INTRODUCTION TO THE CONVERGENCE OF SEQUENCES


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1 INTRODUCTION TO THE CONVERGENCE OF SEQUENCES BECKY LYTLE Abstract. In this paper, we discuss the basic ideas involved in sequences and convergence. We start by defining sequences and follow by explaining convergence and divergence, bounded sequences, continuity, and subsequences. Relevant theorems, such as the BolzanoWeierstrass theorem, will be given and we will apply each concept to a variety of exercises. Contents. Introduction to Sequences 2. Limit of a Sequence 2. Divergence and Bounded Sequences 4 4. Continuity 5 5. Subsequences and the BolzanoWeierstrass Theorem 5 References 7. Introduction to Sequences Definition.. A sequence is a function whose domain is N and whose codomain is R. Given a function f: N R, f(n) is the nth term in the sequence. Example.2. The first example of a sequence is x n = n. In this case, our function f is defined as f(n) = n. As a listed sequence of numbers, this would look like the following: (.) (, 2,, 4, 5, 6, 7... ) Another example of a sequence is x n = 5 n, which would look like the following: (.4) (5, 25, 25, ) We know these are both valid examples of sequences because they are infinite lists of real numbers and hence can be regarded as functions with domain N. Example.5. The following would not be an examples of sequences: (.6) (, 2, ) (.7) (500, 200, 550, 0000) Date: July 2, 205.
2 2 BECKY LYTLE We know that these are not examples of sequences because they are finite lists of real numbers. 2. Limit of a Sequence A limit describes how a sequence x n behaves eventually as n gets very large, in a sense that we make explicit below. Definition 2.. A sequence of real numbers converges to a real number a if, for every positive number ɛ, there exists an N N such that for all n N, a n  a < ɛ. We call such an a the limit of the sequence and write lim n a n = a. Proposition. The sequence n converges to zero. Proof. Let ɛ > 0. We choose N N such that N > ɛ. Such a choice is always possible by the Archimedean property. To verify that this choice of N is appropriate, let n N satisfy n N. Then, n N implies n > ɛ which is equal to n = n 0 < ɛ, proving that n converges to zero by the definition of convergence. Proposition 2. An example of a sequence that does not converge is the following: (2.2) (,,,,...) If a sequence does not converge, it is said to diverge, which we will explain later in the paper, along with the explanation of why the above sequence does not converge. Proposition. If x n y n z n for all n N and lim n x n =lim n z n =l, then lim n y n =l too. Proof. Let ɛ > 0. We want to show there exists an N such that for all n > N, y n l < ɛ. We know that x n l. Therefore, there exists an N such that for all n > N, x n l < ɛ. Also, we know that z n l. Therefore, there exists an N 2 such that for all n > N 2, z n l < ɛ. Let N = max(n, N 2 ) and n > N. Then, n > N so x n l < ɛ. Also, n > N 2 so z n l < ɛ. We want to show that y n l < ɛ. This is equivalent to showing that both y n l < ɛ and l y n < ɛ. We know that y n z n, so y n l z n l < ɛ. Also, we know that y n x n, so l y n l x n < ɛ. Theorem 2. (Algebraic Limit Theorem). Let lim n a n = a and lim n b n = b. Then, (i) lim n ca n = ca for all c R (ii) lim n (a n + b n ) = a+b (iii) lim n (a n b n ) = ab (iv) lim n (a n /b n ) = a/b provided b 0 Example 2.4. If (x n ) 2, then ((2x n  )/). Proof. First, we will start with the information given in the example: x n 2. Next, we simply use the fact that ( 4 )( 2 ) = 2. (2.5) x n ( ) ( ) 4 2
3 INTRODUCTION TO THE CONVERGENCE OF SEQUENCES Now, let a n = x n, and let a = ( 4 )( 2 ), and let c = ( 2 ). From the Algebraic Limit Theorem, we know that ca n ca. Then, ( 2 )(x n) ( 2 )( 4 )( 2 ), which is equal to the following: 2x n (2.6) 4 The next step follows from the fact that 4 = +. 2x n (2.7) + Let 2xn = a n, let (+ ) = a, let b n = (,,...), and let b =. Then, by the Algebraic Limit Theorem, we know that a n + b n a + b. Therefore, we know that 2xn + (+ ) +, which is equal to the following: 2x n (2.8) This last step follows because 2xn  = 2xn. 2x n (2.9) Therefore, using the Algebraic Limit Theorem, we have shown that if (x n ) 2, then ((2x n  )/). Example 2.0. The following sequence converges to the proposed limit (2.) lim ( ) 2n + = 2 5n n Proof. Let 5n+4 be a n, let 5n+4 be b n and let 2n+ 5n+4 be c n, and c n = a n + b n. By Theorem 2., we know that lim(c n ) = lim (a n +b n ) = lim(a n )+lim(b n ). We must therefore determine what lim(a n ) and lim(b n ) are. First, we will show that lim( 5n+4 ) = 0. Let ɛ > 0. By the Archimedean principle, there exists an N N such that N > /ɛ. Then, for n > N, 5n+4 < 5N+4 < /N < ɛ. Therefore, the limit of 5n+4 is zero. Then, because lim(c n ) = lim(a n +b n ), lim(c n ) = lim(a n + 0) = lim(a n ). We will therefore find the limit of a n in order to prove lim( 2n+ 5n+4 ) = 2 5. We now want to show that lim( 2n 5n+4 ) = 2 5. Let ɛ > 0. By the Archimedean Principle, there exists an N such that /ɛ < N. Let n > N. We then want to show the following: (2.2) Then, (2.) 2n 5n < ɛ 2n 5n = 8 5(5n + 4)
4 4 BECKY LYTLE We have to check the following: (2.4) 8 5(5n + 4) < ɛ (2.5) We know that the inequality 8 5(5n + 4) < ɛ 8 5(5n+4) < ɛ is true for every value of n because n > N > /ɛ and ɛ. Therefore we only need to show that the inequality 8 5(5n+4) < ɛ is true. Using the fact that N > /ɛ, we can say the following: (2.6) Then, 8 5(5n + 4) < 8 5(5(/ɛ) + 4) = 8ɛ ɛ 8ɛ 25+20ɛ < 8ɛ 25 < ɛ. Therefore, 8 5(5n+4) < ɛ. Example 2.7. Let x n 0. If (x n ) 0, then ( x n ) 0. Proof. First, we have to prove that lim( x n ) exists. We know that x n is decreasing but is greater than or equal to 0 for all values of n. The square root of a positive number is also positive. Therefore, x n 0. Also, note that if 0< a < b, then 0< a< b. So if x n is decreasing, then so is x n. Therefore, lim( x n ) exists. Next, we must prove that ( x n ) 0. Let lim(x n ) = lim(( x n )( x n )) = 0. By the Algebraic Limit Theorem, we know that if lim(a n ) = a and lim(b n ) = b then lim((a n )(b n )) = ab. By this theorem, lim(( x n )( x n )) = lim( x n )lim( x n ) = 0. Thus, (lim( x n )) 2 =0. This implies that lim( x n )=0.. Divergence and Bounded Sequences Definition.. A sequence that does not have a limit or in other words, does not converge, is said to be divergent. Example.2. Recall proposition 2, which says that the following sequence does not converge: (.) (,,,...) Later in this paper, we will give a concise proof of this fact. Contrast this with the following sequence, which we have seen (.4) (, 2,, 4, 5, 6, 7... ) This converges to zero, as we proved earlier in this paper. However, these sequences do have something in common. They are both bounded. Definition.5. A sequence (x n ) is bounded if there exists a number M>0 such that x n M for all n N. Geometrically, this means we can find an interval [ M,M] that contains every term in the sequence (x n ).
5 INTRODUCTION TO THE CONVERGENCE OF SEQUENCES 5 Example.6. Given the sequence x n = (, 2,, 2,, 2...), we can see that the interval [, 2] contains every term in x n. This sequence is therefore a bounded sequence. Example.7. Given the sequence x n = (0, 00, 000, 0000,...), we can see that there is no real number that serves as an upper bound because lim(x n ) is infinity. Therefore, there does not exist any interval that contains every term in the sequence x n, and x n is not a bounded sequence. Theorem.8. Every convergent sequence is bounded. Example.9. Theorem being illustrated: Let x n = n+ n, which is the following sequence: ( 2 (.0), 2, 4, 5 ) 4... We know this converges to and can verify this using the same logic used in the proof under the definition of convergence showing that n converges to zero. Therefore, as n becomes very large, x n approaches, but is never equal to. By the above theorem, we know that this sequence is bounded because it is convergent. We can see that x n is a decreasing sequence, so the x is the largest value of the sequence and is the upper bound. The limit of the sequence,, is the lower bound. An interval that contains every term in the sequence x n is (,2]. 4. Continuity Theorem 4.. If f: R R is continuous, x n x implies f(x n ) f(x) Example 4.2. Theorem being applied: Let f(x) = x. This function is continuous. Let lim(x n ) = 5. In order words, x n 5. By the above theorem, this implies that f(x n ) f(5). This is equal to x n ()(5) which is also equal to x n 5. Therefore, we are able to see what the limit of f(x n ) is using this theorem. Example 4.. Theorem failing when function is noncontinuous: Let f(x) be x, a noncontinuous function. We know this is noncontinuous because there is an asymptote at x=0. Let x n be n. We know this converges to zero based on a previous proof. Let s see if the continuity theorem fails for a noncontinuous function f. The theorem states that f(x n ) converges to f(x) if x n x. We know that x n 0, so if the theorem works, then f(x n ) f(0). But f(0) = 0 which does not exist. Therefore, f(x n ) cannot converge to /0, and the theorem fails for this noncontinuous function. 5. Subsequences and the BolzanoWeierstrass Theorem Definition 5.. Let (a n ) be a sequence of real numbers, and let n < n 2 < n < n 4... be an increasing sequence of natural numbers. Then, the sequence (a n, a n2, a n, a n4...) is called a subsequence of (a n ) and is denoted by (a nk ), where k N indexes the subsequence.
6 6 BECKY LYTLE Example 5.2. Let x n = n = (, 2,, 4 subsequences:...). Below are two examples of valid (5.) (, 6, 9, 2...) (5.4) ( 20, 200, ) Theorem 5.5. BolzanoWeierstrass Theorem: Every bounded sequence contains a convergent subsequence. Example 5.6. Given a sequence x n = (,2,,4,,2,,4...), a convergent subsequence can be found. Proof. We know that this sequence is bounded by the interval [, 4]. By the BolzanoWeierstrass Theorem, we can say that there indeed exists a convergent subsequence of x n. Just by looking at this sequence, we can see four convergent subsequences: (,,...), (2,2,2...), (,,...), and (4,4,4...). These subsequences converge to, 2,, and 4 respectively. Example 5.7. Given an unbounded sequence x n subsequence of x n does not exist = (,2,,4,5...), a convergent Proof. A convergent subsequence does not necessarily exist because this sequence does not satisfy the BolzanoWeierstrass Theorem. Recall that any subsequence of a sequence is nonrepeating and in the order of the original entries of x n. Notice that x n is increasing for all values of n and is divergent, considering the sequence continues until infinity. Therefore, for any subsequence a n, the values will be increasing toward infinity as well, and the subsequence will also be divergent. Theorem 5.8. Subsequences of a convergent sequence converge to the same limit as the original sequence. Example 5.9. Let us return to the example of a divergent sequence that was given under the definition of divergence. Recall that this sequence, x n, was (, ,, ,, ...). One subsequence of x n is (,,,...). This subsequence converges to. Another subsequence of x n is (, , ,...). This subsequence converges to . Now, we will prove that x n is divergent by contradiction. Assume x n is convergent. Then, by the above theorem, all its subsequences converge to lim(x n ), implying that all its subsequences converge to the same value. The two subsequences of x n stated above converge to different values. Therefore, this contradicts our original hypothesis that x n is convergent. We are then able to conclude that x n is divergent. Acknowledgements. I would like to thank my two mentors, Sean Howe and Abby Ward for meeting with me each week for the duration of my stay at the program. Thank you so much for suggesting this topic, explaining everything to me that I was confused about, editing and helping with my paper, and calming me down when I got overwhelmed. I would also like to thank Peter May for organizing this wonderful REU and for giving me the opportunity to participate.
7 INTRODUCTION TO THE CONVERGENCE OF SEQUENCES 7 References [] Walter Rudin. Principles of Mathematical Analysis. McGrawHill Inc [2] Stephen Abbott. Understanding Analysis. Springer. 205.
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