# Mathematics 615. Definition. If M is a number such that x M for all x in S then M is called an upper bound for S.

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1 Mathematics 615 Review: Real numbers, Sequences, Limits, Lim Sup, Set notation. These notes are intended to be a fast review of elementary facts concerning convergence of sequences of real numbers. The goal is to give a self contained exposition of the notion of completeness of the real number system. Completeness is responsibe for the existence of solutions to ordinary differential equations, partial differential equations, eigenvalue problems, and almost everything else that guides our intuition in analysis. The main ideas in these notes are usually taught in a freshman calculus course. Some of these ideas are developed over and over again in later courses. Chances are that this is at least the third time around for most of you... but it may have been a while.... Recall that a rational numbber is a number of the form m/n where m and n are integers (i.e. whole numbers.) If x = m/n is a rational number it is always possible to choose m and n to have no positive integer factors in common. We say that x is then represented in lowest terms. Example of an irrational number. The suare root of 2 is not rational.proof: Assume that x = m/n is represented in lowest terms and that x 2 = 2. Then m 2 = 2n 2. So m 2 is even (i.e. is a multiple of 2.) If m itself were odd then m 2 would also be odd because (2k +1) 2 = 2(k 2 +2k)+1. So m must be even. Say m = 2j. Therefore (2j) 2 = 2n 2. So 2j 2 = n 2. It now follows that n is also even. This contradicts our assumption that m and n have no common factors. So there is no rational number x satisfying x 2 = 2. Denote by Q the set of rational numbers and by R the set of real numbers. We will be using the symbols + and in this course quite a bit. But it is a universally accepted convention not to call them real numbers. So they are not in R. Some authors like to adjoin them to R and then call the result the extended real number system: R { } { }. We will do this also. Notation: (, ) = R, (, ] = R { }, etc. But R still means the set of real numbers and excludes ±. Addition, subtraction, multplication and division ( but not by zero) make sense within Q and also within R. ( Both are fields.) Moreover Q and R both have an order relation on them. (I.e. a < b has a well defined meaning, which is in fact nicely related to the algebraic operations. E.g. a < b implies a+c < b+c, etc.) The big difference between Q and R is that Q is full of holes. For example the sequence of rational numbers 1, 14/10, 141/100, 1414/1000,... would like to converge to 2 1/2. But 2 1/2 isn t there (in Q.) There is a hole there. The next few pages are devoted to making this anthropomorphic discussion precise.the key concept is that of a Cauchy sequence. Theorem 1 on page 4 says that there are no holes in R. Definition. A set S R (real numbers) is bounded if there is a real number M such that x M for all x in S. S is bounded above if there is a real number M such that x M for all x in S. S is bounded below if for some M in R, x M for all x in S. Definition. If M is a number such that x M for all x in S then M is called an upper bound for S. Definition. If L is an upper bound for S such that L M for all upper bounds M of S then L is called the least upper bound for S. We write L = lubs. 1

2 2 In order to prove anytning about the real numbers one needs, of course, a definition of them. But no axiomatic treatment of the real number system will be given here. ( See Rudin, Principles of Analysis, Chapter 1, if you really want one.) Instead I am just going to list the key property - order completeness- which any definition of the real numbers must include, either in the following form or some equivalent form. R is order complete: that is, if S is a nonempty set of real numbers which is bounded above then S has a least upper bound in R. Remark. The set Q of rational numbers is not order complete. For example if S = {x in Q : x 2 < 2} then S does not have a least upper bound in Q. But S does have a least upper bound in R. What is it? Definition. A sequence is a function from the set Z := {1, 2, 3,... } to R. Notation: s(i) = s i. Definition. If {s n } is a sequence then {t k } is a subsequence if there is a sequence integers n k such that n 1 < n 2 < n 3... and t k = s nk. Definition. A sequence {s n } is said to converge to a number L in R if for each ε > 0 there is an integer N such that s n L < ε whenever n N. Notation. L = lim n s n or s n L as n. Proposition. If s n is an increasing sequence of real numbers bounded above then lim n s n exists and equals lub{s n : n = 1, 2,... }. Proof: Let L = lub{s n } n=1. Then s n L for all n. Let ε > 0. Then L ε is not an upper bound for {s n } n=1. Thus there exists N such that s N > L ε. Since s n s N for all n > N we have s n > L ε for all n > N. Hence s n L > ε. But s n L 0. Hence s n L < ε if n N. Remarks. 1) If S is bounded below, then S is bounded above. By S we mean { x : x S}. 2) Greatest lower bound is defined analogously to least upper bound. Notation. glb S = greatest lower bound of S. 3) Notation. If S is a nonempty set of real numbers which is not bounded above we write lub S = +. If S is nonempty and not bounded below we write glb S =. Exercise. (Write out, but don t hand in) Prove: glb(s) = lub( S) for any nonempty set S. Definition. For any sequence {s n } n=1 we write lim number M there is an integer N such that n s n = + if for each real [Similar definition for lim n s n =.] s n M for all n N.

3 3 Now let {s n } n=1 be an arbitrary sequence of real numbers. Set a k = lub{s n : n k}. Since {s n : n k} {s n : n k + 1} we have a k a k+1. Thus the sequence {a k } k=1 is a decreasing sequence. Using the preceding proposition and definitions, we see that either a) For all k, a k = + or b) For all k, a k is finite (i.e., real) and the set {a k } k=1 is bounded below or c) For all k, a k is finite and{a k : k = 1, 2,... } is not bounded below. In case b) lim k a k exists (and is real) and we write lim sup s n = lim a k. n k In case a) we write In case c) we write lim sup s n = +. n lim sup s n =. n Remarks. 4) sup stands for supremum and means the same thing as lub. inf stands for infinum and means the same as glb. 5) The preceding definition of lim sup s n can be written succinctly in all cases as Similarly we define lim sup s n = lim sup{s n : n k}. n k lim inf s n = lim inf{s n : n k}. n k Proposition 2. If lim n s n := L exists and is real (i.e., finite) then lim sup s n = L = lim inf s n. Conversely if lim sup s n = lim inf s n and both are finite then lim s n exists and is equal to their common value. Proof: Assume lim s n exists and denote it by L. Let ε > 0. N depending on ε such that {s n : n N} [l ε, L + ε]. Thus for all k N, a k := sup{s n : n k} L + ε. Hence lim a k L + ε. Since the inequality holds for all ε > 0 and the k left side is independent of ε we have lim a k L. Thus lim sup s n L. Similarly k n lim inf s n L. But note that n Hence one always has (for any sequence) inf{s n : n k} sup{s n : n k}. lim n inf s n lim n sup s n.

4 4 But we saw that lim sup s n L lim inf s n. n n Hence we have equality. Proof of converse. Assume lim sup s n = lim inf s n = L n n (L is assumed finite, i.e., real). Let a k = inf{s n : n k} and b k = sup{s n : n k}. Then lim a k = lim b k = L. Let ε > 0. There is an integer N 1 (depending on ε, as usual) such that a k L < ε whenever k N 1. Similarly N 2 b k L < ε whenever k N 1. Let N = max{n 1, N 2 }. Then a k L < ε and b k L < ε if k N. Thus L ε < a k b k < L + ε if k N. In particular L ε < a N b N < L + ε. Thus L ε < a N s k b N < L + ε k N. So s k L < ε k N. Definition. A sequence {s n } n=1 is CAUCHY if for each ε > 0 N s n s m < ε n, m N. Proposition 3. Any convergent sequence {s n } n=1 of real numbers is Cauchy. Proof: Given ε > 0 N s n L < ε/2 n N. Then s n s m = s n L + L s m s n L + L s m < ε/2 + ε/2 = ε if n, m N.

5 Theorem 1. Every Cauchy sequence of real numbers converges to a real number. Remarks. 6) The property stated in Theorem 1 is usually referred to as the completeness of the set R of real numbers. The set of rational numbers is not complete. I.e., there is a Cauchy sequence s n in Q which does not have a limit in Q. E.g. take s 1 = 1, s 2 = 1.4, s 3 = 1.41, s 4 = 1.414,..., (so that s n 2, which is not in Q). Proof of Theorem 1: Let {s n } n=1 be a Cauchy sequence. Step 1. The set {s n : n = 1, 2,... } is bounded because if we choose ε = 1 in the definition of Cauchy sequence then we know that there is an integer N s n s N < 1 if n N. Hence s n = s n s N + s N s n s N + s N 1 + s N if n N. Thus if we put K = max{ s 1, s 2,..., s N 1, 1 + s N } then s n K for all n. Step 2. We now know that lim n sup s n is a real number. Let L = lim n sup s n. Claim: lim s n = L. For let ε > 0. Choose N such that s n s m < ε n, m N. Then s N ε < s n < s N + ε n N. Thus if a k = sup{s n : n k} then s N ε a k s N + ε k N. But by definition L = lim k a k. Thus taking the limit as k we get s N ε L s N + ε. So s N L ε. Hence if n N then s n L s n s N + s N L ε + ε = 2ε n N. 5 Set Theory Notation Notation. If A and B are subsets of a set S then the intersection of A and B (A B) is defined by A B = {x S : x A and x B}. The union of A and B is defined by A B = {x S : x A or x B or both)}. The complement of A is A c = {x S : x / A}. The difference is A B = {x A : x / B}. If f : S T is a function and B T then f 1 (B) is defined by f 1 (B) = {x S : f(x) B}.

6 6 Math 615 Homework #1 1. Find the lim sup and lim inf for the following sequences: a) 1, 2, 3, 1, 2, 3, 1, 2, 3,... b) {sin(nπ/2)} n=1 c) {(1 + 1/n) cos nπ} n=1 d) {(1 + 1/n) n } n=1 2. If the lim sup of the sequence {s n } n=1 is equal to M, prove that the lim sup of any subsequence is M. 3. If {s n } n=1 is a bounded sequence of real numbers and lim n inf s n = m, prove that there is a subsequence of {s n } n=1 which converges to m. Also prove that no subsequence of {s m } m=1 can converge to a limit less than m. 4. Write the set of all rational numbers in (0, 1) as r 1, r 2,.... Calculate lim n sup r n and lim n inf r n. 5. If s n +, prove that lim n sup s n = = lim n inf s n. 6. Prove that A (B C) = (A B) (A C). 7. If f : X Y is a function, and A and B are subsets of Y, prove: a) f 1 (A B) = f 1 (A) f 1 (B) b) f 1 (A B) = f 1 (A) f 1 (B) c) f 1 (A c ) = f 1 (A) c 8. Show by example that f(a B) = f(a) f(b) is not always true. Show by example that f(a c ) = f(a) c is not always true.

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