1. Applied Informatics Štefan BEREŽNÝ
Mathematics 1
Applied Informatics
Štefan BEREŽNÝ

2. 7th lecture

3. MATHEMATICS 1 Applied Informatics
Contents
LINEAR ALGEBRA
Matrices
Determinants
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

4. MATHEMATICS 1 Applied Informatics
Matrix
Definition:
A rectangular array of mn real numbers written in m rows and n columns is called a matrix of the type m  n (read: type m by n or shortly an m by n matrix). The numbers which are contained in the matrix are called its entries or its elements. Matrices are usually denoted by capital letters and their entries are denoted by the same small letters with two indices. The indices are related to the position of the entry aij (i-th row and j-th column in matrix A).
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix
Specially type of matrices:
(a) Upper triangular matrix: If all elements under the main diagonal are equal to zero, then matrix A is called the upper triangular matrix.
(b) Lower triangular matrix: If all elements above the main diagonal are equal to zero, then matrix A is called the lower triangular matrix.
(c) Zero matrix: A matrix whose all elements are equal to zero is called zero matrix.
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MATHEMATICS Applied Informatics

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Matrix
Specially type of matrices:
(d) Transposed matrix: The n  m matrix B = bij whose elements satisfy bij = aji for i = 1, 2, ..., m and j = 1, 2, ..., n is called a transposed matrix to matrix A. It is denoted by AT. In other words: the transposed matrix AT to matrix A can be obtained by turning A over the main diagonal.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix
Specially type of matrices:
(e) Square matrix: A matrix with the same number of rows as columns is said to be a square matrix.
(f) Diagonal matrix: The square n  n matrix that is upper and lower triangular matrix together is called diagonal matrix. It is denoted by diag(A) = a11, a22, a33, …, ann.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix
Specially type of matrices:
(g) Identity matrix: The diagonal matrix A whose elements equal 1 is called the identity matrix. It is denoted by E or I.
(h) Symmetric matrix: The square matrix A is symmetric if satisfy: A = AT.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

9. MATHEMATICS 1 Applied Informatics
Matrix
Two matrices are identical if they are of the same type and if they have the same entries at corresponding position.
Suppose that A = aij is an m  n matrix. The entries a11, a22, a33, …, akk (where k = minm, n) form a so called main diagonal in matrix A.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

10. MATHEMATICS 1 Applied Informatics
Matrix operations
Addition of matrices:
If matrices A = aij and B = bij are both m  n then their sum is the m  n matrix C = cij with elements cij = aij + bij for i = 1, 2, …, m and j = 1, 2, …, n. We use the notation C = A + B.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

11. MATHEMATICS 1 Applied Informatics
Matrix operations
Multiplication of matrices by real numbers:
If A = aij is an m  n matrix and   R, then the product of the number  and matrix A is the matrix C = cij of the same type m  n with elements cij = aij for i = 1, 2, …, m and j = 1, 2, …, n. We say: matrix C is -multiple of matrix A. We use the notation C = A or C = A.
Matrices of the same type can also be subtracted. The difference of matrices A and B is the matrix C = A + (1)B = A  B.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

12. MATHEMATICS 1 Applied Informatics
Matrix operations
Multiplication of matrices:
If A = aij is an m  k matrix and B = bij is k  n then the product of the matrices A and B is the m  n matrix C = cij whose elements satisfy:
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix operations
for i = 1, 2, …, m and j = 1, 2, …, n.
We write: C = AB.
You can observe that the element cij in matrix C is the scalar product of i-th row of matrix A with j-th column of matrix B.
Holds: Multiplication of matrices is not commutative!
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix operations
Rules for operations with matrices:
(1) A + B = B + A,
(2) (A + B) + C = A + (B + C),
(3) (A  B)  C = A  (B  C),
(4) (A + B) = A + B,
(5) ( + )A = A + A,
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MATHEMATICS Applied Informatics

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Matrix operations
Definition:
The maximum number of linearly independent rows (or linearly independent columns) is called the rank of matrix A. We denote it rank(A) or r(A). Rows and columns are taken as arithmetic vectors.
Theorem:
Let A be an m  n upper triangular matrix and let all elements on the main diagonal be different from zero. Then the rank of matrix A is equal to the minimum of the numbers m and n.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix operations
Elementary row and column operations:
We can transform the non-upper triangular matrix to an upper triangular matrix using so called elementary row and column operations, which do not change the rank of the matrix. We shall use the following elementary row operations:
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix operations
(a) change of order of rows,
(b) multiplication of some row by a nonzero real number,
(c) addition to some row of a linear combination of the other rows,
(d) omission of a row which is a linear combination of the other rows.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Matrix operations
All the operations can also be performed with columns.
The procedure of transformation of an arbitrary matrix to an upper triangular matrix (all of whose elements on the main diagonal are different from zero) by means of the elementary row and column operations is called the Gauss algorithm.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

19. Determinant Definition:
Let A be a square matrix. The determinant of matrix A is the number which is denoted by det(A) and which is assigned to matrix A in accordance with these rules:
(a) If A = a is a 1  1 square matrix then det(A) = a.
(b) If A = aij is an n  n square matrix (for n  1) then we choose an arbitrary i-th row of matrix A and we put:
det(A) = ai1Ai1 + ai2Ai2 + ai3Ai3 + … + ain1Ain1 + ainAin
where Aij is a so called co-factor of element aij.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
The co-factor is equal to (1)i+jdet(Aij) where det(Aij) is the determinant of the (n1)  (n1) square matrix which arises from A by omission the i-th row and the j-th column. det(Aij)is called the minor, which is the abbreviation for “minor determinant”. The sum ai1Ai1 + ai2Ai2 + ai3Ai3 + … + ain1Ain1 + ainAin is called the expansion of the determinant according to the i-th row. The expansion of the determinant according to the j-th column is:
det(A) = a1jA1j + a2jA2j + a3jA3j + … + an1jAn1j + anjAnj.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Saruss' rule:
The determinant of a 2  2 and 3  3 matrix can also be, apart from the expansion according to some row or column, computed by the so called “Saruss' rule”:
det(A) = a11a22 + a12a21
and
det(A) = a11a22a33 + a12a23a31 + a13a21a32  (a13a22a31 + a11a23a32 + a12a21a33).
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Important facts about determinants:
(1) If all elements in some row or column of matrix A are zero then det(A) = 0.
(2) Interchanging two rows or columns changes the sign of the determinant.
(3) If two rows or columns are identical the determinant is zero.
(4) If we multiply some row or column of matrix A by a real number  then the determinant of the new matrix is equal to det(A).
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Important facts about determinants:
(5) If any row respectively column of matrix A is a multiple of another row respectively column of A, the determinant of A is zero.
(6) If any row respectively column of matrix A is a linear combination of the other rows respectively columns of matrix A, the determinant is zero.
(7) det(A) = det(AT).
(8) If A and B are n  n square matrices then det(A  B) = det(A)  det(B).
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Definition:
An n  n square matrix which has the maximum possible rank is called a regular matrix. (i.e. rank(A) = n)
Suppose that A is an n  n square matrix and E is the n  n identity matrix. An n  n square matrix A1 is called the inverse matrix to matrix A if:
A  A1 = A1  A = E.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Theorem:
Let A be a square matrix. Then the following statements are equivalent:
(1) A is regular.
(2) det(A)  0.
(3) The inverse matrix A1 exists.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

26. MATHEMATICS 1 Applied Informatics
Determinant
Theorem:
If A and B are n  n regular matrices then matrix A  B is also regular matrix. Moreover, it holds:
(A  B)1 = B1  A1.
If matrix A is regular then matrix A1 is also regular. Moreover, it holds:
(1) (A1)1 = A,
(2) A  A1 = A1  A = E.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

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Determinant
Theorem:
(Uniqueness of the inverse matrix)
If a square matrix A has an inverse matrix then the inverse matrix A1 is unique.
Let A is an n  n square regular matrix. For inverse matrix A1 to matrix A holds:
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

28. MATHEMATICS 1 Applied Informatics
Determinant
The adjoint matrix Adj(A) of matrix A is the transpose matrix of the matrix of co-factors of the elements aij in matrix A.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics

29. Thank you for your attention.
Štefan BEREŽNÝ
MATHEMATICS Applied Informatics