Chapter Sets and Functions Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-1 Section 2.1 Basic Set Operations Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-2 The idea of a set or collection of things is common in our everyday experience We speak of a football team, a flock of geese, or a finance committee We not have a formal definition of the concept “set,” but we use the informal understanding that a set is a collection of objects characterized by some defining property that allows us to think of the objects as a whole The objects in a set are called elements or members of the set Notation Meaning a∈S a∉S Object a is an element of set S Object a is not an element of set S To define a particular set, we have to indicate the property that characterizes its elements For a finite set, this can be done by listing its members For example, if set A consists of the elements 1, 2, and 3, then we write A = {1, 2, 3} If B consists of just one member, say 5, then we write B = {5} That is, we distinguish between the element and the set that contains as its only member Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-3 For an infinite set we cannot list all the members, so a defining rule must be given It is customary to set off the rule within braces, as in C = {x : x is prime} Read, “C is the set of all x such that x is prime.” Definition 2.1.3 Let A and B be sets We say that A is a subset of B (or A is contained in B) if every element of A is an element of B, and we denote this by writing A ⊆ B If A is a subset of B and there exists an element in B that is not in A, then A is called a proper subset of B This definition tells us what we must if we want to prove A ⊆ B We must show that “if x ∈ A, then x ∈ B” is a true statement That is, we must show that each element of A satisfies the defining condition that characterizes B Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-4 Definition 2.1.4 Let A and B be sets We say that A is equal to B, written A = B, if A ⊆ B and B ⊆ A When this definition is combined with the definition of subset, we see that proving A = B is equivalent to proving x∈A⇒x∈B and x ∈ B ⇒ x ∈ A Note: in describing a set, the order in which the elements appear does not matter, nor does the number of times they are written So the following sets are all equal: {1, 2, 3, 4} = {2, 4, 1, 3} = {1, 2, 3, 2, 4, 2} Although we cannot give a formal definition of them now, it is convenient to name the following sets: will denote the set of all positive integers (or natural numbers) will denote the set of all rational numbers will denote the set of all real numbers Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-5 In constructing examples of sets it is often helpful to indicate a larger set from which the elements are being chosen {x  : x ∈ and < x < 1} Sometimes we abbreviate the notation becomes {x ∈ : < x < 1} Read, “The set of all x in such that < x < 1.” There is a standard notation that we use for interval subsets of the real numbers: [a, b] = {x ∈   : a ≤ x ≤ b}, [a, b) = {x ∈   : a ≤ x < b}, (a, b) = {x ∈  : a < x < b}, (a, b] = {x ∈   : a < x ≤ b} We use a square bracket if the endpoint is included and a round parenthesis if the endpoint is not included The set [a, b] is called a closed interval and the set (a, b) is called an open interval We also have occasion to refer to the unbounded intervals: [a, ∞) = {x ∈   : x ≥ a}, (− ∞, b] = {x ∈  : x ≤ b}, (a, ∞) = {x ∈   : x > a}, (− ∞, b) = {x ∈   : x < b} At this time no special significance should be attached to the symbols “  ∞ ” and “ – ∞ ” as in [a, ∞) and (– ∞, b] They simply indicate that the interval contains all real numbers greater than or equal to a, or less than or equal to b, as the case may be Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-6 *Similar to Example 2.1.5 in the text Example 2.1.5* Let A = {1, 3}, B = {1, 3, 5}, and C = {x ∈ : x = – 1} Determine whether each statement is true or false 3∈B True 3∈C False A⊆B True B⊆A False C⊆A True is one of the elements listed in B ≠ – Every element of A is an element of B ∈ B, but ∉ A Does C contain any elements that are not in A? No So all the elements of C are contained in A In fact, set C contains no members It is an example of the empty set We denote the empty set by the symbol Theorem 2.1.7 Proof: ∅ Let A be a set Then ∅ ⊆ A To prove that ∅ ⊆ A, we must establish that the implication if x ∈ ∅, then x ∈ A is true Since ∅ has no members, the antecedent “x ∈ ∅” is false for all x Thus, according to our definition of implies, the implication is always true ♦ Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-7 There are three basic ways to form new sets from existing sets Definition 2.1.8 Let A and B be sets The union of A and B (denoted A ∪ B), the intersection of A and B (denoted A ∩ B), and the complement of B in A (denoted A A∪B = {x  \ B) are given by  : x ∈ A or x ∈ B} A ∩ B = {x : x ∈ A and x ∈ B} A \ B = {x : x ∈ A and x ∉ B} If A ∩ B = ∅, then A and B are said to be disjoint These three set operations given above correspond in a natural way to three of the basic logical connectives: x∈A∪B iff (x ∈ A) ∨ (x ∈ B) x∈A∩B iff (x ∈ A) ∧ (x ∈ B) iff (x ∈ A) ∧ ~ (x ∈ B) x ∈ A \ B Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-8 Mathematical concepts and proofs always occur within the context of some mathematical system It is customary for the elements of the system to be called the universal set Then any set under consideration is a subset of this universal set Example 2.1.10 Let A = {1, 2, 3, 4} and B = {2, 4, 6} be subsets of the universal set U = {1, 2, 3, 4, 5, 6} Then A ∪ B = {1, 2, 3, 4, 6} If you toss the elements of A and B into the same bag, this is what you get A ∩ B = {2, 4} These are the elements that A and B have in common A  \ B = {1, 3} If you start with A and throw out anything that’s in B, this is what’s left U \B = {1, 3, 5} If you start with all of U and throw out anything that’s in B, this is what’s left Note: The complement of B in the universal set U, namely U \B, is sometimes called the complement of B Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 1-9 One way to visualize set operations is by use of Venn diagrams as shown below The rectangle represents the universal set U Set A is the blue circle on the left Set B is the yellow circle on the right If we color both circles, the total colored area is the union: A ∪ B And the green area where they overlap is the intersection: A ∩ B U A Copyright © 2013, 2005, 2001 Pearson Education, Inc B Section 2.1, Slide 10 1-10 Theorem 2.1.13 Let A, B, and C be subsets of a universal set U Then the following statements are true (a) A ∪ (U \ A) = U (b) A ∩ (U \ A) = ∅ (c) U \(U \ A) = A (d) A ∪ (B ∩ C ) = (A ∪ B) ∩ (A ∪ C ) (e) A ∩ (B ∪ C ) = (A ∩ B) ∪ (A ∩ C ) (f ) A \(B ∪ C ) = (A \ B) ∩ (A \ C) (g) A \(B ∩ C ) = (A \ B) ∪ (A \ C) The proofs of most of these are left as exercises, but we will part (d) to illustrate the process Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 11 1-11 Theorem 2.1.13 (d) A ∪ (B ∩ C ) = (A ∪ B) ∩ (A ∪ C ) Proof: We begin by showing that A ∪ (B ∩ C ) ⊆ (A ∪ B) ∩ (A ∪ C ) A ∪ (B ∩ C ) If x ∈ _, then either x ∈ A or x ∈ B ∩ C If x ∈ A, then certainly x ∈ A ∪ B and x ∈ A ∪ C Thus x ∈ On the other (A ∪ B) ∩ (A ∪ C ) hand, if _, then x ∈ B and x ∈ C But this implies that (B _, ∩ C) x ∈ A ∪ xB∈and so x ∈ (A ∪ B) ∩ (A ∪ C ) Hence A ∪ (B ∩ C ) ⊆ (A ∪ B) ∩ (A ∪ C ) x ∈ (A ∪ C) Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 12 1-12 Theorem 2.1.13 (d) A ∪ (B ∩ C ) = (A ∪ B) ∩ (A ∪ C ) y ∈are (Atwo ∪ B)cases to Conversely, if y ∈ (A ∪ B) ∩ (A ∪ C ), then and _ There C) when y ∉ A If y ∈ A, then y ∈ A ∪ (B ∩ C ) and this part is done On the consider: wheny ∈y(A ∈∪ A and other hand, if _ , then since y ∈ A ∪ B, we must have y ∈ B Similarly, since y∈ ∉ A that y ∈ A ∪ (B ∩ A ∪ C and y ∉ A, we have _ Thus , and this yimplies C ) Hence (A ∪ B) ∩ (A ∪ C ) ⊆ A ∪ (B ∩ C ) ♦ y∈C y ∈ (B ∩ C) Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 13 1-13 Comments on the proof of Theorem 2.1.13 (d) Notice how the argument divides naturally into parts, the second part being introduced by the word “conversely.” This word is appropriate because the second half of the argument is indeed the converse of the first half In the first part the point in A ∪ (B ∩ C  ) was called x and in the second part the point in (A ∪ B) ∩ (A ∪ C ) was called y Why is this? The choice of a name is completely arbitrary, and in fact the same name could have been used in both parts It is important to realize that the two parts are separate arguments; we start over from scratch in proving the converse and can use nothing that was derived about the point x in the first part By using different names for the points in the two parts we emphasize this separateness It is common practice, however, to use the same name (such as x) for the arbitrary point in both parts Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 14 1-14 Comments on the proof of Theorem 2.1.13 (d) Notice that each half of the argument also has two parts or cases, the second case being introduced by the phrase “on the other hand.” This type of division of the argument is necessary when dealing with unions If x ∈ S ∪ T, then x ∈ S or x ∈ T Each of the possibilities must be followed to its logical conclusion, and both “bridges” must lead to the same desired result (or to a contradiction, which would show that only one alternative could occur) When proving that one set, say S, is a subset of another set, say T, it is common to begin with the phrase “If x ∈ S, then…” It is also acceptable to begin with “Let x ∈ S ” and then conclude that x ∈ T The subtle difference between these phrases is that “Let x ∈ S ” assumes that S is nonempty, so there is an x in S to choose This might seem to be an unwarranted assumption, but really it is not If S is the empty set, then of course S ⊆ T, so the only nontrivial case to prove is when S is nonempty Copyright © 2013, 2005, 2001 Pearson Education, Inc Section 2.1, Slide 15 1-15 Sometimes we wish to form unions or intersections of more than or sets To this we need to extend our previous definitions Definition 2.1.15 If for each element j in a nonempty set J there corresponds a set Aj, then A = {Aj : j ∈ J } is called an indexed family of sets with J as the index set The union of all the sets in A is defined by UA I A = = and the intersection is If J = , we write ∞ and Copyright © 2013, 2005, 2001 Pearson Education, Inc ∞ j =1  } Aj is a nonempty collection of sets, then we let B∈B B∈B  } = {x: UB I = {x: I Uj =1 Aandj In general, if B Uj∈J Ax j∈ Aj for some j ∈ J I j∈J Axj ∈ Aj for all j ∈ J B = {x: = {x: x ∈ B for some B ∈ x ∈ B for all B ∈ B  B  } } Section 2.1, Slide 16 1-16 Example 2.1.17* *Similar to Example 2.1.17 in the text ∞ For each k ∈ N, let Ak = [1/k, 3 – 1/k] We have Uk =1 Ak Find and I ∞ k =1 Ak A1 =  11 , − 11  = [ 1, 2] A2 =  12 , − 12  =  12 , 12  A3 =  13 , − 13  =  13 , 32  A4 =  14 , − 14  =  14 , 43  A5 =  15 , − 15  =  15 , 54  etc To find the union of all these sets, we note that the left endpoint is getting closer and closer to 0, and the right endpoint is approaching (but never reaches) So, ∞ Uk =1 Ak = (0,3) To find the intersection of the sets, we ask, “What numbers are in all of the sets?” We find Copyright © 2013, 2005, 2001 Pearson Education, Inc I ∞ k =1 Ak = [1, 2] Section 2.1, Slide 17 1-17 ... significance should be attached to the symbols “  ∞ ” and “ – ∞ ” as in [a, ∞) and (– ∞, b] They simply indicate that the interval contains all real numbers greater than or equal to a, or less than... elements that A and B have in common A   B = {1, 3} If you start with A and throw out anything that’s in B, this is what’s left U B = {1, 3, 5} If you start with all of U and throw out anything that’s... Mathematical concepts and proofs always occur within the context of some mathematical system It is customary for the elements of the system to be called the universal set Then any set under consideration