FUNCTIONS
Introduction The concept of function is extremely important in discrete mathematics. Functions are used in the definition of such discrete structures as sequences and strings. Functions are also used to represent how long it takes a computer to solve problems of a given size. Recursive functions, which are functions defined in terms of themselves, are used throughout computer science.
Definition 1 Let A and B be sets. A function f from A to B is an assignment of exactly one element of B to each element of A. We write f(a) = b if b is the unique element of B assigned by the function f to the element a of A. If f is a function from A to B, we write f: A B.
Definition 2 If f is a function from A to B, we say that A is the domain of f and B is the codomain of f. If f(a) = b, we say that b is the image of a and a is a pre-image of b. The range of f is the set of all images of elements of A. Also, if f is a function from A to B, we say that f maps A to B.
The Function f Maps A to B b = f(a) A B f
Example 1 Let f be the function that assigns the last two bits of a bit string of length 2 or greater to that string. Then, the domain of f is the set of all bit strings of length 2 or greater, and both the codomain and range are the set {00,01,10,11}.
Example 2 Let f: Z Z assign the square of an integer to this integer. Then f(x) = x2, where the domain of f is the set of all integers, the codomain of f can be chosen to be the set of all integers, and the range of f is the set of all nonnegative integers that are perfect squares, namely, {0,1,4,9,…}.
Example 3 The domain and codomain of functions are often specified in programming languages. For instance, the Java statement int floor(float real){…} int factorial(int x){…}
Definition 3 Let f1 and f2 be functions from A to R. Then f1 + f2 and f1f2 are also functions from A to R defined by: (f1 + f2)(x) = f1(x) + f2(x), (f1f2)(x) = f1(x)f2(x) Two real-valued functions with the same domain can be added and multiplied.
Example 4 Let f1 and f2 be functions from R to R such that f1(x) = x2 and f2(x) = x – x2. What are the functions f1 + f2 and f1f2? (f1 + f2)(x) = f1(x) + f2(x) = x2 + (x – x2) = x and (f1f2)(x) = f1(x)f2(x) = x2(x – x2) = x3 – x4
Definition 4 Let f be a function from the set A to the set B and let S be a subset of A. The image of S is the subset of B that consists of images of the elements of S. We denote the image of S by f(S), so that f(S) = {f(s) | s S}
Example 5 Let A = {a,b,c,d,e} and B = {1,2,3,4} with f(a)=2, f(b)=1, f(c)=4, f(d)=1, and f(e)=1. The image of the subset S = {b,c,d} is the set f(S) = {1,4}.
ONE-TO-ONE AND ONTO FUNCTIONS
Definition 5 A function f is said to be one-to-one, or injective, if and only if f(x) = f(y) implies that x = y for all x and y in the domain of f. A function is said to be an injection if it is one-to-one.
A One-to-One Function a 1 b 2 c 3 d 4 5
Example 6 f(x) = x + 1 is a one-to-one function because x+1 y+1 when x y. f(x) = x2 is not one-to-one because f(1)=f(-1)=1, but 1 -1. The function is one-to-one if its domain is restricted to Z+.
Definition 6 A function f whose domain and codomain are subsets of the set of real numbers is called strictly increasing if f(x) < f(y) whenever x < y and x and y are in the domain of f. Similarly, f is called strictly decreasing if f(x) > f(y) whenever x < y and x and y are in the domain of f. A function that is either strictly increasing or strictly decreasing must be one-to-one.
Definition 7 A function f from A to B is called onto, or surjective, if and only if for every element b B there is an element a A with f(a) = b. A function f is called a surjection if it is onto. Onto functions are function whose range and codomain are equal. That is, every member of the codomain is the image of some element of the domain. yx(f(x) = y)
Example 7 Let f be the function from {a,b,c,d} to {1,2,3} defined by f(a)=3, f(b)=2, f(c)=1, and f(d)=3. Is f an onto function? Is the function f(x)=x2 from the set of integers to the set of integers onto? Is the function f(x)=x+1 from the set of integers to the set of integers onto?
Definition 8 The function f is a one-to-one correspondence, or a bijection, if it is both one-to-one and onto.
Example 8 Let f be the function from {a,b,c,d} to {1,2,3,4} with f(a)=4, f(b)=2, f(c)=1, and f(d)=3. Is f a bijection? The function f is one-to-one and onto. It is one-to-one since the function takes on distinct values. It is onto since all four elements of the codomain are images of elements in the domain. Hence, f is a bijection.
Different Types of Correspondence (a) One-to-one, not onto (b) Onto, not one-to-one 1 2 3 4 a b c 1 2 3 a b c d
Different Types of Correspondence (c) One-to-one, and onto (d) Neither one-to-one nor onto 1 2 3 4 a b c d 1 2 3 4 a b c d
Different Types of Correspondence (e) Not a function 1 2 3 4 a b c
Exercise 1 Why is f not a function from R to R if f(x) = 1/x? f(x) = x ? f(x) = (x2+1) ? Determine whether f is a function from Z to R if f(n) = n. f(n) = (n2+1). f(n) = 1/(n2 – 4).
Exercise 2 Find the domain and range of these functions. the function that assigns to each bit string the difference between the number of ones and the number of zeros the function that assigns to each nonnegative integer its last digit the function that assigns to each bit string twice the number of zeros in that string the function that assigns to each positive integer the largest perfect square not exceeding this integer
Exercise 3 Determine whether each of these functions from Z to Z is one-to-one. f(n) = n – 1 f(n) = n2 + 1 f(n) = n3 f(n) = n/2 Which functions are onto?
INVERSE FUNCTIONS AND COMPOSITIONS OF FUNCTIONS
Definition 9 Let f be a one-to-one correspondence from the set A to the set B. The inverse function of f is the function that assigns to an element b belonging to B the unique element a in A such that f(a)=b. The inverse function of f is denoted by f-1. Hence, f-1(b)=a when f(a)=b.
A one-to-one correspondence is called invertible since we can define an inverse of this function. A funtion is not invertible if it is not a one-to-one correspondence, since the inverse of such a function does not exist.
Example 9 Let f be the function from {a,b,c} to {1,2,3} such that f(a)=2, f(b)=3, f(c)=1. Is f invertible, and if it is, what is the inverse? The function f is invertible since it is one-to-one correspondence. The inverse function f-1 reverses the correspondence given by f, so that f-1(1)=c, f-1(2)=a, and f-1(3)=b.
Example 10 Let f be the function from the set of integers to the set of integers such that f(x)=x+1. Is f invertible, and if it is, what is the inverse? The function f has an inverse since it is a one-to-one correspondence. To reverse the correspondence, suppose that y is the image of x, so that y=x+1. Then x=y-1. This means that y-1 is the unique element of Z that is sent to y by f. Consequently, f-1(y)=y-1.
Definition 10 Let g be a function from the set A to the set B and let f be a function from the set B to the set C. The composition of the functions f and g, denoted by , is defined by
Example 11 Let f and g be the functions from the set of integers to the set of integers defined by f(x)=2x+3 and g(x)=3x+2. What is the composition of f and g? What is the composition of g and f? (f g)(x) = f(g(x)) = f(3x+2) = 2(3x+2)+3 = 6x+7 (g f)(x) = g(f(x)) = g(2x+3) = 3(2x+3)+2 = 6x+11
Remark Note that even though f g and g f are defined for the functions f and g, f g and g f are not necessarily equal. In other words, the commutative law does not hold for the composition of functions.
GRAPH OF FUNCTIONS
Definition 11 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 A and f(a)=b}.
Example 12 Display the graph of the function f(n)=2n+1 from the set of integers to the set of integers. The graph of f is the set of ordered pairs of the form (n, 2n+1) where n is an integer. Display the graph of the function f(x)=x2 from the set of integers to the set of integers. The graph of f is the set of ordered pairs of the form (x, f(x)) = (x, x2) where x is an integer.
SOME IMPORTANT FUNCTIONS
The Floor and Ceiling Functions The floor function assigns to the real number x the largest integer that is less than or equal to x. The value of the floor function at x is denoted by x. The ceiling function assigns to the real number x the smallest integer that is greater than or equal to x. The value of the ceiling function at x is denoted by x. Remark: The floor function is often also called the greatest integer function. It is often denoted by [x].
Table 1. Some Useful Properties of the Floor and Ceiling Functions Table 1. Some Useful Properties of the Floor and Ceiling Functions. (n Z) (1a) x = n iff n x < n+1 (1b) x = n iff n-1 < x n (1c) x = n iff x-1 < n x (1d) x =n iff x n < x+1 (2) x-1 < x x x < x+1 (3a) -x = - x (3b) -x = - x (4a) x+n = x + n (4b) x+n = x + n
The Factorial Function The factorial function f: N Z+, denoted by f(n) = n! is the product of the first n positive integers, so that f(n) = 12(n-1)n [and f(0) = 0! = 1]. Example: f(6) = 6! = 1*2*3*4*5*6 = 720
The Mod and Div Functions Let m and n be natural numbers. The integer division, m div n, returns the whole number result of dividing one natural number, m, by another natural number, n (provided that n is not 0), while m mod n returns the remainder of that division.
Hashing Functions A hashing function h assigns memory location h(k) to the record/item that has k as its key. In practice, many different hashing functions are used. One of the most common is the function h(k) = k mod m. A collision occurs for a hash function H if H(x)=H(y), but xy. To handle collisions, a collision resolution policy is required.
One simple collision resolution policy is to find the next highest unoccupied cell. If we want to locate a stored value k, we compute m=h(k) and begin looking at location m. If k is not in this position, we proceed to the next highest position, and so on. If we reach an empty cell or return to our original position, we conclude that k is not present, otherwise we obtain the position of k.