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Conditional Convergence of Numerical Series

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Conditional Convergence of Numerical Series Classroom notes 267 Conditional convergence of numerical series E. GO MEZ(1) and A. PLAZA(2) (1) E.T.S.I. de Telecomunicacio n, and (2) Department of Mathematics, University of Las Palmas de Gran Canaria, Spain (Received 8 January 2001) One of the most astonishing properties when studying numerical series is that the sum is not commutative, that is the sum may change when the order of its elements is altered. In this note an example is given of such a series. A well- known mathematical proof is given and a MATLAB# program used for di erent rearrangements of the series converging to di erent numbers. It is noted that the code follows the mathematical idea of the proof of the property. Developing programs showing the main ideas in the proofs of the theorems may be a good tool in the understanding of such theorems and proofs. 1. Introduction The sum of an in®nite series is very closely connected to the limit of an in®nite sequence. In fact, an in®nite series is usually de®ned as the limit of the partial sums of a given sequence an n1 1. That is f g ˆ N 1 an lim an lim SN ˆ N ˆ N n 1 !1 n 1 !1 Xˆ Xˆ where N SN an ˆ n 1 Xˆ Finite sums have the property that the sum remains unchanged when the order of the terms is changed, that is N N an bn n 1 ˆ n 1 Xˆ Xˆ for every permutation of 1; 2; . ; N ;’, such that bn a’ n : When we sum in®nite series their sums depenf d on thegorder of the termsˆ. A chang… † e of the order in an in®nite series means that a series n1 1 an a1 a2 a3 . is transformed ˆ ˆ ‡ ‡ ‡ by a rearrangement into a series n1 1 bn b1 b2 b3 . if every term an in the ®rst series appears exactly once in thˆ ePseconˆ d‡and‡conversely‡ . That is, `there is a P bijection from N to N, ’, such that bn a’ n for every n N’. Thus, the only requirement is that everˆ y …elemen† t of the2original series appears, only once, somewhere in the new series. If some of the terms are moved to later positions in the series, other terms must be moved to earlier positions. For example the series 268 Classroom notes 1 1 1 1 1 1 . 1 ‡ 3 ¡ 2 ‡ 5 ‡ 7 ¡ 4 ‡ … † is a rearrangement of the series 1 1 1 1 1 1 1 . 2 ¡ 2 ‡ 3 ¡ 4 ‡ 5 ¡ 6 ‡ 7 ‡ … † However, the sum of the rearrangement (1) is: 1 1 1 1 1 1 . 3 ln 2 3 ‡ 3 ¡ 2 ‡ 5 ‡ 7 ¡ 4 ‡ ˆ 2 … † while series (2) sums to ln 2 (see for example [1, 2]). In order to see (3), following [3] we can write: ln 2 1 1 1 1 1 1 1 1 . ˆ ¡ 2 ‡ 3 ¡ 4 ‡ 5 ¡ 6 ‡ 7 ¡ 8 ‡ 1 ln 2 1 1 1 1 . 2 ˆ 2 ¡ 4 ‡ 6 ¡ 8 Thus, adding both sides of the previous equalities: 3 ln 2 1 1 1 1 1 1 . 4 2 ˆ ‡ 3 ¡ 2 ‡ 5 ‡ 7 ¡ 4 ‡ … † However, if a series is the limit of the sequence of the partial sums of a sequence, one may wonder if it is valid to manipulate two in®nite series as in the example given, in which the order of the terms is taken in two di erent ways. In this note we go over a classic theorem on the importance of the order of terms in in®nite sums, and consider the convergence to any number, the diver- gence or the oscillation of the suitable rearrangement of a series which is con- ditionally convergent but not absolutely convergent. As an illustration of the theorem we conduct experiments with the series n 1 1 1 1 1 1 1 1 1 ‡ 1 2 3 4 5 6 7 . …¡ † ¡ ‡ ¡ ‡ ¡ ‡ ‡ ˆ n 1 n Pˆ 2. The main theorem Let n1 1 an be a conditionally convergent series but not absolutely convergent. ˆ Let n1 1 pn be the series formed by the positive terms of the series and n1 1 qn the series ofˆPthe negative terms. Then, both these series are divergent. A rearrangeˆ - P P ment of the initial series, n1 1 bn, also exists, such that n1 1 bn is divergent, oscillating or convergent to anyˆ predetermined number. ˆ P P Proof. We follow [2]. The proof comprises four steps: Step 1. The general term of the series, an tends to zero when n increases: By writing Classroom notes 269 N N 1 ¡ aN an an SN SN 1 since ¡ ˆ n 1 ¡ n 1 ˆ ¡ Xˆ Xˆ 1 an lim SN ˆ N n 1 !1 Xˆ is convergent, in particular S 1 is a Cauchy sequence, i.e.: for a given positive f N gN 1 number ", it is possible to choosˆe an index K K " , in such a way that the ˆ … † expression S S is less than ", provided only that N > K and M > K. In j N ¡ Mj particular aN SN SN 1 < " if N 1 > K, so that aN 0 when N tends to j j ˆ j ¡ ¡ j ¡ j j ! in®nity. Therefore an; pn; and qn tend to zero when n tends to in®nity. Step 2. The series of positive terms and the series of negative terms are both divergent Now, since n1 1 an is convergent but not absolutely convergent, the series ˆ n1 1 an is divergent, that is n1 1 an increases without bound. Let us write ˆ j j P ˆ j j P NP N1 N2 an pn qn n 1 j j ˆ n 1 ¡ n 1 Xˆ Xˆ. Xˆ As N increases, N1 and N2 increase as well, and the limit of the left-hand side must therefore be equal to the di erence of the two sums on the right. Since the series n1 1 an does not converge absolutely, but does converge conditionally, then the ˆ series n1 1 pn and n1 1 qn must both diverge. Thus, if both were convergent, the serieP s woulˆ d be absolutelˆ y convergent, and if only one of them were divergent the P P series n1 1 an would be divergent, contrary to the hypothesis. ˆ StePp 3. A rearrangement convergent to any predetermined number exists Now let M be a ®xed number. We can rearrange the terms of the series in such a way that the new series n1 1 bn converges to M. Suppose that M is positive (this is not a restriction to the prooˆf because in other cases we proceed in a similar way). P n1 We then add the ®rst n1 positive terms, so that the sum n 1 pn is just greater than n1 ˆ or equal to M. Since n 1 pn increases with n1without bound, it is always possible to make the partial sumˆ greater than or equal to M. PWe now add just enough m1 P n1 m1 negative terms n 1 qn to ensure that the sum n 1 pn n 1 qn is less than M. ˆ ˆ ‡ ˆ This is also possible as follows from the divergence of the series n1 1 qn. We now addPjust enough positive terms n2 Pp , to makPe the partiaˆ l sum again n n1 n greater than or equal to M. ˆ P The values of the sums thus obtained wilPl oscillate around the number M, and when the process is carried far enough the oscillation will take place between arbitrarily narrow bounds; since the terms pn; and qn tend to zero when n is su ciently large, the length of the interval in which the oscillation takes place will also tend to zero. The theorem is thus demonstrated. Step 4. Divergent and oscillating cases The same idea as before can be applied to obtain a rearrangement of the initial series n1 1 an, n1 1 bn, such that n1 1 bn is divergent or oscillating. The key lies ˆ ˆ ˆ in the fact that the positive terms series n1 1 pn and negative terms series n1 1 qn P P P ˆ n1 ˆ are both divergent, so we can build up a rearrangement such that n 1 pn is greater than or equal to M, with M being a ®xePd positive number and ˆ P P 270 Classroom notes n1 m1 pn qn n 1 ‡ n 1 Xˆ Xˆ slightly less than M and n1 m1 n2 pn qn pn n 1 ‡ n 1 ‡ n n 1 Xˆ Xˆ ˆX1‡ greater than or equal to 2M, and so on. In this way the new series is divergent. Finally, in order to build up an oscillating series it is enough to show that we can obtain partial sums such that, for example n1 n1 m1 n1 m1 n2 p > M p q < N p q p > M n n ‡ n n ‡ n ‡ n n 1 n 1 n 1 n 1 n 1 n n1 1 Xˆ Xˆ Xˆ Xˆ Xˆ ˆX‡ and so on. Therefore some partial sums tend to M, and others tend to N. In this way the ®nal rearrangement is neither convergent nor divergent, that is, it is oscillating. 3. An example In this section we collect some examples regarding di erent arrangements of the series n 1 1 1 ‡ 1 1 1 1 1 1 …¡ † 1 . n 1 n ˆ ¡ 2 ‡ 3 ¡ 4 ‡ 5 ¡ 6 ‡ 7 ‡ Xˆ converging to di erent values.
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