Nilpotent Matrix
Nilpotent Matrices are special types of square matrices, they are special because the product of a Nilpotent Matrix with itself is equal to a null matrix. Letβs take a square matrix A of order n Γ n it is considered a nilpotent matrix if Ak = 0. Here k is always less than equal to n.
In this article, we will learn about the Nilpotent Matrix in detail.
Table of Content
- Nilpotent Matrix Definition
- Properties of a Nilpotent Matrix
- Examples on Nilpotent Matrix
- FAQs on Nilpotent Matrix
Nilpotent Matrix Definition
A square matrix is said to be a nilpotent matrix if the product of the matrix with itself is equal to a null matrix. In simple words, a square matrix βAβ of order βn Γ nβ is said to be nilpotent if βAk = O,β where βOβ is a null matrix of order βn Γ nβ and βkβ is a positive integer less than or equal to n.
A nilpotent matrix is a square matrix that has an equal number of rows and columns and also it satisfies matrix multiplication. For example, if βPβ is a nilpotent matrix of order β2 Γ 2,β then its square must be a null matrix. If βPβ is a nilpotent matrix of order β3 Γ 3,β then either its square or cube must be a null matrix.
Nilpotent Matrix Examples
- Matrix given below is a nilpotent matrix of order β2 Γ 2.β
[Tex]A = \left[\begin{array}{cc} 2 & -4\\ 1 & -2 \end{array}\right] [/Tex]
[Tex]A^{2} = A Γ A = \left[\begin{array}{cc} 2 & -4\\ 1 & -2 \end{array}\right] \times \left[\begin{array}{cc} 2 & -4\\ 1 & -2 \end{array}\right] [/Tex]
[Tex]A^{2} =\left[\begin{array}{cc} (4-4) & (-8+8)\\ (2-2) & (-4+4) \end{array}\right] [/Tex]
[Tex]A^{2} = \left[\begin{array}{cc} 0 & 0\\ 0 & 0 \end{array}\right] = O [/Tex]
- Matrix given below is a nilpotent matrix of order β3 Γ 3.β
[Tex]B = \left[\begin{array}{ccc} 0 & -4 & 2\\ 0 & 0 & 4\\ 0 & 0 & 0 \end{array}\right] [/Tex]
As the order of the given matrix is β3 Γ 3,β then either its square or cube of the matrix must be a null matrix if it is nilpotent. Now, let us find its square first.
[Tex]B^{2} =\left[\begin{array}{ccc} 0 & -4 & 2\\ 0 & 0 & 4\\ 0 & 0 & 0 \end{array}\right]\times\left[\begin{array}{ccc} 0 & -4 & 2\\ 0 & 0 & 4\\ 0 & 0 & 0 \end{array}\right] [/Tex]
[Tex]B^{2} =\left[\begin{array}{ccc} 0 & 0 & -16\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] [/Tex]
Square of the matrix is not a null matrix. So, let us find its cube now.
[Tex]B^{3} =\left[\begin{array}{ccc} 0 & 0 & -16\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right]\times\left[\begin{array}{ccc} 0 & 0 & -16\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] = \left[\begin{array}{ccc} 0 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] = O [/Tex]
We can see that cube of the matrix βBβ is a null matrix. So, the given matrix βBβ is nilpotent.
Properties of a Nilpotent Matrix
Following are some important properties of a nilpotent matrix:
- A nilpotent matrix is always a square matrix of order βn Γ n.β
- Nilpotency index of a nilpotent matrix of order βn Γ nβ is always equal to either n or less than n.
- Both the trace and the determinant of a nilpotent matrix are always equal to zero.
- As the determinant of a nilpotent matrix is zero, it is not invertible.
- Null matrix is the only diagonalizable nilpotent matrix.
- A nilpotent matrix is a scalar matrix.
- Any triangular matrices with zeros on the principal diagonal are also nilpotent.
- Eigenvalues of a nilpotent matrix are always equal to zero.
Articles Related to Nilpotent Matrix:
Examples on Nilpotent Matrix
Example 1: Verify whether the matrix given below is nilpotent or not.
[Tex]P = \left[\begin{array}{ccc} 1 & -3 & 1\\ 2 & 0 & 2\\ -1 & 3 & -1 \end{array}\right] [/Tex]
Solution:
Order of the given matrix is β3 Γ 3.β If the given matrix is nilpotent, then either its square or cube of the matrix must be a null matrix.
Now, let us find its square first.
[Tex]P^{2} = \left[\begin{array}{ccc} 1 & -3 & 1\\ 2 & 0 & 2\\ -1 & 3 & -1 \end{array}\right] \times\left[\begin{array}{ccc} 1 & -3 & 1\\ 2 & 0 & 2\\ -1 & 3 & -1 \end{array}\right] [/Tex]
[Tex]P^{2} = \left[\begin{array}{ccc} (1-6-1) & (-3+0+3) & (1-6-1)\\ (2+0-2) & (-6+0+6) & (2+0-2)\\ (-1+6+1) & (3+0-3) & (-1+6+1) \end{array}\right] [/Tex]
[Tex]P^{2} = \left[\begin{array}{ccc} -6 & 0 & -6\\ 0 & 0 & 0\\ 6 & 0 & 6 \end{array}\right] [/Tex]
Square of the matrix is not a null matrix. So, let us find its cube now.
[Tex]P^{3} = \left[\begin{array}{ccc} 1 & -3 & 1\\ 2 & 0 & 2\\ -1 & 3 & -1 \end{array}\right]\times\left[\begin{array}{ccc} -6 & 0 & -6\\ 0 & 0 & 0\\ 6 & 0 & 6 \end{array}\right] [/Tex]
[Tex]P^{3}= \left[\begin{array}{ccc} (-6-0+6) & 0 & (-6-0+6)\\ (-12+0+12) & 0 & (-12+0+12)\\ (6+0-6) & 0 & (6+0-6) \end{array}\right] [/Tex]
[Tex]P^{3}=\left[\begin{array}{ccc} 0 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] = O [/Tex]
We can see that cube of the matrix βPβ is a null matrix. So, the given matrix βPβ is nilpotent.
Example 2: Verify whether the matrix given below is nilpotent or not.
[Tex]M = \left[\begin{array}{cc} 5 & -5\\ 5 & -5 \end{array}\right] [/Tex]
Solution:
The order of the given matrix is β2 Γ 2.β If the given matrix is nilpotent, then its square must be a null matrix.
[Tex]M^{2} = \left[\begin{array}{cc} 5 & -5\\ 5 & -5 \end{array}\right]\times\left[\begin{array}{cc} 5 & -5\\ 5 & -5 \end{array}\right] [/Tex]
[Tex]M^{2} = \left[\begin{array}{cc} (25-25) & (-25+25)\\ (25-25) & (-25+25) \end{array}\right] [/Tex]
[Tex]M^{2} = \left[\begin{array}{cc} 0 & 0\\ 0 & 0 \end{array}\right] = O [/Tex]
We can see that square of the matrix βMβ is a null matrix. So, the given matrix βMβ is nilpotent.
Example 3: Determine whether the matrix given below is nilpotent or not.
[Tex]A = \left[\begin{array}{ccc} 0 & 15 & -17\\ 0 & 0 & 12\\ 0 & 0 & 0 \end{array}\right] [/Tex]
Solution:
Order of the given matrix is β3 Γ 3.β If the given matrix is nilpotent, then either its square or cube of the matrix must be a null matrix. Now, let us find its square first.
[Tex]A^{2} = \left[\begin{array}{ccc} 0 & 15 & -17\\ 0 & 0 & 12\\ 0 & 0 & 0 \end{array}\right] \times \left[\begin{array}{ccc} 0 & 15 & -17\\ 0 & 0 & 12\\ 0 & 0 & 0 \end{array}\right] [/Tex]
[Tex]A^{2} = \left[\begin{array}{ccc} 0 & 0 & 180\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] [/Tex]
The square of the matrix is not a null matrix. So, let us find its cube now.
[Tex]A^{3} = A^{2} Γ A [/Tex]
[Tex]A^{3} = \left[\begin{array}{ccc} 0 & 0 & 180\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right]\times\left[\begin{array}{ccc} 0 & 15 & -17\\ 0 & 0 & 12\\ 0 & 0 & 0 \end{array}\right] [/Tex]
[Tex]A^{3} = \left[\begin{array}{ccc} 0 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{array}\right] = O [/Tex]
We can see that cube of the matrix βAβ is a null matrix. So, the given matrix βAβ is nilpotent.
FAQs on Nilpotent Matrix
What is meant by a nilpotent matrix?
A square matrix is said to be a nilpotent matrix if the product of the matrix with itself is equal to a null matrix.
Is a nilpotent matrix singular?
Yes, a nilpotent matrix is singular, as the determinant of the nilpotent matrix is always zero.
What is the order of a Nilpotent Matrix?
Order of a nilpotent matrix is determined by the number of rows and columns that it has. In general, a nilpotent matrix has an equal number of rows and columns because it is a square matrix.
How to find whether a matrix is nilpotent or not?
To find whether the given matrix is nilpotent or not, we have to check if the product of the matrix with itself is a null matrix.
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