Types of Matrices

There are different types of matrices, and they are basically categorised on the basis of the value of their elements, their order, the number of rows and columns, etc. Now, using different conditions, the various matrix types are categorised below, along with their definition and examples.

All Contents in Matrices

• Introduction to Matrices
• Types of Matrices
• Matrix Operations
• Adjoint and Inverse of a Matrix
• Rank of a Matrix and Special Matrices
• Solving Linear Equations Using Matrix

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Matrix Types: Overview

The different types of matrices are given below:

Types of Matrices: Explanations

Row Matrix

A matrix having only one row is called a row matrix. Thus A = [a_ij]_mxn is a row matrix if m = 1. So, a row matrix can be represented as A = [a_ij]_1×n. It is called so because it has only one row, and the order of a row matrix will hence be 1 × n. For example, A = [1 2 4 5] is a row matrix of order 1 x 4. Another example of the row matrix is P = [ -4 -21 -17 ] which is of the order 1×3.

Column Matrix

A matrix having only one column is called a column matrix. Thus, A = [a_ij]_mxn is a column matrix if n = 1. So, the value of a column matrix will be 1. Hence, the order is m × 1.

An example of a column matrix is:

Just like the row matrices had only one row, column matrices have only one column. Thus, the value of a column matrix will be 1. Hence, the order is m × 1. The general form of a column matrix is given by A = [a_ij]_m×1. Other examples of a column matrix include:

In the above example, P and Q are 3 ×1 and 5 × 1 order matrices, respectively.

Zero or Null Matrix

If all the elements are zero in a matrix, then it is called a zero matrix and generally denoted by 0. Thus, A = [a_ij]_mxn is a zero-matrix if a_ij = 0 for all i and j; E.g.

is a 3 x 2 null matrix and

Singleton Matrix

If there is only one element in a matrix, it is called a singleton matrix. Thus, A = [a_ij]_mxn is a singleton matrix if m = n = 1. E.g. [2], [3], [a], [] are singleton matrices.

Horizontal Matrix

A matrix of order m x n is a horizontal matrix if n > m; E.g.

Vertical Matrix

A matrix of order m x n is a vertical matrix if m > n; E.g.

Square Matrix

If the number of rows and the number of columns in a matrix are equal, then it is called a square matrix.

Thus, A = [a_ij]_mxn is a square matrix if m = n; E.g.

is a square matrix of order 3 × 3.

The sum of the diagonal elements in a square matrix A is called the trace of matrix A, and which is denoted by tr(A); 

Another example of a square matrix is:

The order of P and Q is 2 ×2 and 3 × 3, respectively.

Diagonal Matrix

If all the elements, except the principal diagonal, in a square matrix, are zero, it is called a diagonal matrix. Thus, a square matrix A = [a_ij] is a diagonal matrix if a_ij = 0,when i ≠ j.

(i) A diagonal matrix is always a square matrix

(ii) The diagonal elements are characterized by this general form: a_ij where i = j. This means that a matrix can have only one diagonal.

A few more examples of a diagonal matrix are:

P = [9]

In the above examples, P, Q, and R are diagonal matrices with orders 1 × 1, 2 × 2 and 3 × 3, respectively. When all the diagonal elements of a diagonal matrix are the same, it goes by a different name, the scalar matrix, which is explained below.

Scalar Matrix

If all the elements in the diagonal of a diagonal matrix are equal, it is called a scalar matrix. Thus, a square matrix

More examples of scalar matrices are:

Now, what if all the diagonal elements are equal to 1? That will still be a scalar matrix and obviously a diagonal matrix. It has got a special name which is known as the identity matrix.

Unit Matrix or Identity Matrix

If all the elements of a principal diagonal in a diagonal matrix are 1, it is called a unit matrix. A unit matrix of order n is denoted by I_n. Thus, a square matrix A = [a_ij]_m×n is an identity matrix if

E.g.

Conclusions:

• All identity matrices are scalar matrices
• All scalar matrices are diagonal matrices
• All diagonal matrices are square matrices

It should be noted that the converse of the above statements is not true for any of the cases.

Equal Matrices

Equal matrices are those matrices which are equal in terms of their elements. The conditions for matrix equality are discussed below.

• Equality of Matrices Conditions

Two matrices A and B are said to be equal if they are of the same order and their corresponding elements are equal, i.e. two matrices A = [a_ij]_m×n and B = [b_ij]_r×s are equal if:

(a) m = r, i.e., the number of rows in A = the number of rows in B.

(b) n = s, i.e. the number of columns in A = the number of columns in B

(c) a_ij = b_ij, for i = 1, 2, ….., m and j = 1, 2, ….., n, i.e. the corresponding elements are equal;

For example, Matrices

But, If

a₁ = 1, a₂ = 6, a₃ = 3, b₁ = 5, b₂ = 2, b₃ = 1.

Triangular Matrix

A square matrix is said to be a triangular matrix if the elements above or below the principal diagonal are zero, and there are of two types:

• Upper Triangular Matrix

A square matrix [a_ij] is called an upper triangular matrix, if a_ij = 0, when i > j.

• Lower Triangular Matrix

A square matrix is called a lower triangular matrix, if a_ij = 0 when i < j.

Singular Matrix and Non-Singular Matrix

Matrix A is said to be a singular matrix if it’s determinant |A| = 0; otherwise, a non-singular matrix, i.e. if for det |A| = 0, it is singular matrix and for det |A| ≠ 0, it is non-singular.

Symmetric and Skew Symmetric Matrices

• Symmetric matrix: A square matrix A = [a_ij] is called a symmetric matrix if a_ij = a_ji, for all i,j values;

Eg.

is symmetric, because a₁₂ = 2 = a₂₁, a₃₁ = 3 = a₁₃ etc.

Note: A is symmetric if A’ = A (where ‘A’ is the transpose of the matrix)

• Skew-Symmetric Matrix: A square matrix A = [a_ij] is a skew-symmetric matrix if a_ij = a_ji, for all values of i,j.

Thus, in a skew-symmetric matrix, all diagonal elements are zero; E.g.

are skew-symmetric matrices.

Note: A square matrix A is a skew-symmetric matrix A’ = -A.

Some Important Conclusions on Symmetric and Skew-Symmetric Matrices

1. If A is any square matrix, then A + A’ is a symmetric matrix and A – A’ is a skew-symmetric matrix.
2. Every square matrix can be uniquely expressed as the sum of a symmetric matrix and a skew-symmetric matrix.
   \(\begin{array}{l}A=\frac{1}{2}(A+A’)+\frac{1}{2}(A-A’)=\frac{1}{2}(B+C),\end{array} \)
   where B is symmetric, and C is a skew-symmetric matrix.
3. If a and B are symmetric matrices, then AB is symmetric AB = BA, i.e., A & B commute.
4. The matrix B’AB is symmetric or skew-symmetric in correspondence if A is symmetric or skew-symmetric.
5. All positive integral powers of a symmetric matrix are symmetric.
6. Positive odd integral powers of a skew-symmetric matrix are skew-symmetric, and positive even integral powers of a skew-symmetric matrix are symmetric.

Hermitian and Skew-Hermitian Matrices

A square matrix A = [a_ij] is said to be a Hermitian matrix if

Important Notes:

• If A is a Hermitian matrix, then
  \(\begin{array}{l}{{a}_{ii}}={{\overline{a}}_{ii}}\Rightarrow {{a}_{ii}}\,is\,real\,\forall \,i,\end{array} \)
  thus every diagonal element of a Hermitian Matrix must be real.

• If a Hermitian matrix over the set of real numbers is actually a real symmetric matrix; and A a square matrix, A = [a_ij] is said to be a skew-Hermitian if
  \(\begin{array}{l}{{a}_{ij}}=-{{\overline{a}}_{ji}},\,\forall \,i,j;\end{array} \)

i.e., A^θ = – A;

E.g.

are skew-Hermitian matrices.

• If A is a skew-Hermitian matrix then
  \(\begin{array}{l}{{a}_{ii}}=-{{\overline{a}}_{ii}}\Rightarrow a_{ii}+\overline{a_{ii}}=0\end{array} \)

i.e., a_ii must be purely imaginary or zero.

• A skew-Hermitian matrix over the set of real numbers is actually a real skew-symmetric matrix.

Special Matrices

(a) Idempotent Matrix:

A square matrix is idempotent, provided A² = A. For an idempotent matrix

For an idempotent matrix A, det A = 0 or x.

(b) Nilpotent Matrix:

A nilpotent matrix is said to be nilpotent of index p,

(c) Periodic Matrix:

A square matrix which satisfies the relation A^{k + 1} = A, for some positive integer K, then A is periodic with period K, i.e. if K is the least positive integer for which A^{k + 1} = A, and A is said to be periodic with period K. If K =1, then A is called idempotent.

E.g. the matrix

has period 1.

Notes:

(i) Period of a square null matrix is not defined.

(ii) Period of an idempotent matrix is 1.

(d) Involutory Matrix:

If A² = I, the matrix is said to be an involutory matrix. An involutory matrix with its own inverse.

E.g.

Matrices and Determinants – Important Topics

Matrices and Determinants – Important Questions