givens(.8) * X)

• What is the null space and range of the Givens matrix?

• Is the Givens matrix orthogonal? What does that mean?

Reflection

Using a construct of the form $I-\alpha v{v}^T$, create a reflection across the plane defined by unit normal vector $v=[\cos \theta ,\sin \theta ]$. Try different values of $\alpha$ and convince yourself of why it works or sketch a diagram for the projections $v{v}^T$ and $I-v{v}^T$ and decide how to modify it to make a reflection.

function reflect(theta)
    v = [cos(theta), sin(theta)]
    I - ??? * v * v'
end

tplot(X, reflect(pi/2) * X)

using Test
@test mapslices(norm, X; dims=1) ≈ mapslices(norm, reflect(0.3) * X; dims=1)
@test X ≈ reflect(0.5) * reflect(0.5) * X

Test Passed
  Expression: X ≈ reflect(0.5) * reflect(0.5) * X
   Evaluated: [1.0 1.291607690975099 … 0.6919723366713914 0.9999999999999993; 0.0 0.16653437041080127 … -0.08921995295821092 -2.449293598294705e-16] ≈ [1.0 1.291607690975099 … 0.6919723366713914 0.9999999999999993; 0.0 0.16653437041080127 … -0.08921995295821092 -2.449293598294705e-16]

• Where is the vector $v$ on the figure above?

• Does the reflection matrix change if you replace $v(\theta )$ with $v(\theta +\pi )$? Why or why not?

• What happens if you reflect twice?

• What is the null space and range of the reflection matrix?

• Is the reflection matrix orthogonal?