Eigendecomposition, Eigenvectors and Eigenvalues

Eigendecomposition

Eigendecomposition(pronounced eye-gan) of a matrix is a type of decomposition that involves decomposing a square matrix into a set of eigenvectors and eigenvalues.

Note: Decomposition does NOT result in a compression of the matrix; instead, it breaks it down into constituent parts to make certain operations on the matrix easier to perform.

$A$ : Parent square matrix $Q$ : Matrix comprised of the eigenvectors $\Lambda$: Diagonal matrix comprised of the eigenvalues $Q^{-1}$: Inverse of the matrix comprised of the eigenvectors

Eigenvectors and Eigenvalues

An eigenvector is a vector whose direction remains unchanged when a linear transformation is applied to it. The factor by which an eigenvector is stretched or squished after a linear transformation is known as eigenvalue.

The eigenvalue lambda tells whether the special vector x is stretched or shrunk or reversed(negative value) or left unchanged.

Eigenvectors (red) do not change direction when a linear transformation is applied to them

Eigenvector of a matrix must satisfy the following equation:

$$A . \vec{v} = \lambda . \vec{v} \newline Matrix-vector\hspace{0.1cm}multiplication\hspace{0.2cm}Vs\hspace{0.2cm}Scalar-vector\hspace{0.1cm}multiplication \newline Convert\hspace{0.1cm}scalar \lambda\hspace{0.1cm}to\hspace{0.1cm}Matrix\hspace{0.1cm}: \lambda.I(Identity Matrix) \newline A . \vec{v} = (\lambda I) . \vec{v}\newline A . \vec{v} - (\lambda I) . \vec{v} = \vec{0} \newline (A - \lambda I) . \vec{v} = \vec{0}$$

$A$ : Parent square matrix $\vec{v}$ : Eigenvector of the matrix $\lambda$: Scalar eigenvalue

For eigenvector $\vec{v}$ to be non-zero, the only possible solution where a matrix($A$-$\lambda$$I$) and vector ($\vec{v}$) multiplication that results in zero is during squishification. i.e det(matrix) = 0 or $det(A - \lambda I) = 0$

Positive and negative definite matrix

A matrix that has only positive eigenvalues is referred to as a positive definite matrix, whereas if the eigenvalues are all negative, it is referred to as a negative definite matrix.

Calculate an eigendecomposition with NumPy

from numpy import array
from numpy.linalg import eig

A = array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
eigenvalues, eigenvectors = eig(A)

print(A)
print(eigenvalues)
print(eigenvectors)

[[1 2 3]
 [4 5 6]
 [7 8 9]]
 
[ 1.61168440e+01 -1.11684397e+00 -9.75918483e-16]

[[-0.23197069 -0.78583024  0.40824829]
 [-0.52532209 -0.08675134 -0.81649658]
 [-0.8186735   0.61232756  0.40824829]]

Confirm a vector is an eigenvector

$$A . \vec{v} = \lambda . \vec{v}$$

from numpy import array
from numpy.linalg import eig
from numpy import dot

A = array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
eigenvalues, eigenvectors = eig(A)

# Verify eigenvector equation for the first eigenvalue and eigenvector
firstEigenValue = eigenvalues[0]
firstEigenVector = eigenvectors[:, 0]

# Eigenvector equation
# A . v = lambda . v
lhs = dot(A, firstEigenVector)
rhs = firstEigenValue * firstEigenVector

print(lhs)
print(rhs)​

[ -3.73863537  -8.46653421 -13.19443305]
[ -3.73863537  -8.46653421 -13.19443305]

Reconstruct a matrix from eigenvectors and eigenvalues

from numpy import array
from numpy.linalg import eig
from numpy import dot
from numpy.linalg import inv
from numpy import diag

A = array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
eigenvalues, eigenvectors = eig(A)​

# A = Q . diag(eigenvalues) . Q^-1
Q = eigenvectors
diagEigenVals = diag(eigenvalues)
invQ = inv(Q)

compose = Q.dot(diagEigenVals).dot(invQ)
print(A)
print(compose)​

[[1 2 3]
 [4 5 6]
 [7 8 9]]
[[1. 2. 3.]
 [4. 5. 6.]
 [7. 8. 9.]]

Link:

• Video: 3Blue1Brown
• Video: What are Eigenvalues and Eigenvectors?
• Computer Vision for dummies: Eigenvalues and Eigenvectors
• Machine Learning Mastery