Eigenvalues are a fundamental concept in linear algebra with numerous applications in various fields, such as chemistry, physics, engineering, and computer science. They are used to understand the behavior of linear transformations and matrices.

Eigenvalues help in predicting the behavior of electrons in atoms and molecules, shedding light on chemical reactions, spectroscopy, and the overall behavior of matter at the quantum level.

The Schrödinger equation, a fundamental equation in quantum mechanics, relies on eigenvalues and eigenvectors to describe the energy levels and corresponding wave functions of electrons in atomic and molecular systems. These energy levels, represented by eigenvalues, dictate the quantization of energy, which, in turn, influences chemical bonding, molecular geometry, and spectral properties.

Outline

## What Are Eigenvalues?

Eigenvalues are numbers associated with a square matrix. They provide valuable information about the matrix’s behavior when it is used to transform vectors. The eigenvalues represent the scaling factor by which a vector is stretched or compressed when multiplied by the matrix.

### Basics Terms

Some foundational concepts related to eigenvalues are discussed here:

**Square Matrix**: Eigenvalues are calculated for square matrices, which have an equal number of rows and columns.**Eigenvectors**: Eigenvectors are the associated vectors to eigenvalues. These vectors represent the directions in which the original vector is stretched or compressed.

## The Eigenvalue Equation

To find the eigenvalues of a square matrix A, you must solve the following equation:

A v = λ v

In this equation:

- A is the square matrix
- v is the eigenvector
- λ (lambda) is the eigenvalue we want to find

### Rearranging the Equation

To isolate λ, rearrange the equation:

(A – λ * I) * v = 0

Where I is the identity matrix with the same dimensions as A.

### Finding Determinant

Calculate the determinant of (A – λ * I). This is known as the characteristic polynomial of the matrix. The equation is:

det(A – λ * I) = 0

This polynomial equation will have λ as a variable.

### Solve for λ

Solve the characteristic polynomial equation for λ. The solutions to this equation are the eigenvalues of the matrix A. You may use various methods such as factorization or numerical methods to find the eigenvalues.

### Repeat for Each Eigenvalue

If the matrix A has n eigenvalues, you will need to repeat steps 3 to 5 for each λ, finding all the eigenvalues.

### Verify the Eigenvalues

It’s important to check your results. To verify the eigenvalues, substitute each eigenvalue into the original equation (A * v = λ * v) and ensure that the equation holds true. This step helps confirm the accuracy of your calculations.

## Calculating Eigenvalues Example

Consider the following 2×2 matrix A:

A = [4 2] [1 3]

**Step 1: Start with the Eigenvalue Equation**

The eigenvalue equation for a 2×2 matrix A is:

A * v = λ * v

**Step 2: Rearrange the Equation**

To isolate λ, rearrange the equation:

(A - λ I) * v = 0

**Step 3: Calculate the Characteristic Polynomial**

Now, let’s form the characteristic polynomial and set it equal to zero:

det(A - λ I) = det([4-λ 2] [1 3-λ]) = 0

**Step 4: Solve for λ**

We now solve this quadratic equation for λ:

λ^2 - 7λ + 8 = 0

Factoring the quadratic equation:

(λ - 1)(λ - 8) = 0

Now, we have two potential eigenvalues: λ₁ = 1 and λ₂ = 8.

**Step 5: Verify the Eigenvalues**

To verify the eigenvalues, substitute them back into the original equation:

For λ₁ = 1:

(A - 1I) * v = [3 2] * v = 0 [1 2]

For λ₂ = 8:

(A - 8I) * v = [-4 2] * v = 0 [1 -5]

You can use techniques such as Gaussian elimination or matrix inversion to find the eigenvectors corresponding to each eigenvalue.

In this example, the eigenvalues of the 2×2 matrix A are λ₁ = 1 and λ₂ = 8. Eigenvalues provide information about how the matrix scales and transforms vectors, which can have various applications in mathematical modeling and science.

### Practical Tips

**Use software or calculators:**Calculating eigenvalues by hand can be complex, especially for large matrices. Utilize mathematical software or calculators for faster and more accurate results.**Understand the geometric interpretation:**Eigenvalues represent the scaling factors of the associated eigenvectors. Larger eigenvalues indicate more significant scaling.**Eigenvalues of special matrices:**Certain matrices, like symmetric or Hermitian matrices, have special properties that simplify the calculation of eigenvalues.

## Eigenvalues and Eigenvectors

Eigenvalues and eigenvectors are intimately related concepts in linear algebra and matrix theory. They go hand in hand when analyzing the properties and behavior of matrices.

- Eigenvalues and eigenvectors are intertwined through the eigenvalue equation, which is expressed as A * v = λ * v, where A is the matrix, λ is the eigenvalue, and v is the eigenvector.
- Eigenvalues determine the scaling factor (λ) associated with the corresponding eigenvectors (v). In other words, the eigenvalues quantify how much the eigenvectors are scaled when A is applied to them.
- Eigenvectors, in turn, provide the directions or patterns along which this scaling occurs. They describe the “shape” of the transformation represented by matrix A.

## Properties of Eigen Values

Here are some important properties of eigenvalues:

**Eigenvalues Are Scalar Values**

Eigenvalues are real or complex numbers. They represent the scaling factor by which an eigenvector is stretched or compressed when a matrix operates on it. This scaling factor can be positive, negative, or zero.

**Eigenvalues of Identity Matrix**

The eigenvalues of an identity matrix are all equal to 1. This property is a direct consequence of the definition of an identity matrix, which has no effect on the vectors it operates on.

**Eigenvalues of Diagonal Matrix**

The eigenvalues of a diagonal matrix are simply the diagonal elements of the matrix itself. This is because diagonal matrices only scale the corresponding eigenvectors by the values on their diagonal.

**Eigenvalues of Transpose**

The eigenvalues of a matrix and its transpose are the same. This property holds for both real and complex matrices.

**Eigenvalues of Inverse Matrix**

The eigenvalues of the inverse of a matrix A are the reciprocals of the eigenvalues of A. In mathematical terms, if λ is an eigenvalue of A, then 1/λ is an eigenvalue of A^{-1}.

**Eigenvalues of Sum and Product**

For two matrices A and B, the eigenvalues of their sum (A + B) are not necessarily the sums of their eigenvalues, and the eigenvalues of their product (AB) are not necessarily the products of their eigenvalues. However, there are some special cases and conditions where specific relationships can be established.

**Eigenvalues and Matrix Trace**

The sum of the eigenvalues of a matrix A is equal to the trace of the matrix. In mathematical notation, if λ₁, λ₂, …, λn are the eigenvalues of A, then Tr(A)

= λ₁ + λ₂ + … + λn.

**Eigenvalues and Matrix Determinants**

The product of the eigenvalues of a matrix A is equal to the determinant of the matrix. In mathematical notation, if λ₁, λ₂, …, λn are the eigenvalues of A, then;

det(A) = λ₁ * λ₂ * … * λn

**Eigenvalues and Matrix Rank**

The rank of a matrix is related to its eigenvalues. If a matrix has n nonzero eigenvalues, then its rank is equal to n. This property is useful in solving systems of linear equations.

**Eigenvalues and Symmetric Matrices**

Symmetric matrices have real eigenvalues, and their eigenvectors can be chosen to be orthogonal. This property has many applications in physics and engineering, particularly in the context of the diagonalization of symmetric matrices.

## Concepts Berg

**How do I find eigenvalues and eigenvectors of a matrix?**

To find eigenvalues, you need to solve the characteristic polynomial equation, and eigenvectors can be found by solving linear systems of equations using eigenvalues. Alternatively, you can use computational tools or software for practical calculations.

**Do all matrices have eigenvalues and eigenvectors?**

Every square matrix has at least one eigenvalue and an associated eigenvector. However, not all matrices have a complete set of linearly independent eigenvectors, which is required for diagonalization.

**Are all matrices diagonalizable?**

Not all matrices are diagonalizable. A matrix is diagonalizable if it has a full set of linearly independent eigenvectors. Diagonalizable matrices are often useful for simplifying mathematical operations.

**How are eigenvalues and eigenvectors used in practical applications?**

Eigenvalues and eigenvectors are used in various fields. In physics, they help describe the behavior of quantum systems. In engineering, they are used for structural analysis and control systems. In data analysis, they can be used for dimensionality reduction and feature extraction.

**Are eigenvectors unique for a given eigenvalue?**

Eigenvectors are not unique for a given eigenvalue. Different linearly independent eigenvectors can be associated with the same eigenvalue.

**Reference links**

- Eigenvalues and Eigenvectors (swarthmore.edu)
- The Schrödinger Equation is an Eigenvalue Problem (Libretexts Chemistry)