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Functions

- Definition: A functionf from set A to set B, denoted f: A→B,is an assignment of each element of A to exactly one element of B.
- We writef(a) = bif b is the unique element of B assigned to the element a of A.

- Functions are also called mappings

Students

Grades

A

Carlota Rodriguez

B

Sandeep Patel

C

Jalen Williams

D

F

Kathy Scott

Functions

Given a function f: A→ B

- A is called the domain of f
- B is called the codomain of f
- f is a mapping from Ato B
- If f(a) = b
- then bis called the image of aunder f
- a is called the preimage of b

- The range (or image) of f is the set of all images of points in A. We denote it by f(A).

Example

A

B

- The domain of f is A
- The codomain of f is B
- The image of b is y
- f(b) = y

- The preimage of y is b
- The preimageof z is {a,c,d}
- The range/image of A is {y,z}
- f(A) = {y,z}

a

x

b

y

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z

d

Representing Functions

- Functions may be specified in different ways:
- An explicit statement of the assignment.
- Students and grades example.

- A formula.
- f(x) = x+ 1

- A computer program.
- A Java program that when given an integer n, produces the nth Fibonacci Number

- An explicit statement of the assignment.

Injections

B

A

- Definition: A function f is one-to-one, or injective, iffa ≠ b implies that f(a) ≠ f(b) for all a and b in the domain of f.

x

a

v

b

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z

d

w

Surjections

A

B

- Definition: A function f from A to B is called onto or surjective, iff for every element b ∈ B there exists an element a ∈A with f(a) = b

x

a

b

y

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z

d

Bijections

A

B

- Definition: A function f is a one-to-one correspondence, or a bijection, if it is both one-to-one and onto (surjective and injective)

a

x

b

y

c

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w

Showing that f is/is not injective or surjective

- Consider a function f: A→ B
- f is injectiveiff:
∀x,y∈ A ( x ≠ y → f(x) ≠ f(y) )

- f is not injective iff:
∃x,y∈ A( x ≠ y ∧ f(x) = f(y) )

- fis surjectiveiff:
∀y ∈ B ∃x ∈ A( f(x) = y )

- f is not surjectiveiff:
∃y ∈ B ∀x ∈ A (f(x) ≠ b)

Inverse Functions

- Definition: Let f be a bijection from A to B. Then the inverse of f, denoted f –1, is the function from B to Adefined as
- No inverse exists unless f is a bijection.

Inverse Functions

- Example 1:
- Let f be the function from {a,b,c} to {1,2,3}
- f(a)=2, f(b)=3, f(c)=1.
- Is f invertible and if so what is its inverse?

- Solution:
- fis invertible because it is a bijection
- f –1reverses the correspondence given by f:
- f –1(1)=c, f –1(2)=a, f –1(3)=b.

Inverse Functions

- Example 2:
- Let f: R→Rbe such that f(x) = x2
- Is f invertible, and if so, what is its inverse?

- Solution:
- The function f is not invertible because it is not one-to-one

Inverse Functions

- Example 3:
- Let f: Z Zbe such that f(x) = x + 1
- Is f invertible and if so what is its inverse?

- Solution:
- The function f is invertible because it is a bijection
- f –1reverses the correspondence:
- f –1(y) = y – 1

Composition

- Definition: Let f: B→C, g: A→B. The composition of f with g, denoted f∘g is the function from A to C defined by

Composition

- Example: If and then
and

Graphs of Functions

- Let f be a function from the set A to the set B. The graph of the function f is the set of ordered pairs
{(a,b) | a ∈Aandf(a) = b}

Graph of f(x) = x2

from Z to Z

Graph of f(n) = 2n+1

from Z to Z

Some Important Functions

- The floor function, denoted
is the largest integer less than or equal to x.

- The ceilingfunction, denoted
is the smallest integer greater than or equal to x

- Examples:

Some Important Functions

Floor Function (≤x) Ceiling Function (≥x)

Factorial Function

- Definition: f: N → Z+, denoted by f(n) = n! is the product of the first n positive integers:
f(n) = 1 ∙ 2 ∙∙∙ (n–1) ∙ nforn>0

f(0) = 0! = 1

- Examples:
- f(1) = 1! = 1
- f(2) = 2! = 1 ∙ 2 = 2
- f(6) = 6! = 1 ∙ 2 ∙ 3 ∙ 4 ∙ 5 ∙ 6 = 720
- f(20) = 2,432,902,008,176,640,000

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