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Scalar Matrix

Last Updated : 24 Jan, 2024
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A matrix is defined as a rectangular array of numbers that are arranged in rows and columns. The size of a matrix can be determined by the number of rows and columns in it. A matrix is said to be an “m by n” matrix when it has “m” rows and “n” columns and is written as an “m × n” matrix. For example, if a matrix has three rows and four columns, then the order of the matrix is “3 × 4.” We have different types of matrices, such as rectangular, square, triangular, symmetric, singular, etc.

Definition of Scalar Matrix

A scalar matrix is a square matrix in which all of the principal diagonal elements are equal and the remaining elements are zero. It is a special case of a diagonal matrix and can be obtained when an identity matrix is multiplied by a constant numeric value. The matrix given below is a scalar matrix of order “4 × 4.” We can observe that all its main diagonal elements are the same, while the rest of the elements are zeros.

A =\left[\begin{array}{cccc} 5 & 0 & 0 & 0\\ 0 & 5 & 0 & 0\\ 0 & 0 & 5 & 0\\ 0 & 0 & 0 & 5 \end{array}\right]

A scalar matrix can be obtained when an identity matrix is multiplied by a constant numeric value. In the image given below, we can observe that when an identity matrix is multiplied with a constant “k,” a scalar matrix is obtained.

Scalar Matrix = k × Identity Matrix

\left[\begin{array}{ccc} k & 0 & 0\\ 0 & k & 0\\ 0 & 0 & k \end{array}\right] = k \times\left[\begin{array}{ccc} 1 & 0 & 0\\ 0 & 1 & 0\\ 0 & 0 & 1 \end{array}\right]

Condition for a Scalar Matrix

Consider a square matrix A that has “i” rows and “j” columns, and let “aij” be an element of the matrix at row number “i” and column number “j.” The following two requirements must be satisfied for matrix A to be a scalar matrix:

  • aij = k for i = j and k ≠ 0, where i = j = 0, 1, 2, ……., n.
  • aij = 0 for i ≠ j, where i = j = 0, 1, 2, ……., n

Examples of Scalar Matrix

  •  The matrix given below is a scalar matrix of order “2 × 2”

A = \left[\begin{array}{cc} -6 & 0\\ 0 & -6 \end{array}\right]

  • The matrix given below is a scalar matrix of order “3 × 3”

P = \left[\begin{array}{ccc} k & 0 & 0\\ 0 & k & 0\\ 0 & 0 & k \end{array}\right]

Properties of a Scalar Matrix

Following are the properties of the scalar matrix

  • As the transpose of a scalar matrix is equal to the matrix itself, it is a symmetric matrix.
  • As the entries above and below the principal diagonal are zero in a scalar matrix, it is both an upper triangular matrix and a lower triangular matrix.
  • An identity matrix or a unit matrix is a scalar matrix.
  • Any scalar matrix can be obtained when an identity matrix is multiplied by a constant numeric value.
  • The determinant of a scalar matrix of any order is equal to the product of the principal diagonal elements.
  • The inverse of a scalar matrix is also a scalar matrix whose principal diagonal elements are the reciprocals of the numbers of the original matrix. Remember that the inverse of a scalar matrix exists only if all the principal diagonal elements are not equal to zero.

If A = \left[\begin{array}{cc} k & 0\\ 0 & k \end{array}\right]        , then A-1\left[\begin{array}{cc} \frac{1}{k} & 0\\ 0 & \frac{1}{k} \end{array}\right]         (for k ≠ 0).

Also, Check

Operation on Scaler Matrix

For any two matrices of the order m × n, let us say, A = [aij] and B = [bij] and take two scalers ‘a’ and ‘b’ Then the scalar multiplication is:

  • a(A + B) = aA + aB
  • (a + b)A = a A + b A

Multiplication of a scalar matrix (say A) with another matrix (say B) is equal to the multiplication of the constant element of the scalar matrix (A) with all the elements of the matrix (B).

Solved Examples on Scalar Matrix

Example 1: Calculate the determinant of a scalar matrix given below.

A = \left[\begin{array}{ccc} -3 & 0 & 0\\ 0 & -3 & 0\\ 0 & 0 & -3 \end{array}\right]

Solution:

Given matrix  A = \left[\begin{array}{ccc} -3 & 0 & 0\\ 0 & -3 & 0\\ 0 & 0 & -3 \end{array}\right]

|A| = −3[(−3 × −3) − 0] − 0 + 0

|A| = −3(9) = −27

Hence, the determinant of the given scalar matrix is −27.

Example 2: Give an example of a scalar matrix that has three rows and three columns.

Solution:

The order of a scalar matrix that has three rows and three columns is “3 × 3.” The matrix given below represents a scalar matrix of order “3 × 3,” where all the principal diagonal elements are equal, and the rest of the elements are zeros.

B = \left[\begin{array}{ccc} 6 & 0 & 0\\ 0 & 6 & 0\\ 0 & 0 & 6 \end{array}\right]

Example 3: Determine the inverse of the scalar matrix given below.

P = \left[\begin{array}{cc} \frac{1}{2} & 0\\ 0 & \frac{1}{2} \end{array}\right]

Solution:

The given matrix P = \left[\begin{array}{cc} \frac{1}{2} & 0\\ 0 & \frac{1}{2} \end{array}\right]

Now, P-1 = Adj P/|P|

|P| = 1/2(1/2 − 0) − 0 = 1/4

P-1\left[\begin{array}{cc} \frac{1}{2} & 0\\ 0 & \frac{1}{2} \end{array}\right]        / (1/1/4) 

P-1 = 4 × \left[\begin{array}{cc} \frac{1}{2} & 0\\ 0 & \frac{1}{2} \end{array}\right]

P-1\left[\begin{array}{cc} 2 & 0\\ 0 & 2 \end{array}\right]

Example 4: Find the value of (a + b + c) if the matrix given below, is a scalar matrix.

C = \left[\begin{array}{ccc} a & 0 & 0\\ 0 & -2 & b+3\\ c-5 & 0 & -2 \end{array}\right]

Solution:

If the given matrix is a scalar matrix, then all its principal diagonal elements are equal, and the rest of the elements are zeros.

So, a = −2

b + 1 = 0 q = −3

c − 2 = 0 c = 5

Now, a + b + c = −2 + (−3) + 5

= −5 + 5 = 0

Hence, the value of (a + b + c) is 0 if matrix A is a scalar matrix.

FAQs on Scalar Matrix

Question 1: What is a Scalar Matrix?

Answer:

A scalar matrix is a square matrix in which all of the principal diagonal elements are equal and the remaining elements are zero.

Question 2: What is the condition of a square matrix to be a scalar matrix?

Answer:

Consider a square matrix A that has “i” rows and “j” columns, and let “aij” be an element of the matrix at row number “i” and column number “j.” The following two requirements must be satisfied for matrix A to be a scalar matrix:

  • aij = k for i = j and k ≠ 0, where i = j = 0, 1, 2, ……., n.
  • aij = 0 for i ≠ j, where i = j = 0, 1, 2, ……., n.

Question 3: Is an identity matrix a scalar matrix?

Answer:

A scalar matrix is a square matrix whose principal diagonal elements are equal, and the rest of the elements of the matrix are zeros. We know that an identity matrix is a square matrix whose principal diagonal elements are ones, and the rest of the elements of the matrix are zeros. So, an identity matrix or a unit matrix is a scalar matrix.

Question 4: Is a null matrix a scalar matrix?

Answer:

A null matrix or a zero matrix is a square matrix whose all elements are equal to zero. So, we can’t say that it is not a scalar matrix.



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