Functions

1. Functions

2. 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

3. 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).

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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)

10. 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.

11. 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.

12. 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

13. 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

14. 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

15. 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

16. Composition • Example: If and then and

17. 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

18. 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:

19. Some Important Functions Floor Function (≤x) Ceiling Function (≥x)

20. 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