# CHAPTER 1. Basic Ideas

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1 CHPTER 1 asic Ideas In the end, all mathematics can be boiled down to logic and set theory. ecause of this, any careful presentation of fundamental mathematical ideas is inevitably couched in the language of logic and sets. This chapter defines enough of that language to allow the presentation of basic real analysis. Much of it will be familiar to you, but look at it anyway to make sure you understand the notation. 1. Sets Set theory is a large and complicated subject in its own right. There is no time in this course to touch on any but the simplest parts of it. Instead, we ll just look at a few topics from what is often called naive set theory. We begin with a few definitions. set is a collection of objects called elements. Usually, sets are denoted by the capital letters,,, Z. set can consist of any type and number of elements. Even other sets can be elements of a set. The sets dealt with here usually have real numbers as their elements. If a is an element of the set, we write a 2. Ifa is not an element of the set, we write a. If all the elements of are also elements of, then is a subset of. In this case, we write Ω or æ. In particular, notice that whenever is a set, then Ω. Two sets and are equal, if they have the same elements. In this case we write =. It is easy to see that = iff Ω and Ω. Establishing that both of these containments are true is the most common way to show that two sets are equal. If Ω and 6=, then is a proper subset of. In cases when this is important, it is written ( instead of just Ω. There are several ways to describe a set. set can be described in words such as P is the set of all presidents of the United States. This is cumbersome for complicated sets. ll the elements of the set could be listed in curly braces as S = {2,0, a}. If the set has many elements, this is impractical, or impossible. More common in mathematics is set builder notation. Some examples are P = {p : p is a president of the United states} = {Washington, dams, Jefferson,, Clinton, ush, Obama} 1-1

2 1-2 CHPTER 1. SIC IDES and = {n : n is a prime number} = {2,3,5,7,11, }. In general, the set builder notation defines a set in the form {formula for a typical element : objects to plug into the formula}. more complicated example is the set of perfect squares: S = {n 2 : n is an integer} = {0,1,4,9, }. The existence of several sets will be assumed. The simplest of these is the empty set, which is the set with no elements. It is denoted as ;. The natural numbers is the set N = {1,2,3, } consisting of the positive integers. The set Z = {, 2, 1,0,1,2, } is the set of all integers.! = {n 2 Z : n 0} = {0,1,2, } is the nonnegative integers. Clearly, ;Ω, for any set and ;ΩN Ω! Ω Z. DEFINITION 1.1. Given any set, the power set of, written P (), is the set consisting of all subsets of ; i. e., P () = { : Ω }. For example, P ({a,b}) = {;,{a},{b},{a,b}}. lso, for any set, it is always true that ;2P () and 2 P (). If a 2, it is rarely true that a 2 P (), but it is always true that {a} Ω P (). Make sure you understand why! n amusing example is P (;) = {;}. (Don t confuse ; with {;}! The former is empty and the latter has one element.) Now, consider P (;) = {;} P (P (;)) = {;,{;}} P (P (P (;))) = {;,{;},{{;}},{;,{;}}} fter continuing this n times, for some n 2 N, the resulting set, P (P ( P (;) )), is very large. In fact, since a set with k elements has 2 k elements in its power set, there are = 65,536 elements after only five iterations of the example. eyond this, the numbers are too large to print. Number sequences such as this one are sometimes called tetrations. 2. lgebra of Sets Let and be sets. There are four common binary operations used on sets. 1 1 In the following, some logical notation is used. The symbol _ is the logical nonexclusive or. The symbol ^ is the logical and. Their truth tables are as follows: ^ T F T T F F F F _ T F T T T F T F

3 2. LGER OF SETS 1-3 \ FIGURE 1. These are Venn diagrams showing the four standard binary operations on sets. In this figure, the set which results from the operation is shaded. The union of and is the set containing all the elements in either or : [ = {x : x 2 _ x 2 }. The intersection of and is the set containing the elements contained in both and : \ = {x : x 2 ^ x 2 }. The difference of and is the set of elements in and not in : \ = {x : x 2 ^ x }. The symmetric difference of and is the set of elements in one of the sets, but not the other: = ( [ )\( \ ). nother common set operation is complementation. The complement of a set is usually thought of as the set consisting of all elements which are not in. ut, a little thinking will convice you this is not a meaningful definition because the collection of elements not in is not a precisely understood collection. To make sense of the complement of a set, there must be a well-defined universal set U which contains all the sets in question. Then the complement of a set Ω U is c = U \. It is usually the case that the universal set U is evident from the context in which it is used. With these operations, an extensive algebra for the manipulation of sets can be developed. It s usually done hand in hand with formal logic because the two subjects share much in common. These topics are studied as part of oolean algebra. 2 Several examples of set algebra are given in the following theorem and its corollary. 2 George oole ( )

4 1-4 CHPTER 1. SIC IDES THEOREM 1.2. Let, and C be sets. (a) \( [C) = ( \ ) \ ( \C) (b) \( \C) = ( \ ) [ ( \C) PROOF. (a) This is proved as a sequence of equivalences. 3 x 2 \( [C) () x 2 ^ x ( [C) () x 2 ^ x ^ x C () (x 2 ^ x ) ^ (x 2 ^ x C) () x 2 ( \ ) \ ( \C) (b) This is also proved as a sequence of equivalences. x 2 \( \C) () x 2 ^ x ( \C) () x 2 ^ (x _ x C) () (x 2 ^ x ) _ (x 2 ^ x C) () x 2 ( \ ) [ ( \C) Theorem 1.2 is a version of a pair of set equations which are often called De Morgan s Laws. 4 The more usual statement of De Morgan s Laws is in Corollary 1.3. Corollary 1.3 is an obvious consequence of Theorem 1.2 when there is a universal set to make the complementation well-defined. COROLLRY 1.3 (De Morgan s Laws). Let and be sets. (a) ( [ ) c = c \ c (b) ( \ ) c = c [ c 3. Indexed Sets We often have occasion to work with large collections of sets. For example, we could have a sequence of sets 1, 2, 3,, where there is a set n associated with each n 2 N. In general, let be a set and suppose for each 2 there is a set. The set { : 2 } is called a collection of sets indexed by. In this case, is called the indexing set for the collection. EXMPLE 1.1. For each n 2 N, let n = {k 2 Z : k 2 n}. Then 1 = 2 = 3 = { 1,0,1}, 4 = { 2, 1,0,1,2},, is a collection of sets indexed by N. 61 = { 7, 6, 5, 4, 3, 2, 1,0,1,2,3,4,5,6,7}, 3 The logical symbol () is the same as if, and only if. If and are any two statements, then () is the same as saying implies and implies. It is also common to use iff in this way. 4 ugustus De Morgan ( )

5 4. FUNCTIONS ND RELTIONS 1-5 Two of the basic binary operations can be extended to work with indexed collections. In particular, using the indexed collection from the previous paragraph, we define [ = {x : x 2 for some 2 } and 2 \ = {x : x 2 for all 2 }. 2 De Morgan s Laws can be generalized to indexed collections. THEOREM 1.4. If { : 2 } is an indexed collection of sets and is a set, then \ [ = \ \ ) 2 2 ( and \ \ = [ \ ). 2 2 ( PROOF. The proof of this theorem is Exercise Functions and Relations 4.1. Tuples. When listing the elements of a set, the order in which they are listed is unimportant; e. g., {e,l, v,i,s} = {l,i,v,e, s}. If the order in which n items are listed is important, the list is called an n-tuple. (Strictly speaking, an n-tuple is not a set.) We denote an n-tuple by enclosing the ordered list in parentheses. For example, if x 1, x 2, x 3, x 4 are four items, the 4-tuple (x 1, x 2, x 3, x 4 ) is different from the 4-tuple (x 2, x 1, x 3, x 4 ). ecause they are used so often, the cases when n = 2 and n = 3 have special names: 2-tuples are called ordered pairs and a 3-tuple is called an ordered triple. DEFINITION 1.5. The set of all ordered pairs = {(a,b):a 2 ^ b 2 } is called the Cartesian product of and. and EXMPLE 1.2. If = {a,b,c} and = {1,2}, then = {(a,1),(a,2),(b,1),(b,2),(c,1),(c,2)}. = {(1, a),(1,b),(1,c),(2, a),(2,b),(2,c)}. Notice that 6= because of the importance of order in the ordered pairs. useful way to visualize the Cartesian product of two sets is as a table. The Cartesian product from Example 1.2 is listed as the entries of the following table. 1 2 a (a, 1) (a,2) b (b, 1) (b, 2) c (c, 1) (c,2)

6 1-6 CHPTER 1. SIC IDES Of course, the common Cartesian plane from your analytic geometry course is nothing more than a generalization of this idea of listing the elements of a Cartesian product as a table. The definition of Cartesian product can be extended to the case of more than two sets. If { 1, 2,, n } are sets, then 1 2 n = {(a 1, a 2,, a n ):a k 2 k for 1 k n} is a set of n-tuples. This is often written as ny k = 1 2 n Relations. k=1 DEFINITION 1.6. If and are sets, then any R Ω is a relation from to. If(a,b) 2 R,wewriteaRb. In this case, dom(r) = {a :(a,b) 2 R} is the domain of R and is the range of R. ran(r) = {b :(a,b) 2 R} In the special case when R Ω, for some set, there is some additional terminology. R is symmetric, if arb () bra. R is reflexive, if ara whenever a 2 dom(). R is transitive,ifarb ^ brc =) arc. R is an equivalence relation on, if it is symmetric, reflexive and transitive. EXMPLE 1.3. Let R be the relation on Z Z defined by arb () a b. Then R is reflexive and transitive, but not symmetric. EXMPLE 1.4. Let R be the relation on Z Z defined by arb () a < b. Then R is transitive, but neither reflexive nor symmetric. EXMPLE 1.5. Let R be the relation on Z Z defined by arb () a 2 = b 2.In this case, R is an equivalence relation. It is evident that arb iff b = a or b = a Functions. DEFINITION 1.7. relation R Ω is a function if arb 1 ^ arb 2 =) b 1 = b 2. If f Ω is a function and dom f =, then we usually write f :! and use the usual notation f (a) = b instead of afb. If f :! is a function, the usual intuitive interpretation is to regard f as a rule that associates each element of with a unique element of. It s not necessarily the case that each element of is associated with something from ; i.e., may not be ran f.

7 4. FUNCTIONS ND RELTIONS 1-7 f b a c f f g b a c g g FIGURE 2. These diagrams show two functions, f :! and g :!. The function g is injective and f is not because f (a) = f (c). EXMPLE 1.6. Define f : N! Z by f (n) = n 2 and g : Z! Z by g (n) = n 2.In this case ran f = {n 2 : n 2 N} and ran g = ran f [ {0}. Notice that even though f and g use the same formula, they are actually different functions. DEFINITION 1.8. If f :! and g :! C, then the composition of g with f is the function g ± f :! C defined by g ± f (a) = g (f (a)). In Example 1.6, g ± f (n) = g (f (n)) = g (n 2 ) = (n 2 ) 2 = n 4 makes sense for all n 2 N, but f ± g is undefined at n = 0. There are several important types of functions. DEFINITION 1.9. function f :! is a constant function, if ran f has a single element; i. e., there is a b 2 such that f (a) = b for all a 2. The function f is surjective (or onto ), if ran f =. In a sense, constant and surjective functions are the opposite extremes. constant function has the smallest possible range and a surjective function has the largest possible range. Of course, a function f :! can be both constant and surjective, if has only one element. DEFINITION function f :! is injective (or one-to-one), if f (a) = f (b) implies a = b. The terminology one-to-one is very descriptive because such a function uniquely pairs up the elements of its domain and range. n illustration of this definition is in Figure 2. In Example 1.6, f is injective while g is not.

8 1-8 CHPTER 1. SIC IDES f f 1 f 1 f FIGURE 3. This is one way to visualize a general invertible function. First f does something to a and then f 1 undoes it. DEFINITION function f :! is bijective, if it is both surjective and injective. bijective function can be visualized as uniquely pairing up all the elements of and. Some authors, favoring less pretentious language, use the more descriptive terminology one-to-one correspondence instead of bijection. This pairing up of the elements from each set is like counting them and finding they have the same number of elements. Given any two sets, no matter how many elements they have, the intuitive idea is they have the same number of elements if, and only if, there is a bijection between them. The following theorem shows that this property of counting the number of elements works in a familiar way. (Its proof is left as an easy exercise.) THEOREM If f :! and g :! C are bijections, then g ± f :! C is a bijection Inverse Functions. DEFINITION If f :!, C Ω and D Ω, then the image of C is the set f (C) = {f (a):a 2 C}. The inverse image of D is the set f 1 (D) = {a : f (a) 2 D}. Definitions 1.11 and 1.13 work together in the following way. Suppose f :! is bijective and b 2. The fact that f is surjective guarantees that f 1 ({b}) 6= ;. Since f is injective, f 1 ({b}) contains only one element, say a, where f (a) = b. In this way, it is seen that f 1 is a rule that assigns each element of to exactly one element of ; i. e., f 1 is a function with domain and range. DEFINITION If f :! is bijective, the inverse of f is the function f 1 :! with the property that f 1 ± f (a) = a for all a 2 and f ± f 1 (b) = b for all b 2. There is some ambiguity in the meaning of f 1 between 1.13 and The former is an operation working with subsets of and ; the latter is a function working with elements of and. It s usually clear from the context which meaning is being used. EXMPLE 1.7. Let = N and be the even natural numbers. If f :! is f (n) = 2n and g :! is g (n) = n/2, it is clear f is bijective. Since f ± g (n) =

9 4. FUNCTIONS ND RELTIONS 1-9 f (n/2) = 2n/2 = n and g ± f (n) = g (2n) = 2n/2 = n, we see g = f 1. (Of course, it is also true that f = g 1.) EXMPLE 1.8. Let f : N! Z be defined by ( (n 1)/2, n odd, f (n) = n/2, n even It s quite easy to see that f is bijective and ( 2n + 1, n 0, f 1 (n) = 2n, n < 0 Given any set, it s obvious there is a bijection f :! and, if g :! is a bijection, then so is g 1 :!. Combining these observations with Theorem 1.12, an easy theorem follows. THEOREM Let S be a collection of sets. The relation on S defined by is an equivalence relation. ª () there is a bijection f :! 4.5. Schröder-ernstein Theorem. The following theorem is a powerful tool in set theory, and shows that a seemingly intuitively obvious statement is sometimes difficult to verify. It will be used in Section 5. THEOREM 1.16 (Schröder-ernstein 5 ). Let and be sets. If there are injective functions f :! and g :!, then there is a bijective function h :! f() FIGURE 4. Here are the first few steps from the construction used in the proof of Theorem This is often called the Cantor-Schröder-ernstein or Cantor-ernstein Theorem, despite the fact that it was apparently first proved by Richard Dedekind.

10 1-10 CHPTER 1. SIC IDES PROOF. Let 1 = \ f (). If k Ω is defined for some k 2 N, let k = g ( k ) and k+1 = f ( k ). This inductively defines k and k for all k 2 N. Use these sets to define Ã = S k2n k and h :! as ( g 1 (x), x 2 Ã h(x) = f (x), x 2 \ Ã. It must be shown that h is well-defined, injective and surjective. To show h is well-defined, let x 2. Ifx 2 \ Ã, then it is clear h(x) = f (x)is defined. On the other hand, if x 2 Ã, then x 2 k for some k. Since x 2 k = g ( k ), we see h(x) = g 1 (x) is defined. Therefore, h is well-defined. To show h is injective, let x, y 2 with x 6= y. If both x, y 2 Ã or x, y 2 \ Ã, then the assumptions that g and f are injective, respectively, imply h(x) 6= h(y). The remaining case is when x 2 Ã and y 2 \ Ã. Suppose x 2 k and h(x) = h(y). If k = 1, then h(x) = g 1 (x) 2 1 and h(y) = f (y) 2 f () = \ 1. This is clearly incompatible with the assumption that h(x) = h(y). Now, suppose k > 1. Then there is an x such that This implies so that x = g ± f ± g ± f ± ±f ± g (x 1 ). {z } k 1 f s and kg s h(x) = g 1 (x) = f ± g ± f ± ±f ± g (x 1 ) = f (y) {z } k 1 f s and k 1 g s y = g ± f ± g ± f ± ±f ± g (x 1 ) 2 {z } k 1 Ω Ã. k 2 f s and k 1 g s This contradiction shows that h(x) 6= h(y). We conclude h is injective. To show h is surjective, let y 2. Ify 2 k for some k, then h( k ) = g 1 ( k ) = k shows y 2 h(). If y k for any k, y 2 f () because 1 = \ f (), and g (y) Ã, so y = h(x) = f (x) for some x 2. This shows h is surjective. The Schröder-ernstein theorem has many consequences, some of which are at first a bit unintuitive, such as the following theorem. COROLLRY There is a bijective function h : N! N N PROOF. If f : N! N N is f (n) = (n,1), then f is clearly injective. On the other hand, suppose g : N N! N is defined by g ((a,b)) = 2 a 3 b. The uniqueness of prime factorizations guarantees g is injective. n application of Theorem 1.16 yields h. To appreciate the power of the Schröder-ernstein theorem, try to find an explicit bijection h : N! N N.

11 5. CRDINLITY Cardinality There is a way to use sets and functions to formalize and generalize how we count. For example, suppose we want to count how many elements are in the set {a,b,c}. The natural way to do this is to point at each element in succession and say one, two, three. What we re doing is defining a bijective function between {a,b,c} and the set {1,2,3}. This idea can be generalized. DEFINITION Given n 2 N, the set n = {1,2,,n} is called an initial segment of N. The trivial initial segment is 0 =;. set S has cardinality n, if there is a bijective function f : S! n. In this case, we write card(s) = n. The cardinalities defined in Definition 1.18 are called the finite cardinal numbers. They correspond to the everyday counting numbers we usually use. The idea can be generalized still further. DEFINITION Let and be two sets. If there is an injective function f :!, we say card() card(). ccording to Theorem 1.16, the Schröder-ernstein Theorem, if card() card() and card() card(), then there is a bijective function f :!. s expected, in this case we write card() = card(). When card() card(), but no such bijection exists, we write card() < card(). Theorem 1.15 shows that card() = card() is an equivalence relation between sets. The idea here, of course, is that card() = card() means and have the same number of elements and card() < card() means is a smaller set than. This simple intuitive understanding has some surprising consequences when the sets involved do not have finite cardinality. In particular, the set is countably infinite, if card() = card(n). In this case, it is common to write card(n) 0. 6 When 0, then is said to be a countable set. In other words, the countable sets are those having finite or countably infinite cardinality. EXMPLE 1.9. Let f : N! Z be defined as ( n+1 f (n) = 2, when n is odd 1 n. 2, when n is even It s easy to show f is a bijection, so card(n) = card(z) 0. THEOREM Suppose and are countable sets. (a) is countable. (b) [ is countable. PROOF. (a) This is a consequence of Theorem (b) This is Exercise The is the Hebrew letter aleph is usually pronounced aleph nought.

12 1-12 CHPTER 1. SIC IDES n alert reader will have noticed from previous examples 0 = card(z) = card(!) = card(n) = card(n N) = card(n N N) = logical question is whether all sets either have finite cardinality, or are countably infinite. That this is not so is seen by letting S = N in the following theorem. THEOREM If S is a set, card(s) < card(p (S)). PROOF. Noting that 0 = card(;) < 1 = card(p (;)), the theorem is true when S is empty. Suppose S 6= ;. Since {a} 2 P (S) for all a 2 S, it follows that card(s) card(p (S)). Therefore, it suffices to prove there is no surjective function f : S! P (S). To see this, assume there is such a function f and let T = {x 2 S : x f (x)}. Since f is surjective, there is a t 2 S such that f (t) = T. Either t 2 T or t T. If t 2 T = f (t), then the definition of T implies t T, a contradiction. On the other hand, if t T = f (t), then the definition of T implies t 2 T, another contradiction. These contradictions lead to the conclusion that no such function f can exist. set S is said to be uncountably infinite, or just uncountable, 0 < card(s). Theorem < card(p (N)), so P (N) is uncountable. In fact, the same argument 0 = card(n) < card(p (N)) < card(p (P (N))) < So, there are an infinite number of distinct infinite cardinalities. In 1874 Georg Cantor [5] proved card(r) = card(p (N)) and card(r) 0, where R is the set of real numbers. ( version of Cantor s theorem appears in Theorem 2.26 below.) This naturally led to the question whether there are sets S such 0 < card(s) < card(r). Cantor spent many years trying answer this question and never succeeded. His assumption that no such sets exist came to be called the continuum hypothesis. The importance of the continuum hypothesis was highlighted by David Hilbert at the 1900 International Congress of Mathematicians in Paris, when he put it first on his famous list of the 23 most important open problems in mathematics. Kurt Gödel proved in 1940 that the continuum hypothesis cannot be disproved using standard set theory, but he did not prove it was true. In 1963 it was proved by Paul Cohen that the continuum hypothesis is actually unprovable as a theorem in standard set theory. So, the continuum hypothesis is a statement with the strange property that it is neither true nor false within the framework of ordinary set theory. This means that in the standard axiomatic development of set theory, the continuum hypothesis, or a suitable negation of it, can be taken as an additional axiom without causing any contradictions. The technical terminology is that the continuum hypothesis is independent of the axioms of set theory.

13 6. EXERCISES 1-13 The proofs of these theorems are extremely difficult and entire broad areas of mathematics were invented just to make their proofs possible. Even today, there are some deep philosophical questions swirling around them. more technical introduction to many of these ideas is contained in the book by Ciesielski [8]. nontechnical and very readable history of the efforts by mathematicians to understand the continuum hypothesis is the book by czel [1]. readable account of Cantor s work is in an article by Dauben [9]. 6. Exercises 1.1. If a set S has n elements for n 2!, then how many elements are in P (S)? 1.2. Is there a set S such that S \ P (S) 6= ;? 1.3. Prove that for any sets and, (a) = ( \ ) [ ( \ ) (b) [ = ( \ ) [ ( \ ) [ ( \ ) and that the sets \, \ and \ are pairwise disjoint. (c) \ = \ c Prove Theorem For any sets,, C and D, and ( ) \ (C D) = ( \C) ( \ D) ( ) [ (C D) Ω ( [C) ( [ D). Why does equality not hold in the second expression? 1.6. Prove Theorem Suppose R is an equivalence relation on. For each x 2 define C x = {y 2 : xry}. Prove that if x, y 2, then either C x = C y or C x \C y =;. (The collection {C x : x 2 } is the set of equivalence classes induced by R.) 1.8. If f :! and g :! C are bijections, then so is g ± f :! C Prove or give a counter example: f : X! Y is injective iff whenever and are disjoint subsets of Y, then f 1 () \ f 1 () =; If f :! is bijective, then f 1 is unique Prove that f : X! Y is surjective iff for each subset Ω X, Y \ f () Ω f (X \ ) Suppose that k is a set for each positive integer k. (a) Show that x 2 T 1 n=1 S 1 k=n k iff x 2 k for infinitely many sets k.

14 1-14 CHPTER 1. SIC IDES (b) Show that x 2 S 1 n=1 T 1 k=n k iff x 2 k for all but finitely many of the sets k. The set T 1 n=1 S 1 k=n k from (a) is often called the superior limit of the sets k and S 1 n=1 T 1 k=n k is often called the inferior limit of the sets k Given two sets and, it is common to let denote the set of all functions f :!. Prove that for any set, card 2 = card(p ()). This is why many authors use 2 as their notation for P () Let S be a set. Prove the following two statements are equivalent: (a) S is infinite; and, (b) there is a proper subset T of S and a bijection f : S! T. This statement is often used as the definition of when a set is infinite If S is an infinite set, then there is a countably infinite collection of nonempty pairwise disjoint infinite sets T n, n 2 N such that S = S n2n T n Without using the Schröder-ernstein theorem, find a bijection f : [0, 1]! (0,1) If f :[0,1)! (0,1) and g : (0,1)! [0,1) are given by f (x) = x + 1 and g (x) = x, then the proof of the Schrŏder-ernstein theorem yields what bijection h : [0,1)! (0,1)? Find a function f : R \ {0}! R \ {0} such that f 1 = 1/f Find a bijection f : [0,1)! (0,1) If f :! and g :! are functions such that f ± g (x) = x for all x 2 and g ± f (x) = x for all x 2, then f 1 = g If and are sets such that card() = card() 0, then card( [ ) Using the notation from the proof of the Schröder-ernstein Theorem, let = [0,1), = (0,1), f (x) = x + 1 and g (x) = x. Determine h(x) Using the notation from the proof of the Schröder-ernstein Theorem, let = N, = Z, f (n) = n and ( 1 3n, n 0 g (n) = 3n 1, n > 0. Calculate h(6) and h(7) Suppose that in the statement of the Schröder-ernstein theorem = = Z and f (n) = g (n) = 2n. Following the procedure in the proof yields what function h?

15 6. EXERCISES If { n : n 2 N} is a collection of countable sets, then S n2n n is countable If and are sets, the set of all functions f :! is often denoted by. If S is a set, prove that card 2 S = card(p (S)) card(s)), then there is an injective function f : S! S that is not surjective If card(s) 0, then there is a sequence of pairwise disjoint sets T n, n 2 N such that card(t n ) 0 for every n 2 N and S n2n T n = S.

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