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Limits and Topology of Metric Spaces LIMITS AND TOPOLOGY OF METRIC SPACES PAUL SCHRIMPF SEPTEMBER 26, 2013 UNIVERSITY OF BRITISH COLUMBIA ECONOMICS 526 This lecture focuses on sequences, limits, and topology. Similar material is covered in chapters 12 and 29 of Simon and Blume, or 1.3 of Carter. 1. SEQUENCES AND LIMITS f g f g¥ f g A sequence is a list of elements, x1, x2, ... or xn n=1 or sometimes just xn . Al- though the notation for a sequence is similar to the notation for a set, they should not be confused. Sequences are different from sets in that the order of elements in a sequence matters, and the same element can appear many times in a sequence. Some examples of 2 R sequences with xi include (1) f1, 1, 2, 3, 5, 8, ...g f 1 1 1 g (2) 1, 2 , 3 , 4 , ... f 1 −2 3 −4 5 g (3) 2 , 3 , 4 , 5 , 6 , ... Some sequences, like 2, have elements that all get closer and closer to some fixed point. We say that these types of sequences converge. A sequence that does not converge di- verges. Some divergent sequences like 1, increase without bound. Other divergent se- quences, like 3, are bounded, but they do not converge to any single point. To analyze sequences with elements that are not necessarily real numbers, we need to be able to say how far apart the entries in the sequence are. Definition 1.1. A metric space is a set, X, and function d : X × X ! R called a metric (or distance) such that 8x, y, z 2 X (1) d(x, y) > 0 unless x = y and then d(x, x) = 0 (2) (symmetry) d(x, y) = d(y, x) (3) (triangle inequality) d(x, y) ≤ d(x, z) + d(z, y). Example 1.1. R is a metric space with d(x, y) = jx − yj. Example 1.2. Any normed vector space is a metric space with d(x, y) = kx − yk. The most common metricp space that we will encounter will be Rn with the Euclidean k − k ∑n − 2 metric, d(x, y) = x y = i=1(xi yi) . f g¥ 8e 9 Definition 1.2. A sequence xn n=1 in a metric space converges to x if > 0 N such that d(xn, x) < e ¥ ≥ f g !¥ ! for all n N. We call x the limit of xn n=1 and write limn xn = x or xn x. 1 LIMITS AND TOPOLOGY OF METRIC SPACES f 1 1 1 g f g¥ Example 1.3. The sequence 1, 2 , 3 , 4 , ... = 1/n n=1 converges. To see this, take any e > 0. Then 9 N such that 1/N < e. For all n ≥ N, d(1/n, 0) = 1/n < e. If a sequence does not converge, it diverges. Example 1.4. The Fibonacci sequence, f1, 1, 2, 3, 5, 8, ...g diverges. f g¥ 8e 9 Definition 1.3. a is an accumulation point of xn n=1 if > 0 infinitely many xi such that e d(a, xi) < . f 1 −2 3 −4 5 g − Example 1.5. The sequence 2 , 3 , 4 , 5 , 6 , ... has two accumulation points, 1 and 1. The limit of any convergent sequence is an accumulation point of the sequence. In fact, it is the only accumulation point. ! f g¥ Lemma 1.1. If xn x, then x is the only accumulation point of xn n=1. Proof. Let e > 0 be given. By the definition of convergence, 9N such that d(xn, x) < e for all n ≥ N. fn 2 N : n ≥ Ng is infinite, so x is an accumulation point. 0 0 Suppose x is another accumulation point. Then 8e > 0 9N and N such that if n ≥ 0 0 N and n ≥ N , then d(xn, x) < e/2 and d(xn, x ) < e/2). By the triangle inequality, 0 0 d(x, x ) ≤ d(xn, x ) + d(xn, x) < e. Since this inequality holds for any e, it must be that 0 0 d(x, x ) = 0. d is a metric, so then x = x , and the limit of sequence is the sequence’s unique accumulation point. □ The third example of a sequence at the start of this section, 3, shows that the converse of this lemma is false. Not every accumulation point is a limit. f g¥ f g Definition 1.4. Given xn n=1 and any sequence of positive integers, nk such that n1 < < f g f g¥ n2 ... we call xnk a subsequence of xn n=1. In example 3, there are two accumulation points, −1 and 1, and you can find subse- quences that converge to these points. Lemma 1.2. Let a be an accumulation point of fxng. Then 9 a subsequence that converges to a. fe g Proof. We can construct a subsequence as follows. Let k be a sequence that converges e 8 e to zero with k > 0 k, (for example, k = 1/k). By the definition of accumulation point, e 9 for each k infinitely many xn such that e d(xn, a) < k (1) e 6 Pick any xn1 such that (1) holds for 1. For k > 1, pick nk = nj for all j < k and such that e (1) holds for k. Such an nk always exists because there are infinite xn that satisfy (1). By □ construction, limk!¥ xnk = a (you should verify this using the definition of limit). Convergence of sequences is often preserved by arithmetic operations, as in the follow- ing two theorems. 2 LIMITS AND TOPOLOGY OF METRIC SPACES Theorem 1.1. Let fxng and fyng be sequences in a normed vector space V. If xn ! x and yn ! y, then xn + yn ! x + y. Proof. Let e > 0 be given. Then 9 Nx such that for all n ≥ Nx, d(xn, x) < e/2, and 9Ny such that for all n ≥ Ny, d(yn, y) < e/2. Let N = maxfNx, Nyg. Then for all n ≥ N, d(xn + yn, x + y) = k(xn + yn) − (x + y)k ≤ kxn − xk + kyn − yk <e/2 + e/2 = e. □ Theorem 1.2. Let fxng be a sequence in a normed vector space with scalar field R and let fcng be a sequence in R. If xn ! x and cn ! c then xncn ! xc. Proof. On problem set. □ In fact, in the next lecture we will see that if f (·, ·) is continuous, then lim f (xn, yn) = f (x, y). The previous two theorems are examples of this with f (x, y) = x + y and f (c, x) = cx, respectively. 1.1. Series. Infinite sums or series are formally defined as the limit of the sequence of partial sums. f g¥ ∑n Definition 1.5. Let xn n=1 be a sequence in a normed vector space. Let sn = i=1 xi denote the sum of the first n elements of the sequence. We call sn the nth partial sum. We define the sum of all the x s as i ¥ ≡ ∑ xi lim sn n!¥ i=1 This is called a(n infinite) series. b 2 R ∑¥ bi Example 1.6. Let . i=0 is called a geometric series. Geometric series appear often in economics, where b will be the subjective discount factor or perhaps 1/(1 + r). Notice that 2 n sn =1 + b + b + ··· + b − =1 + b(1 + b + ··· + bn 1) − =1 + b(1 + b + ··· + bn 1 + bn) − bn+1 n+1 sn(1 − b) =1 − b 1 − bn+1 s = , n 1 − b 3 LIMITS AND TOPOLOGY OF METRIC SPACES so, ¥ i ∑ b = lim sn i=0 1 − bn+1 = lim 1 − b 1 = if jbj < 1. 1 − b 1.2. Cauchy sequences. We have defined convergent sequences as ones whose entries all get close to a fixed limit point. This means that all the entries of the sequence are also getting closer together. You might imagine a sequence where the entries get close together without necessarily reaching a fixed limit. f g¥ e 9 Definition 1.6. A sequence xn n=1 is a Cauchy sequence if for any > 0 N such that ≥ e for all i, j N, d(xi, xj) < . It turns that in Rn Cauchy sequences and convergent sequences are the same. This is a consequence of R having the least upper bound property. Theorem 1.3. A sequence in Rn converges if and only if it is a Cauchy sequence. There is a proof of this in Chapter 29.1 of Simon and Blume. If you want more practice with the sort of proofs in this lecture, it would be good to read that section. The conver- gence of Cauchy sequences in the real numbers is a consequence of the least upper bound property that we discussed in lecture 1. Cauchy sequences do not converge in all metric spaces. For example, the rational numbers are a metric space, and any sequence of ratio- nals that converges to an irrational number in R is a Cauchy sequence in Q but has no limit in Q. Having Cauchy sequences converge is necessary for proving many theorems, so we have a special name for metric spaces where Cauchy sequences converge. Definition 1.7. A metric space, X, is complete if every Cauchy sequence of points in X converges in X. Completeness is important for so many results that complete version of vector spaces are named after the mathematicians who first studied them extensively. A Banach space is a complete normed vector space. A Hilbert space is a complete inner productp space.
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