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| Meta Title | Eigenvalues - Examples | How to Find Eigenvalues of Matrix? | |||||||||
| Meta Description | The eigenvalues of a matrix are the scalars by which eigenvectors change when some transformation is applied to them. Learn how to find the eigenvalues of 2x2 and 3x3 matrices using the characteristic equation with examples. | |||||||||
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| Boilerpipe Text | We come up with the terms eigenvalues and eigenvectors when we study the linear transformations. Some vectors change almost by their scale factors when some linear transformation (matrix) is applied to it. Such vectors are known as eigenvectors and the corresponding scale factors are known as eigenvalues of matrix.
Let us learn more about the eigenvalues of matrix along with their properties and examples.
1.
What are Eigenvalues of Matrix?
2.
How to Find Eigenvalues?
3.
Eigenvalues of a 2x2 Matrix
4.
Eigenvalues of a 3x3 Matrix
5.
Properties of Eigenvalues
6.
Applications of Eigenvalues
7.
FAQs on Eigenvalues
The
eigenvalues of matrix
are scalars by which some vectors (eigenvectors) change when the matrix (transformation) is applied to it. In other words, if A is a square matrix of order n x n and
v
is a non-zero column vector of order n x 1 such that
Av
=
λv
(it means that the product of A and
v
is just a scalar multiple of
v
), then the scalar (real number) λ is called an eigenvalue of the matrix A that corresponds to the eigenvector
v
.
The word "eigen" is from the German language and it means "characteristic", "proper", or "own". Thus, eigenvalues are also known as "characteristic values" (or) "characteristic roots" (or) "proper values", etc. The mathematical definition of eigenvalues is as shown below.
From the definition of eigenvalues, if λ is an eigenvalue of a square matrix A, then
A
v
= λ
v
If I is the
identity matrix
of the same order as A, then we can write the above equation as
A
v
= λ (I
v
) (because
v
= I
v
)
A
v
- λ (I
v
) = 0
Taking
v
as common factor,
v
(A - λI) = 0
This represents a
homogeneous system of linear equations
and it has a non-trivial solution only when the determinant of the coefficient matrix is 0.
i.e.,
|A - λI| = 0
This equation is called the
characteristic equation
(where |A - λI| is called the characteristic polynomial) and by solving this for λ, we get the eigenvalues. Here is the step-by-step process used to find the eigenvalues of a square matrix A.
Take the identity matrix I whose order is the same as A.
Multiply every element of I by λ to get λI.
Subtract λI from A to get A - λI.
Find its
determinant
.
Set the determinant to zero and solve for λ.
Let us apply these steps to find the eigenvalues of matrices of different orders.
Let us see the process of finding the eigenvalues of a 2x2 matrix with an example where we will find the eigenvalues of A =
[
5
4
1
2
]
. Let λ represents its eigenvalue(s). The identity matrix of order 2x2 is, I =
[
1
0
0
1
]
. Then
λI = λ
[
1
0
0
1
]
=
[
λ
0
0
λ
]
A - λI =
[
5
4
1
2
]
-
[
λ
0
0
λ
]
=
[
5
−
λ
4
1
2
−
λ
]
Its determinant is,
|A - λI| = (5 - λ) (2 - λ) - (1)(4)
= 10 - 5λ - 2λ + λ
2
- 4
= λ
2
- 7λ + 6
The characteristic equation is,
|A - λI| = 0
λ
2
- 7λ + 6 = 0
(λ - 6)(λ - 1) = 0
λ - 6 = 0; λ - 1 = 0
λ = 6; λ = 1
Thus, the eigenvalues of matrix A are 1 and 6.
Let us just observe the result of A - λI in the previous section. Isn't it just the matrix obtained by subtracting λ from all diagonal elements of A? Yes, so we will use this fact here and find the eigenvalues of 3x3 matrix A =
[
3
1
1
2
4
2
1
1
3
]
.
The characteristic equation is,
|A - λI| = 0
|
3
−
λ
1
1
2
4
−
λ
2
1
1
3
−
λ
|
= 0
(3 - λ) [(4 - λ)(3 - λ) - 2(1) ] - 1 [ 2(3 - λ) - 2(1) ] + 1 [2 (1) - 1 (4 - λ) ] = 0
(3 - λ) [12 - 4λ - 3λ + λ
2
- 2] - 6 + 2λ + 2 + 2 - 4 + λ = 0
(3 - λ) [10 - 7λ + λ
2
] - 6 + 3λ = 0
30 - 21λ + 3λ
2
- 10λ + 7λ
2
- λ
3
- 6 + 3λ = 0
-λ
3
+ 10λ
2
- 28λ + 24 = 0
Multiplying both sides by -1,
λ
3
- 10λ
2
+ 28λ - 24 = 0
This is a
cubic equation
. We will find one of its roots by trial and error and the other roots by synthetic division. By trial and error, we can see that λ = 2 satisfies the above equation (substitute and check whether we get 0 = 0). Now, using
synthetic division
,
Set the quotient equal to 0.
λ
2
- 8λ + 12 = 0
(λ - 6)(λ - 2) = 0
λ = 6; λ = 2
Thus, the eigenvalues of the given 3x3 matrix are 2, 2, and 6.
A
square matrix
of order n has at most n eigenvalues.
An identity matrix has only one eigenvalue which is 1.
The eigenvalues of triangular matrices and
diagonal matrices
are nothing but the elements of their principal diagonal.
The sum of eigenvalues of matrix A is equal to the sum of its diagonal elements.
The product of eigenvalues of matrix A is equal to its
determinant
.
The eigenvalues of
hermitian
and
symmetric
matrices are real.
The eigenvalues of
skew hermitian
and
skew-symmetric
matrices are either zeros are purely
imaginary numbers
.
A matrix and its
transpose
have the same eigenvalues.
If A and B are two square matrices of the same order, then AB and BA have the same eigenvalues.
The eigenvalues of an
orthogonal matrix
are 1 and -1.
If λ is an eigenvalue of A, then kλ is an eigenvalue of kA, where 'k' is a scalar.
If λ is an eigenvalue of A, then λ
k
is an eigenvalue of A
k
.
If λ is an eigenvalue of A, then 1/λ is an eigenvalue of A
-1
(if the
inverse
of A exists).
If λ is an eigenvalue of A, then |A| / λ is an eigenvalue of the
adjoint
of A.
Apart from these properties, we have another theorem related to eigenvalues called the "
Caley-Hamilton Theorem
"
. It says, "every square matrix satisfies its characteristic equation". i.e., if A is a square matrix then it satisfies |A - λI| = 0. For example, if λ
2
- 8λ + 12 = 0 is the characteristic equation of a square matrix A, then A
2
- 8A + 12 = 0.
Eigenvalues are used in electric circuits, quantum mechanics, control theory, etc.
They are used in the design of car stereo systems.
They are also used to design bridges.
It is not surprising to know that eigenvalues are also used in determining Google's page rank.
They are used in geometric transformations.
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# Eigenvalues
We come up with the terms eigenvalues and eigenvectors when we study the linear transformations. Some vectors change almost by their scale factors when some linear transformation (matrix) is applied to it. Such vectors are known as eigenvectors and the corresponding scale factors are known as eigenvalues of matrix.
Let us learn more about the eigenvalues of matrix along with their properties and examples.
| | |
|---|---|
| 1\. | [What are Eigenvalues of Matrix?](https://www.cuemath.com/algebra/eigenvalues/#what-are-eigenvalues-of-matrix) |
| 2\. | [How to Find Eigenvalues?](https://www.cuemath.com/algebra/eigenvalues/#how-to-find-eigenvalues) |
| 3\. | [Eigenvalues of a 2x2 Matrix](https://www.cuemath.com/algebra/eigenvalues/#eigenvalues-of-a-2x2-matrix) |
| 4\. | [Eigenvalues of a 3x3 Matrix](https://www.cuemath.com/algebra/eigenvalues/#eigenvalues-of-a-3x3-matrix) |
| 5\. | [Properties of Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#properties-of-eigenvalues) |
| 6\. | [Applications of Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#applications) |
| 7\. | [FAQs on Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#faqs) |
## What are Eigenvalues of Matrix?
The **eigenvalues of matrix** are scalars by which some vectors (eigenvectors) change when the matrix (transformation) is applied to it. In other words, if A is a square matrix of order n x n and **v** is a non-zero column vector of order n x 1 such that **Av** = **λv** (it means that the product of A and **v** is just a scalar multiple of **v**), then the scalar (real number) λ is called an eigenvalue of the matrix A that corresponds to the eigenvector **v**.
The word "eigen" is from the German language and it means "characteristic", "proper", or "own". Thus, eigenvalues are also known as "characteristic values" (or) "characteristic roots" (or) "proper values", etc. The mathematical definition of eigenvalues is as shown below.

## How to Find Eigenvalues?
From the definition of eigenvalues, if λ is an eigenvalue of a square matrix A, then
A**v** = λ**v**
If I is the [identity matrix](https://www.cuemath.com/algebra/identity-matrix/) of the same order as A, then we can write the above equation as
A**v** = λ (I**v**) (because **v** = I**v**)
A**v** - λ (I**v**) = 0
Taking **v** as common factor,
**v** (A - λI) = 0
This represents a [homogeneous system of linear equations](https://www.cuemath.com/algebra/homogeneous-system-of-linear-equations/) and it has a non-trivial solution only when the determinant of the coefficient matrix is 0.
i.e., **\|A - λI\| = 0**
This equation is called the **characteristic equation** (where \|A - λI\| is called the characteristic polynomial) and by solving this for λ, we get the eigenvalues. Here is the step-by-step process used to find the eigenvalues of a square matrix A.
1. Take the identity matrix I whose order is the same as A.
2. Multiply every element of I by λ to get λI.
3. Subtract λI from A to get A - λI.
4. Find its [determinant](https://www.cuemath.com/algebra/determinants/).
5. Set the determinant to zero and solve for λ.
Let us apply these steps to find the eigenvalues of matrices of different orders.
## Eigenvalues of a 2x2 Matrix
Let us see the process of finding the eigenvalues of a 2x2 matrix with an example where we will find the eigenvalues of A = ⎡⎢⎣5412⎤⎥⎦ \[ 5 4 1 2 \]. Let λ represents its eigenvalue(s). The identity matrix of order 2x2 is, I = ⎡⎢⎣1001⎤⎥⎦ \[ 1 0 0 1 \]. Then
λI = λ ⎡⎢⎣1001⎤⎥⎦ \[ 1 0 0 1 \] = ⎡⎢⎣λ00λ⎤⎥⎦ \[ λ 0 0 λ \]
A - λI = ⎡⎢⎣5412⎤⎥⎦ \[ 5 4 1 2 \] - ⎡⎢⎣λ00λ⎤⎥⎦ \[ λ 0 0 λ \]
\= ⎡⎢⎣5−λ412−λ⎤⎥⎦ \[ 5 − λ 4 1 2 − λ \]
Its determinant is,
\|A - λI\| = (5 - λ) (2 - λ) - (1)(4)
\= 10 - 5λ - 2λ + λ2 - 4
\= λ2 - 7λ + 6
The characteristic equation is,
\|A - λI\| = 0
λ2 - 7λ + 6 = 0
(λ - 6)(λ - 1) = 0
λ - 6 = 0; λ - 1 = 0
λ = 6; λ = 1
Thus, the eigenvalues of matrix A are 1 and 6.
## Eigenvalues of a 3x3 Matrix
Let us just observe the result of A - λI in the previous section. Isn't it just the matrix obtained by subtracting λ from all diagonal elements of A? Yes, so we will use this fact here and find the eigenvalues of 3x3 matrix A = ⎡⎢⎣311242113⎤⎥⎦ \[ 3 1 1 2 4 2 1 1 3 \].
The characteristic equation is,
\|A - λI\| = 0
∣∣ ∣∣3−λ1124−λ2113−λ∣∣ ∣∣ \| 3 − λ 1 1 2 4 − λ 2 1 1 3 − λ \| = 0
(3 - λ) \[(4 - λ)(3 - λ) - 2(1) \] - 1 \[ 2(3 - λ) - 2(1) \] + 1 \[2 (1) - 1 (4 - λ) \] = 0
(3 - λ) \[12 - 4λ - 3λ + λ2 - 2\] - 6 + 2λ + 2 + 2 - 4 + λ = 0
(3 - λ) \[10 - 7λ + λ2\] - 6 + 3λ = 0
30 - 21λ + 3λ2 \- 10λ + 7λ2 - λ3 - 6 + 3λ = 0
\-λ3 + 10λ2 \- 28λ + 24 = 0
Multiplying both sides by -1,
λ3 - 10λ2 \+ 28λ - 24 = 0
This is a [cubic equation](https://www.cuemath.com/calculators/cubic-equation-solver/). We will find one of its roots by trial and error and the other roots by synthetic division. By trial and error, we can see that λ = 2 satisfies the above equation (substitute and check whether we get 0 = 0). Now, using [synthetic division](https://www.cuemath.com/algebra/synthetic-division-of-polynomial/),

Set the quotient equal to 0.
λ2 - 8λ + 12 = 0
(λ - 6)(λ - 2) = 0
λ = 6; λ = 2
Thus, the eigenvalues of the given 3x3 matrix are 2, 2, and 6.
## Properties of Eigenvalues
- A [square matrix](https://www.cuemath.com/algebra/square-matrix/) of order n has at most n eigenvalues.
- An identity matrix has only one eigenvalue which is 1.
- The eigenvalues of triangular matrices and [diagonal matrices](https://www.cuemath.com/algebra/diagonal-matrix/) are nothing but the elements of their principal diagonal.
- The sum of eigenvalues of matrix A is equal to the sum of its diagonal elements.
- The product of eigenvalues of matrix A is equal to its [determinant](https://www.cuemath.com/algebra/determinant-of-matrix/).
- The eigenvalues of [hermitian](https://www.cuemath.com/algebra/hermitian-matrix/) and [symmetric](https://www.cuemath.com/algebra/symmetric-matrix/) matrices are real.
- The eigenvalues of [skew hermitian](https://www.cuemath.com/algebra/skew-hermitian-matrix/) and [skew-symmetric](https://www.cuemath.com/algebra/skew-symmetric-matrix/) matrices are either zeros are purely [imaginary numbers](https://www.cuemath.com/numbers/imaginary-numbers/).
- A matrix and its [transpose](https://www.cuemath.com/algebra/transpose-of-a-matrix/) have the same eigenvalues.
- If A and B are two square matrices of the same order, then AB and BA have the same eigenvalues.
- The eigenvalues of an [orthogonal matrix](https://www.cuemath.com/algebra/orthogonal-matrix/) are 1 and -1.
- If λ is an eigenvalue of A, then kλ is an eigenvalue of kA, where 'k' is a scalar.
- If λ is an eigenvalue of A, then λk is an eigenvalue of Ak.
- If λ is an eigenvalue of A, then 1/λ is an eigenvalue of A\-1 (if the [inverse](https://www.cuemath.com/algebra/inverse-of-a-matrix/) of A exists).
- If λ is an eigenvalue of A, then \|A\| / λ is an eigenvalue of the [adjoint](https://www.cuemath.com/algebra/adjoint-of-a-matrix/) of A.
Apart from these properties, we have another theorem related to eigenvalues called the "**[Caley-Hamilton Theorem](https://www.cuemath.com/algebra/cayley-hamilton-theorem/)"**. It says, "every square matrix satisfies its characteristic equation". i.e., if A is a square matrix then it satisfies \|A - λI\| = 0. For example, if λ2 - 8λ + 12 = 0 is the characteristic equation of a square matrix A, then A2 - 8A + 12 = 0.
## Applications of Eigenvalues
- Eigenvalues are used in electric circuits, quantum mechanics, control theory, etc.
- They are used in the design of car stereo systems.
- They are also used to design bridges.
- It is not surprising to know that eigenvalues are also used in determining Google's page rank.
- They are used in geometric transformations.
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## Examples of Eigenvalues
1. **Example 1:** Find the eigenvalues of the matrix ⎡⎢⎣300−12020−3⎤⎥⎦ \[ 3 0 0 − 1 2 0 2 0 − 3 \].
**Solution:**
**Method 1:**
The given matrix is a lower triangular matrix. Hence its eigenvalues are nothing but its diagonal elements which are 3, 2, and -3.
**Method 2:**
The characteristic equation of the given matrix is:
\|A - λI\| = 0
∣∣ ∣∣3−λ00−12−λ020−3−λ∣∣ ∣∣ \| 3 − λ 0 0 − 1 2 − λ 0 2 0 − 3 − λ \| = 0
Expanding the determinant using the first row:
(3 - λ)(2 - λ)(-3 - λ) = 0
3 - λ = 0; 2 - λ = 0; -3 - λ = 0
λ = 3; λ = 2; λ = -3
**Answer:** 3, 2, and -3.
2. **Example 2:** Prove that the eigenvalues of the 2x2 matrix ⎡⎢⎣0−110⎤⎥⎦ \[ 0 − 1 1 0 \]
**Solution:**
The characteristic equation of the given matrix is:
\|A - λI\| = 0
∣∣ ∣∣0−λ−110−λ∣∣ ∣∣ \| 0 − λ − 1 1 0 − λ \| = 0
∣∣ ∣∣−λ−11−λ∣∣ ∣∣ \| − λ − 1 1 − λ \| = 0
λ2 + 1 = 0
λ2 = -1
λ = ± √(-1) = ± i
But eigenvalues should be [real numbers](https://www.cuemath.com/numbers/real-numbers/).
**Answer:** The given matrix has no eigenvalues.
3. **Example 3:** Find the eigenvalues of ∣∣ ∣∣111111111∣∣ ∣∣ \| 1 1 1 1 1 1 1 1 1 \|.
**Solution:**
The characteristic equation of the given matrix is:
\|A - λI\| = 0
⎡⎢⎣1−λ1111−λ1111−λ⎤⎥⎦ \[ 1 − λ 1 1 1 1 − λ 1 1 1 1 − λ \]
(1 - λ) \[ (1 - λ)(1 - λ) - 1 \] - 1 (1 - λ - 1) + 1 (1 - 1 + λ) = 0
(1 - λ) (1 - 2λ + λ2 - 1) + λ + λ = 0
(1 - λ) (λ2 -2λ) + 2λ = 0
λ2 - 2λ - λ3 + 2λ2 + 2λ = 0
\-λ3 + 3λ2 = 0
λ2 (-λ + 3) = 0
λ2 = 0; -λ + 3 = 0
λ = 0, 0, 3
**Answer:** 0, 0, 3.
View Answer \>

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## Practice Questions on Eigenvalues
Check Answer \>
## FAQs on Eigenvalues
### What is the Definition of Eigenvalues?
**Eigenvalues** of a matrix are scalars by which eigenvectors change when the matrix or transformation is applied to it. Mathematically, if A**v** = λ**v**, then
- λ is called the eigenvalue
- **v** is called the corresponding eigenvector
### How can We Find the Eigenvalues of Matrix?
To find the eigenvalues of a square matrix A:
- Find its characteristic equation using \|A - λI\| = 0, where I is the identity matrix of same order A.
- Solve it for λ and the solutions would give the eigenvalues.
### What are the Eigenvalues of a Diagonal Matrix?
We know that all the elements of a diagonal matrix other than its diagonal elements are zeros. Hence, the eigen values of a diagonal matrix are just its diagonal elements.
### How to Find Eigenvalues and Eigenvectors?
For any square matrix A:
- Solve \|A - λI\| = 0 for λ to find eigenvalues.
- Solve (A - λI) **v** = 0 for **v** to get corresponding eigenvectors.
### Where Can We Find Eigenvalue Calculator?
We can find the eigenvalue calculator by clicking [here](https://www.cuemath.com/calculators/eigenvalue-calculator/). Here, you can enter any 2x2 matrix, then it will show you the eigenvalues along with steps.
### What is Characteristic Equation For Finding Eigenvalues?
If A is a square matrix and λ represents its eigenvalues then \|A - λI\| = 0 represents its characteristic equation and by solving this would result in the eigenvalues.
### What are the Eigenvalues of an Upper Triangular Matrix?
Since in [upper triangular matrix](https://www.cuemath.com/algebra/triangular-matrix/), all elements under the principal diagonal are zeros, the eigenvalues are nothing but the diagonal elements of the matrix.
### What are the Eigenvalues of a Unitary Matrix?
A [unitary matrix](https://www.cuemath.com/algebra/unitary-matrix/) is a complex matrix such that its inverse is equal to its conjugate transpose. The eigenvalues of a unitary matrix are -1 and 1.
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| Readable Markdown | We come up with the terms eigenvalues and eigenvectors when we study the linear transformations. Some vectors change almost by their scale factors when some linear transformation (matrix) is applied to it. Such vectors are known as eigenvectors and the corresponding scale factors are known as eigenvalues of matrix.
Let us learn more about the eigenvalues of matrix along with their properties and examples.
| | |
|---|---|
| 1\. | [What are Eigenvalues of Matrix?](https://www.cuemath.com/algebra/eigenvalues/#what-are-eigenvalues-of-matrix) |
| 2\. | [How to Find Eigenvalues?](https://www.cuemath.com/algebra/eigenvalues/#how-to-find-eigenvalues) |
| 3\. | [Eigenvalues of a 2x2 Matrix](https://www.cuemath.com/algebra/eigenvalues/#eigenvalues-of-a-2x2-matrix) |
| 4\. | [Eigenvalues of a 3x3 Matrix](https://www.cuemath.com/algebra/eigenvalues/#eigenvalues-of-a-3x3-matrix) |
| 5\. | [Properties of Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#properties-of-eigenvalues) |
| 6\. | [Applications of Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#applications) |
| 7\. | [FAQs on Eigenvalues](https://www.cuemath.com/algebra/eigenvalues/#faqs) |
The **eigenvalues of matrix** are scalars by which some vectors (eigenvectors) change when the matrix (transformation) is applied to it. In other words, if A is a square matrix of order n x n and **v** is a non-zero column vector of order n x 1 such that **Av** = **λv** (it means that the product of A and **v** is just a scalar multiple of **v**), then the scalar (real number) λ is called an eigenvalue of the matrix A that corresponds to the eigenvector **v**.
The word "eigen" is from the German language and it means "characteristic", "proper", or "own". Thus, eigenvalues are also known as "characteristic values" (or) "characteristic roots" (or) "proper values", etc. The mathematical definition of eigenvalues is as shown below.
From the definition of eigenvalues, if λ is an eigenvalue of a square matrix A, then
A**v** = λ**v**
If I is the [identity matrix](https://www.cuemath.com/algebra/identity-matrix/) of the same order as A, then we can write the above equation as
A**v** = λ (I**v**) (because **v** = I**v**)
A**v** - λ (I**v**) = 0
Taking **v** as common factor,
**v** (A - λI) = 0
This represents a [homogeneous system of linear equations](https://www.cuemath.com/algebra/homogeneous-system-of-linear-equations/) and it has a non-trivial solution only when the determinant of the coefficient matrix is 0.
i.e., **\|A - λI\| = 0**
This equation is called the **characteristic equation** (where \|A - λI\| is called the characteristic polynomial) and by solving this for λ, we get the eigenvalues. Here is the step-by-step process used to find the eigenvalues of a square matrix A.
1. Take the identity matrix I whose order is the same as A.
2. Multiply every element of I by λ to get λI.
3. Subtract λI from A to get A - λI.
4. Find its [determinant](https://www.cuemath.com/algebra/determinants/).
5. Set the determinant to zero and solve for λ.
Let us apply these steps to find the eigenvalues of matrices of different orders.
Let us see the process of finding the eigenvalues of a 2x2 matrix with an example where we will find the eigenvalues of A = \[ 5 4 1 2 \]. Let λ represents its eigenvalue(s). The identity matrix of order 2x2 is, I = \[ 1 0 0 1 \]. Then
λI = λ \[ 1 0 0 1 \] = \[ λ 0 0 λ \]
A - λI = \[ 5 4 1 2 \] - \[ λ 0 0 λ \]
\= \[ 5 − λ 4 1 2 − λ \]
Its determinant is,
\|A - λI\| = (5 - λ) (2 - λ) - (1)(4)
\= 10 - 5λ - 2λ + λ2 - 4
\= λ2 - 7λ + 6
The characteristic equation is,
\|A - λI\| = 0
λ2 - 7λ + 6 = 0
(λ - 6)(λ - 1) = 0
λ - 6 = 0; λ - 1 = 0
λ = 6; λ = 1
Thus, the eigenvalues of matrix A are 1 and 6.
Let us just observe the result of A - λI in the previous section. Isn't it just the matrix obtained by subtracting λ from all diagonal elements of A? Yes, so we will use this fact here and find the eigenvalues of 3x3 matrix A = \[ 3 1 1 2 4 2 1 1 3 \].
The characteristic equation is,
\|A - λI\| = 0
\| 3 − λ 1 1 2 4 − λ 2 1 1 3 − λ \| = 0
(3 - λ) \[(4 - λ)(3 - λ) - 2(1) \] - 1 \[ 2(3 - λ) - 2(1) \] + 1 \[2 (1) - 1 (4 - λ) \] = 0
(3 - λ) \[12 - 4λ - 3λ + λ2 - 2\] - 6 + 2λ + 2 + 2 - 4 + λ = 0
(3 - λ) \[10 - 7λ + λ2\] - 6 + 3λ = 0
30 - 21λ + 3λ2 \- 10λ + 7λ2 - λ3 - 6 + 3λ = 0
\-λ3 + 10λ2 \- 28λ + 24 = 0
Multiplying both sides by -1,
λ3 - 10λ2 \+ 28λ - 24 = 0
This is a [cubic equation](https://www.cuemath.com/calculators/cubic-equation-solver/). We will find one of its roots by trial and error and the other roots by synthetic division. By trial and error, we can see that λ = 2 satisfies the above equation (substitute and check whether we get 0 = 0). Now, using [synthetic division](https://www.cuemath.com/algebra/synthetic-division-of-polynomial/),
Set the quotient equal to 0.
λ2 - 8λ + 12 = 0
(λ - 6)(λ - 2) = 0
λ = 6; λ = 2
Thus, the eigenvalues of the given 3x3 matrix are 2, 2, and 6.
- A [square matrix](https://www.cuemath.com/algebra/square-matrix/) of order n has at most n eigenvalues.
- An identity matrix has only one eigenvalue which is 1.
- The eigenvalues of triangular matrices and [diagonal matrices](https://www.cuemath.com/algebra/diagonal-matrix/) are nothing but the elements of their principal diagonal.
- The sum of eigenvalues of matrix A is equal to the sum of its diagonal elements.
- The product of eigenvalues of matrix A is equal to its [determinant](https://www.cuemath.com/algebra/determinant-of-matrix/).
- The eigenvalues of [hermitian](https://www.cuemath.com/algebra/hermitian-matrix/) and [symmetric](https://www.cuemath.com/algebra/symmetric-matrix/) matrices are real.
- The eigenvalues of [skew hermitian](https://www.cuemath.com/algebra/skew-hermitian-matrix/) and [skew-symmetric](https://www.cuemath.com/algebra/skew-symmetric-matrix/) matrices are either zeros are purely [imaginary numbers](https://www.cuemath.com/numbers/imaginary-numbers/).
- A matrix and its [transpose](https://www.cuemath.com/algebra/transpose-of-a-matrix/) have the same eigenvalues.
- If A and B are two square matrices of the same order, then AB and BA have the same eigenvalues.
- The eigenvalues of an [orthogonal matrix](https://www.cuemath.com/algebra/orthogonal-matrix/) are 1 and -1.
- If λ is an eigenvalue of A, then kλ is an eigenvalue of kA, where 'k' is a scalar.
- If λ is an eigenvalue of A, then λk is an eigenvalue of Ak.
- If λ is an eigenvalue of A, then 1/λ is an eigenvalue of A\-1 (if the [inverse](https://www.cuemath.com/algebra/inverse-of-a-matrix/) of A exists).
- If λ is an eigenvalue of A, then \|A\| / λ is an eigenvalue of the [adjoint](https://www.cuemath.com/algebra/adjoint-of-a-matrix/) of A.
Apart from these properties, we have another theorem related to eigenvalues called the "**[Caley-Hamilton Theorem](https://www.cuemath.com/algebra/cayley-hamilton-theorem/)"**. It says, "every square matrix satisfies its characteristic equation". i.e., if A is a square matrix then it satisfies \|A - λI\| = 0. For example, if λ2 - 8λ + 12 = 0 is the characteristic equation of a square matrix A, then A2 - 8A + 12 = 0.
- Eigenvalues are used in electric circuits, quantum mechanics, control theory, etc.
- They are used in the design of car stereo systems.
- They are also used to design bridges.
- It is not surprising to know that eigenvalues are also used in determining Google's page rank.
- They are used in geometric transformations.
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