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Mathematics | Matrix Introduction
• Difficulty Level : Easy
• Last Updated : 18 Sep, 2020

A matrix represents a collection of numbers arranged in an order of rows and columns. It is necessary to enclose the elements of a matrix in parentheses or brackets.
A matrix with 9 elements is shown below. This Matrix [M] has 3 rows and 3 columns. Each element of matrix [M] can be referred to by its row and column number. For example, a23=6

Order of a Matrix :
The order of a matrix is defined in terms of its number of rows and columns.
Order of a matrix = No. of rows ×No. of columns
Therefore Matrix [M] is a matrix of order 3 × 3.

Transpose of a Matrix :
The transpose [M]T of an m x n matrix [M] is the n x m matrix obtained by interchanging the rows and columns of [M].
if A= [aij] mxn , then AT = [bij] nxm where bij = aji

Properties of transpose of a matrix:

• (AT)T = A
• (A+B)T = AT + BT
• (AB)T = BTAT

Singular and Nonsingular Matrix:

1. Singular Matrix: A square matrix is said to be singular matrix if its determinant is zero i.e. |A|=0
2. Nonsingular Matrix: A square matrix is said to be non-singular matrix if its determinant is non-zero.

Properties of Matrix addition and multiplication:

1. A+B = B+A (Commutative)
2. (A+B)+C = A+ (B+C) (Associative)
3. AB ? BA (Not Commutative)
4. (AB) C = A (BC) (Associative)
5. A (B+C) = AB+AC (Distributive)

Square Matrix: A square Matrix has as many rows as it has columns. i.e. no of rows = no of columns.
Symmetric matrix: A square matrix is said to be symmetric if the transpose of original matrix is equal to its original matrix. i.e. (AT) = A.
Skew-symmetric: A skew-symmetric (or antisymmetric or antimetric) matrix is a square matrix whose transpose equals its negative.i.e. (AT) = -A.

Diagonal Matrix:A diagonal matrix is a matrix in which the entries outside the main diagonal are all zero. The term usually refers to square matrices.
Identity Matrix:A square matrix in which all the elements of the principal diagonal are ones and all other elements are zeros.Identity matrix is denoted as I.
Orthogonal Matrix: A matrix is said to be orthogonal if AAT = ATA = I
Idemponent Matrix: A matrix is said to be idemponent if A2 = A
Involutary Matrix: A matrix is said to be Involutary if A2 = I.

Note: Every Square Matrix can uniquely be expressed as the sum of a symmetric matrix and skew-symmetric matrix. A = 1/2 (AT + A) + 1/2 (A – AT).

Adjoint of a square matrix: The adjoint of a matrix A is the transpose of the cofactor matrix of A   Properties of Adjoint:

1. A(Adj A) = (Adj A) A = |A| In
2. Adj(AB) = (Adj B).(Adj A)
3. |Adj A|= |A|n-1
4. Adj(kA) = kn-1 Adj(A)
5. |adj(adj(A))|= |A|^(n-1)^2
6. adj(adj(A))=|A|^(n-2)     *  A
7. If A = [L,M,N] then adj(A) = [MN, LN, LM]
8. adj(I) = I

Where,     “n = number of rows = number of columns”

Inverse of a square matrix: Here |A| should not be equal to zero, means matrix A should be non-singular.

Properties of inverse:

1. (A-1)-1 = A
2. (AB)-1 = B-1A-1
3. only a non singular square matrix can have an inverse.

Where should we use the inverse matrix?

If you have a set of simultaneous equations:

7x + 2y + z = 21
3y – z = 5
-3x + 4y – 2x = -1

As we know when AX = B, then X = A-1B so we calculate the inverse of A and by multiplying it B, we can get the values of x, y, and z.

Trace of a matrix: trace of a matrix is denoted as tr(A) which is used only for square matrix and equals the sum of the diagonal elements of the matrix. Remember trace of a matrix is also equal to sum of eigen value of the matrix. For example: This article is contributed by Nitika Bansal. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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