Idempotent matrix

A matrix (plural: matrices) is a rectangular arrangement of symbols along rows and columns that might be real or complex numbers. Thus, a system of m x n symbols arranged in a rectangular formation along m rows and n columns is called an m by n matrix (which is written as m x n matrix). There are special types of matrices like Orthogonal matrices, Unitary matrices, and Idempotent matrices. In real life, we use unitary matrices in quantum mechanics.

In this article, we will cover the concept of unitary matrices. This category falls under the broader category of Matrices, which is a crucial Chapter in class 12 Mathematics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination(JEE Main) and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more. A total of twelve questions have been asked on this topic in JEE MAINS(2013 - 2023) including one in 2021 and one in 2023.

Square matrix

The square matrix is the matrix in which the number of rows = number of columns. So a matrix is said to be a square matrix when m = n.

E.g.

Idempotent matrix

An idempotent matrix is defined as a square matrix that remains unchanged when multiplied by itself.

A square matrix is said to be an idempotent matrix if it satisfies the condition A² = A.

Properties of Idempotent Matrix

1) Every identity matrix is also an idempotent matrix, as the identity matrix gives the same matrix when multiplied by itself.

2) All idempotent matrices are singular matrices, except for the identity matrix.

3) The determinant of an idempotent matrix is either one or zero. If A is an idempotent matrix then |A| = 1 or 0.

4) The non-diagonal entries of an idempotent matrix can be non-zero entries.

5) The trace of an idempotent matrix is always an integer and equal to the rank of the matrix

6) The relationship between idempotent and involuntary matrices is if A is a square matrix “A” is said to be an idempotent matrix if and only if P = 2A − I is an involuntary matrix.

Solved Examples Based on Idempotent Matrices

Example 1: What is an index of the idempotent matrix :

1) 2

2) 3

3) 4

4) 5

Solution: We know that the idempotent matrix -

now if we multiply

we get .

Thus A is an idempotent matrix of order = 4

Hence, the answer is the option (3).

Example 2: Which of the following matrices is idempotent?

1) \( \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \)
2) \( \begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix} \)
3) \( \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} \)
4) \( \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix} \)

Solution:
A matrix \( A \) is idempotent if \( A^2 = A \).

1. Matrix A: \( \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \)

$A^2 = \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} = \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}$

\( A^2 = A \), so Matrix A is idempotent.

2. Matrix B: \( \begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix} \)

$B^2 = \begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix} \begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix} = \begin{pmatrix} 1 & 2 \\ 0 & 1 \end{pmatrix}$

\( B^2 \neq B \), so Matrix B is not idempotent.

3. Matrix C: \( \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} \)

$ C^2 = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} = \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}$

\( C^2 \neq C \), so Matrix C is not idempotent.

4. **Matrix D: \( \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix} \)**

$D^2 = \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix} \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix} = \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix}$

\( D^2 = D \), so Matrix D is idempotent.

Hence, matrices that are idempotent are 1 and 4.

Example 3: Which of the following is a property of an idempotent matrix \( A \)?

1) \( A \) is invertible.
2) \( A \) has a trace equal to its rank.
3) The eigenvalues of \( A \) are purely imaginary.
4) \( A \) is always diagonalizable.

Solution:
1. False. An idempotent matrix is not necessarily invertible. For instance, a matrix with eigenvalue 0 is not invertible.
2. True. For an idempotent matrix, the trace (sum of eigenvalues) equals the rank because the eigenvalues are either 0 or 1.
3. False. The eigenvalues of an idempotent matrix are 0 and 1, which are not purely imaginary.
4. False. An idempotent matrix is not necessarily diagonalizable, though it can be.

Hence, the answer is option 2.

Example 4: Find the value of a for which the matrix is idempotent.

1) 2

2) -1

3) 1

4) 0

Solution: We know that, For Idempotent A²=A

Solving any element we get a=1

Hence, the answer is the option 3.

Example 5: Consider the matrix \( B \):
$B = \begin{pmatrix}
0 & 1 \\
0 & 0
\end{pmatrix}$
Is \( B \) an idempotent matrix?
1) Yes
2) No

Solution:
Compute \( B^2 \):
$B^2 = \begin{pmatrix}
0 & 1 \\
0 & 0
\end{pmatrix} \times \begin{pmatrix}
0 & 1 \\
0 & 0
\end{pmatrix} = \begin{pmatrix}
0 & 0 \\
0 & 0
\end{pmatrix}$
Since \( B^2 \neq B \), \( B \) is not idempotent.

Summary

Idempotent matrices are unique matrices because of their unique property of being equal to squares of itself. This characteristic simplifies computations. It also provides insights into various mathematical and practical scenarios, such as projections, optimization problems, and stability analysis in dynamical systems.