Riemann series theorem

Summary

In mathematics, the Riemann series theorem, also called the Riemann rearrangement theorem, named after 19th-century German mathematician Bernhard Riemann, says that if an infinite series of real numbers is conditionally convergent, then its terms can be arranged in a permutation so that the new series converges to an arbitrary real number, or diverges. This implies that a series of real numbers is absolutely convergent if and only if it is unconditionally convergent.

As an example, the series 1 − 1 + 1/2 − 1/2 + 1/3 − 1/3 + ⋯ converges to 0 (for a sufficiently large number of terms, the partial sum gets arbitrarily near to 0); but replacing all terms with their absolute values gives 1 + 1 + 1/2 + 1/2 + 1/3 + 1/3 + ⋯, which sums to infinity. Thus the original series is conditionally convergent, and can be rearranged (by taking the first two positive terms followed by the first negative term, followed by the next two positive terms and then the next negative term, etc.) to give a series that converges to a different sum: 1 + 1/2 − 1 + 1/3 + 1/4 − 1/2 + ⋯ = ln 2. More generally, using this procedure with p positives followed by q negatives gives the sum ln(p/q). Other rearrangements give other finite sums or do not converge to any sum.

History edit

It is a basic result that the sum of finitely many numbers does not depend on the order in which they are added. For example, 2 + 6 + 7 = 7 + 2 + 6. The observation that the sum of an infinite sequence of numbers can depend on the ordering of the summands is commonly attributed to Augustin-Louis Cauchy in 1833.[1] He analyzed the alternating harmonic series, showing that certain rearrangements of its summands result in different limits. Around the same time, Peter Gustav Lejeune Dirichlet highlighted that such phenomena are ruled out in the context of absolute convergence, and gave further examples of Cauchy's phenomenon for some other series which fail to be absolutely convergent.[2]

In the course of his analysis of Fourier series and the theory of Riemann integration, Bernhard Riemann gave a full characterization of the rearrangement phenomena.[3] He proved that in the case of a convergent series which does not converge absolutely (known as conditional convergence), rearrangements can be found so that the new series converges to any arbitrarily prescribed real number.[4] Riemann's theorem is now considered as a basic part of the field of mathematical analysis.[5]

For any series, one may consider the set of all possible sums, corresponding to all possible rearrangements of the summands. Riemann’s theorem can be formulated as saying that, for a series of real numbers, this set is either empty, a single point (in the case of absolute convergence), or the entire real number line (in the case of conditional convergence). In this formulation, Riemann’s theorem was extended by Paul Lévy and Ernst Steinitz to series whose summands are complex numbers or, even more generally, elements of a finite-dimensional real vector space. They proved that the set of possible sums forms a real affine subspace. Extensions of the Lévy–Steinitz theorem to series in infinite-dimensional spaces have been considered by a number of authors.[6]

Definitions edit

A series   converges if there exists a value   such that the sequence of the partial sums

 

converges to  . That is, for any ε > 0, there exists an integer N such that if n ≥ N, then

 

A series converges conditionally if the series   converges but the series   diverges.

A permutation is simply a bijection from the set of positive integers to itself. This means that if   is a permutation, then for any positive integer   there exists exactly one positive integer   such that   In particular, if  , then  .

Statement of the theorem edit

Suppose that   is a sequence of real numbers, and that   is conditionally convergent. Let   be a real number. Then there exists a permutation   such that

 

There also exists a permutation   such that

 

The sum can also be rearranged to diverge to   or to fail to approach any limit, finite or infinite.

Alternating harmonic series edit

Changing the sum edit

The alternating harmonic series is a classic example of a conditionally convergent series:

 
is convergent, whereas
 
is the ordinary harmonic series, which diverges. Although in standard presentation the alternating harmonic series converges to ln(2), its terms can be arranged to converge to any number, or even to diverge.


One instance of this is as follows. Begin with the series written in the usual order,

 

and rearrange and regroup the terms as:

 

where the pattern is: the first two terms are 1 and −1/2, whose sum is 1/2. The next term is −1/4. The next two terms are 1/3 and −1/6, whose sum is 1/6. The next term is −1/8. The next two terms are 1/5 and −1/10, whose sum is 1/10. In general, since every odd integer occurs once positively and every even integers occur once negatively (half of them as multiples of 4, the other half as twice odd integers), the sum is composed of blocks of three which can be simplified as:

 


Hence, the above series can in fact be written as:

 

which is half the sum originally, and can only equate to the original sequence if the value were zero. This series can be demonstrated to be greater than zero by the proof of Leibniz's theorem using that the second partial sum is half.[7] Alternatively, the value of   which it converges to, cannot be zero. Hence, the value of the sequence is shown to depend on the order in which series is computed.

It is true that the sequence:

 

contains all elements in the sequence:

 


However, since the summation is defined as   and  , the order of the terms can influence the limit.[7]

Getting an arbitrary sum edit

An efficient way to recover and generalize the result of the previous section is to use the fact that

 

where γ is the Euler–Mascheroni constant, and where the notation o(1) denotes a quantity that depends upon the current variable (here, the variable is n) in such a way that this quantity goes to 0 when the variable tends to infinity.

It follows that the sum of q even terms satisfies

 

and by taking the difference, one sees that the sum of p odd terms satisfies

 

Suppose that two positive integers a and b are given, and that a rearrangement of the alternating harmonic series is formed by taking, in order, a positive terms from the alternating harmonic series, followed by b negative terms, and repeating this pattern at infinity (the alternating series itself corresponds to a = b = 1, the example in the preceding section corresponds to a = 1, b = 2):

 

Then the partial sum of order (a + b)n of this rearranged series contains p = an positive odd terms and q = bn negative even terms, hence

 

It follows that the sum of this rearranged series is[8]

 

Suppose now that, more generally, a rearranged series of the alternating harmonic series is organized in such a way that the ratio pn/qn between the number of positive and negative terms in the partial sum of order n tends to a positive limit r. Then, the sum of such a rearrangement will be

 

and this explains that any real number x can be obtained as sum of a rearranged series of the alternating harmonic series: it suffices to form a rearrangement for which the limit r is equal to e2x/ 4.

Proof edit

Existence of a rearrangement that sums to any positive real M edit

Riemann's description of the theorem and its proof reads in full:[9]

… infinite series fall into two distinct classes, depending on whether or not they remain convergent when all the terms are made positive. In the first class the terms can be arbitrarily rearranged; in the second, on the other hand, the value is dependent on the ordering of the terms. Indeed, if we denote the positive terms of a series in the second class by a1, a2, a3, ... and the negative terms by b1, −b2, −b3, ... then it is clear that Σa as well as Σb must be infinite. For if they were both finite, the series would still be convergent after making all the signs the same. If only one were infinite, then the series would diverge. Clearly now an arbitrarily given value C can be obtained by a suitable reordering of the terms. We take alternately the positive terms of the series until the sum is greater than C, and then the negative terms until the sum is less than C. The deviation from C never amounts to more than the size of the term at the last place the signs were switched. Now, since the number a as well as the numbers b become infinitely small with increasing index, so also are the deviations from C. If we proceed sufficiently far in the series, the deviation becomes arbitrarily small, that is, the series converges to C.

This can be given more detail as follows.[10] Recall that a conditionally convergent series of real terms has both infinitely many negative terms and infinitely many positive terms. First, define two quantities,   and   by:

 

That is, the series   includes all an positive, with all negative terms replaced by zeroes, and the series   includes all an negative, with all positive terms replaced by zeroes. Since   is conditionally convergent, both the 'positive' and the 'negative' series diverge. Let M be any real number. Take just enough of the positive terms   so that their sum exceeds M. That is, let p1 be the smallest positive integer such that

 

This is possible because the partial sums of the   series tend to  . Now let q1 be the smallest positive integer such that

 

This number exists because the partial sums of   tend to  . Now continue inductively, defining p2 as the smallest integer larger than p1 such that

 

and so on. The result may be viewed as a new sequence

 

Furthermore the partial sums of this new sequence converge to M. This can be seen from the fact that for any i,

 

with the first inequality holding due to the fact that pi+1 has been defined as the smallest number larger than pi which makes the second inequality true; as a consequence, it holds that

 

Since the right-hand side converges to zero due to the assumption of conditional convergence, this shows that the (pi+1 + qi)'th partial sum of the new sequence converges to M as i increases. Similarly, the (pi+1 + qi+1)'th partial sum also converges to M. Since the (pi+1 + qi + 1)'th, (pi+1 + qi + 2)'th, ... (pi+1 + qi+1 − 1)'th partial sums are valued between the (pi+1 + qi)'th and (pi+1 + qi+1)'th partial sums, it follows that the whole sequence of partial sums converges to M.

Every entry in the original sequence an appears in this new sequence whose partial sums converge to M. Those entries of the original sequence which are zero will appear twice in the new sequence (once in the 'positive' sequence and once in the 'negative' sequence), and every second such appearance can be removed, which does not affect the summation in any way. The new sequence is thus a permutation of the original sequence.

Existence of a rearrangement that diverges to infinity edit

Let   be a conditionally convergent series. The following is a proof that there exists a rearrangement of this series that tends to   (a similar argument can be used to show that   can also be attained).

The above proof of Riemann's original formulation only needs to be modified so that pi+1 is selected as the smallest integer larger than pi such that

 

and with qi+1 selected as the smallest integer larger than qi such that

 

The choice of i+1 on the left-hand sides is immaterial, as it could be replaced by any sequence increasing to infinity. Since   converges to zero as n increases, for sufficiently large i there is

 

and this proves (just as with the analysis of convergence above) that the sequence of partial sums of the new sequence diverge to infinity.

Existence of a rearrangement that fails to approach any limit, finite or infinite edit

The above proof only needs to be modified so that pi+1 is selected as the smallest integer larger than pi such that

 

and with qi+1 selected as the smallest integer larger than qi such that

 

This directly shows that the sequence of partial sums contains infinitely many entries which are larger than 1, and also infinitely many entries which are less than −1, so that the sequence of partial sums cannot converge.

Generalizations edit

Sierpiński theorem edit

Given an infinite series  , we may consider a set of "fixed points"  , and study the real numbers that the series can sum to if we are only allowed to permute indices in  . That is, we let

 
With this notation, we have:
  • If   is finite, then  . Here   means symmetric difference.
  • If   then  .
  • If the series is an absolutely convergent sum, then   for any  .
  • If the series is a conditionally convergent sum, then by Riemann series theorem,  .

Sierpiński proved that rearranging only the positive terms one can obtain a series converging to any prescribed value less than or equal to the sum of the original series, but larger values in general can not be attained.[11][12][13] That is, let   be a conditionally convergent sum, then   contains  , but there is no guarantee that it contains any other number.

More generally, let   be an ideal of  , then we can define  .

Let   be the set of all asymptotic density zero sets  , that is,  . It's clear that   is an ideal of  .

(Władysław, 2007)[14] —  If   is a conditionally convergent sum, then   (that is, it is sufficient to rearrange a set of indices of asymptotic density zero).

Proof sketch: Given  , a conditionally convergent sum, construct some   such that   and   are both conditionally convergent. Then, rearranging   suffices to converge to any number in  .

Filipów and Szuca proved that other ideals also have this property.[15]

Steinitz's theorem edit

Given a converging series  of complex numbers, several cases can occur when considering the set of possible sums for all series   obtained by rearranging (permuting) the terms of that series:

  • the series   may converge unconditionally; then, all rearranged series converge, and have the same sum: the set of sums of the rearranged series reduces to one point;
  • the series   may fail to converge unconditionally; if S denotes the set of sums of those rearranged series that converge, then, either the set S is a line L in the complex plane C, of the form
     
    or the set S is the whole complex plane C.

More generally, given a converging series of vectors in a finite-dimensional real vector space E, the set of sums of converging rearranged series is an affine subspace of E.

See also edit

References edit

  1. ^ Cauchy 1833, Section 8; Apostol 1967, p. 411.
  2. ^ Dirichlet 1837, Section 1.
  3. ^ Riemann 1868.
  4. ^ Kline 1990, p. 966.
  5. ^ Apostol 1967, Section 10.21; Apostol 1974, Section 8.18; Rudin 1976, Theorem 3.54; Whittaker & Watson 2021, Section II.17.
  6. ^ Banaszczyk 1991, Section 10; Mauldin 2015, Problem 28 and Problem 106.
  7. ^ a b Spivak, Michael (2008). Calculus (4th ed.). Houston, Texas: Publish or Perish. pp. 482–483. ISBN 978-0-914098-91-1.
  8. ^ Apostol, Tom M. (1991-01-16). Calculus, Volume 1. John Wiley & Sons. p. 416. ISBN 978-0-471-00005-1.
  9. ^ Riemann 1868, p. 97, quoted from the 2004 English translation.
  10. ^ Apostol 1967, Section 10.21; Whittaker & Watson 2021, Section II.17.
  11. ^ Sierpiński, Wacław (1910). "Przyczynek do teoryi szeregów rozbieżnych [Contribution à la théorie des séries divergentes]" [Contribution to the theory of divergent series]. Sprawozdania Z Posiedzen Towarzystwa Naukowego Warszawskiego (in Polish). 3: 89–93.
  12. ^ Sierpiński, Wacław (1910). "Uwaga do twierdzenia Riemanna o szeregach warunkowo zbieżnych [Remarque sur le théorème de Riemann relatif aux séries semiconvergentes]" [Remark on Riemann's theorem relating to semi-convergent series]. Prace Matematyczno-Fizyczne (in Polish). 21 (1): 17–20.
  13. ^ Sierpiński, Wacław (1911). "Sur une propriété des séries qui ne sont pas absolument convergentes [O pewnej własności szeregów warunkowo zbieżnych]". Bulletin International de l'Académie des Sciences de Cracovie, Séries A: 149–158.
  14. ^ Wilczyński, Władysław (2007). "On Riemann derangement theorem". Słupskie Prace Matematyczno-Fizyczne. 4: 79–82.
  15. ^ Filipów, Rafał; Szuca, Piotr (February 2010). "Rearrangement of conditionally convergent series on a small set". Journal of Mathematical Analysis and Applications. 362 (1): 64–71. doi:10.1016/j.jmaa.2009.07.029.

External links edit