Relations and Functions

March 28th, 2024

Relations and Functions

In mathematics, relations and functions are concepts that describe how elements of one set relate to elements of another set.

Relations:

- A relation between two sets A and B is any subset of their Cartesian product A×BA×B. In simpler terms, a relation is a set of ordered pairs, where the first element is from set A and the second element is from set B.

- Relations can be represented as tables, graphs, mappings, or algebraic expressions.

- Examples of relations include "less than," "greater than," "equal to," "divides evenly," etc.

- Relations can also be classified based on properties like reflexivity, symmetry, and transitivity.

Functions:

- A function is a special type of relation where each input from the domain set maps to exactly one output in the range set.

- Formally, a function f from set A to set B is a relation that assigns to each element x in set A exactly one element yy in set B, denoted as f(x)=y.

- The set of all possible inputs is called the domain, and the set of all possible outputs is called the codomain. The actual set of outputs attained by the function is called the range.

- Functions can be represented in various forms, including algebraic expressions, tables, graphs, and mappings.

- Functions are often denoted by f, g, h, etc., and the input variable is typically denoted as x or any other letter.

- Functions play a fundamental role in mathematics and have many applications in various fields, including science, engineering, economics, and computer science.

Ordered Pair

An ordered pair is a pair of elements where the order in which the elements are written matters. It is a fundamental concept in set theory and forms the basis for defining relations, functions, and Cartesian products.

Formally, an ordered pair (a, b) consists of two elements 'a' and 'b' enclosed in parentheses. The order of elements is significant, meaning (a, b) is distinct from (b, a) if a ≠ b or if the elements are distinct.

Ordered pairs are often used to represent points in the Cartesian plane, where the first element represents the x-coordinate and the second element represents the y-coordinate.

For example:

- (3, 4) represents the point with x-coordinate 3 and y-coordinate 4.

- (2, -1) represents the point with x-coordinate 2 and y-coordinate -1.

Ordered pairs are also used to define relations between sets. In a relation, each ordered pair (a, b) indicates that element 'a' is related to element 'b'. For example, in the relation "is less than," (3, 5) represents the statement "3 is less than 5."

Cartesian Product

The Cartesian product of two sets A and B, denoted as A × B, is the set of all possible ordered pairs where the first element comes from set A and the second element comes from set B.

Formally, if A={a1,a2,...,am}A={a1,a2,...,am} and B={b1,b2,...,bn}B={b1,b2,...,bn}, then the Cartesian product A×BA×B is defined as:

A X B = {(a,b) | a ∈ A and b ∈ B}

In simpler terms, if you take every element from set A and pair it with every element from set B, you get the Cartesian product of A and B.

For example:

- If A={1,2} and B={x,y}, then A×B={(1,x),(1,y),(2,x),(2,y)}.

- If A={a,b} and B={1,2,3}, then A×B={(a,1),(a,2),(a,3),(b,1),(b,2),(b,3)}.

The **number of elements **in the Cartesian product of two finite sets A and B is simply the product of the number of elements in each set.

If set A has mm elements and set B has nn elements, then the Cartesian product A×B will have m×n elements.

- For example, if set A={1,2,3}and set B={x,y}, then the Cartesian product A×B will have 3×2=6 elements.

Now, for the Cartesian product of the real numbers with itself (up to R×R×R), each real number is infinitely dense. Therefore, the Cartesian product of the real numbers with itself will also be infinitely dense, and the number of elements will be infinite.

Symbolically, R×R×R represents all possible ordered triples of real numbers. Since there are infinitely many real numbers in each dimension, the Cartesian product will contain infinitely many elements. Therefore, the Cartesian product of the real numbers with itself is **infinite**.

Here are the key components associated with relations and their definitions:

Relation:

A relation RR from set AA to set BB is a subset of the Cartesian product A×BA×B, where each element of AA is related to at least one element of BB. In other words, a relation is a set of ordered pairs (a,b)(a,b), where aa belongs to the first set and bb belongs to the second set.

Pictorial Diagrams:

Pictorial diagrams, such as directed graphs or arrow diagrams, are often used to visually represent relations. In these diagrams, elements of the first set are typically represented by nodes or points, and connections between elements of the first and second sets are depicted using arrows or directed edges.

The domain of a relation is the set of all first components (or inputs) of the ordered pairs in the relation. It represents all the elements from the initial set that are involved in the relation.

Co-**Domain:**

The co-domain of a relation is the set of all possible second components (or outputs) of the ordered pairs in the relation. It represents all the possible elements that could be related to elements in the domain.

The range of a relation is the set of all second components (or outputs) that are actually related to at least one element in the domain. It represents the set of all elements in the co-domain that are actually reached by the relation.

In summary:

- The relation itself is a set of ordered pairs representing connections between elements of two sets.

- Pictorial diagrams help visualize these connections.

- The domain is the set of all inputs.

- The co-domain is the set of all possible outputs.

- The range is the set of all actual outputs.

Let's go through each type of function you've mentioned and discuss their properties, domain, range, and provide graphs where applicable.

Constant Function:

**Definition:**A function that returns the same constant value for every input.

**Form:**f(x)=c, where c is a constant.

**Domain:**RR (all real numbers).

**Range:**{c} (a single constant value).

**Graph:**A horizontal line parallel to the x-axis.

Identity Function:

**Definition:**A function where the output is equal to the input.

**Form:**f(x)=x.

**Domain:**R (all real numbers).

**Range:**R (all real numbers).

**Graph:**A diagonal line passing through the origin with a slope of 1.

Polynomial Function:

**Definition:**A function consisting of one or more terms, each being a constant multiplied by a variable raised to a non-negative integer power.

**Form:**f(x)=anxn+an−1xn−1+...+a1x+a0, where n is a non-negative integer and a0,a1,...,an are constants.

**Domain:**R (all real numbers) unless restricted.

**Range:**R (all real numbers) unless restricted.

**Graph:**Varies depending on the degree and coefficients of the polynomial.

Rational Function:

**Definition:**A function defined as the ratio of two polynomials.

**Form:**f(x)=p(x)/q(x), where p(x) and q(x) are polynomials and q(x) ≠ 0.

**Domain:**All real numbers x such that q(x) ≠ 0.

**Range:**All real numbers y such that y≠ values that make the denominator q(x) equal to 0.

**Graph:**Can have various shapes, including asymptotes where the function approaches certain values but never reaches them due to division by zero.

Modulus Function (Absolute Value Function):

**Definition:**A function that returns the distance of a number from zero on the real number line.

**Form:**f(x)=|x|.

**Domain:**R (all real numbers).

**Range:**[0,+∞] (all non-negative real numbers).

**Graph:**V-shaped graph centered at the origin.

Signum Function (Sign Function):

**Definition:**A function that returns the sign of a real number.

**Form:**f(x)=sgn(x).

**Domain:**R (all real numbers).

**Range:**{−1,0,1}.

**Graph:**A horizontal line at y=−1 for x<0, y=0 for x=0, and y=1 for x>0

Greatest Integer Function (Floor Function):

**Definition:**A function that returns the greatest integer less than or equal to a given number.

**Form:**f(x)=|x| f(x)=floor(x).

**Domain:**R (all real numbers).

**Range:**Z (all integers).

**Graph:**A step function where the value jumps to the greatest integer less than or equal to the input.

When it comes to combining these functions through addition, subtraction, multiplication, and division, the resulting functions can take on various forms depending on the specific functions involved. For instance, the sum, difference, product, and quotient of two polynomial functions will yield another polynomial function, provided the denominator in the quotient is not zero. Similarly, combinations of other types of functions will result in functions with their own properties, domains, ranges, and graphs.

Videos Sections