Functions
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Functions. Functions. Definition : A function f 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 write f ( a ) = b if b is the unique element of B assigned to the element a of A . .

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Functions

Functions


Functions1

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


Functions2

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

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

c

z

d


Representing functions

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


Injections

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

y

c

z

d

w


Surjections

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

c

z

d


Bijections

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

z

d

w


Showing that f is is not injective or surjective

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

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 functions1

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 functions2

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 functions3

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

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


Composition1

Composition

A

g

B

f

C

A

C

v

a

h

a

b

i

h

w

c

b

i

j

x

d

c

j

y

d


Composition2

Composition

  • Example: If and then

    and


Graphs of functions

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

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 functions1

Some Important Functions

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


Factorial function

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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