See main article: Grandi's series.
The formal manipulations that lead to 1 − 1 + 1 − 1 + · · · being assigned a value of 1⁄2 include:
These are all legal manipulations for sums of convergent series, but 1 − 1 + 1 − 1 + · · · is not a convergent series.
Nonetheless, there are many summation methods that respect these manipulations and that do assign a "sum" to Grandi's series. Two of the simplest methods are Cesàro summation and Abel summation.[1]
The first rigorous method for summing divergent series was published by Ernesto Cesàro in 1890. The basic idea is similar to Leibniz's probabilistic approach: essentially, the Cesàro sum of a series is the average of all of its partial sums. Formally one computes, for each n, the average σn of the first n partial sums, and takes the limit of these Cesàro means as n goes to infinity.
For Grandi's series, the sequence of arithmetic means is
1, 1⁄2, 2⁄3, 2⁄4, 3⁄5, 3⁄6, 4⁄7, 4⁄8, …or, more suggestively,
(1⁄2+1⁄2), 1⁄2, (1⁄2+1⁄6), 1⁄2, (1⁄2+1⁄10), 1⁄2, (1⁄2+1⁄14), 1⁄2, …where
| \sigma | ||||
|
| \sigma | |||||||
|
The Cesàro sum of 1 + 0 − 1 + 1 + 0 − 1 + · · · is 2⁄3. So the Cesàro sum of a series can be altered by inserting infinitely many 0s as well as infinitely many brackets.[3]
The series can also be summed by the more general fractional (C, a) methods.[4]
Abel summation is similar to Euler's attempted definition of sums of divergent series, but it avoids Callet's and N. Bernoulli's objections by precisely constructing the function to use. In fact, Euler likely meant to limit his definition to power series,[5] and in practice he used it almost exclusively[6] in a form now known as Abel's method.
Given a series a0 + a1 + a2 + · · ·, one forms a new series a0 + a1x + a2x2 + · · ·. If the latter series converges for 0 < x < 1 to a function with a limit as x tends to 1, then this limit is called the Abel sum of the original series, after Abel's theorem which guarantees that the procedure is consistent with ordinary summation. For Grandi's series one has
| infty(-1) | |
| A\sum | |
| n=0 |
n=\limx →
| infty(-x) | |
| \sum | |
| n=0 |
n=\limx →
| 1 | = | |
| 1+x |
| 12. | |
The corresponding calculation that the Abel sum of 1 + 0 − 1 + 1 + 0 − 1 + · · · is 2⁄3 involves the function (1 + x)/(1 + x + x2).
Whenever a series is Cesàro summable, it is also Abel summable and has the same sum. On the other hand, taking the Cauchy product of Grandi's series with itself yields a series which is Abel summable but not Cesàro summable:
has Abel sum 1⁄4.[8]
That the ordinary Abel sum of 1 + 0 − 1 + 1 + 0 − 1 + · · · is 2⁄3 can also be phrased as the (A, λ) sum of the original series 1 − 1 + 1 − 1 + · · · where (λn) = (0, 2, 3, 5, 6, …). Likewise the (A, λ) sum of 1 − 1 + 1 − 1 + · · · where (λn) = (0, 1, 3, 4, 6, …) is 1⁄3.[9]
The summability of 1 − 1 + 1 − 1 + · · · can be frustrated by separating its terms with exponentially longer and longer groups of zeros. The simplest example to describe is the series where (−1)n appears in the rank 2n:
0 + 1 − 1 + 0 + 1 + 0 + 0 + 0 − 1 + 0 + 0 + 0 + 0 + 0 + 0 + 0 + 1 + 0 + · · ·.This series is not Cesaro summable. After each nonzero term, the partial sums spend enough time lingering at either 0 or 1 to bring the average partial sum halfway to that point from its previous value. Over the interval following a (− 1) term, the nth arithmetic means vary over the range
| 2 | \left( | |
| 3 |
| 22m-1 | \right) to | |
| 22m+2 |
| 1 | |
| 3 |
(1-2-2m),
In fact, the exponentially spaced series is not Abel summable either. Its Abel sum is the limit as x approaches 1 of the function
F(x) = 0 + x − x2 + 0 + x4 + 0 + 0 + 0 − x8 + 0 + 0 + 0 + 0 + 0 + 0 + 0 + x16 + 0 + · · ·.
This function satisfies a functional equation:
\begin{array}{rcl} F(x)&=&\displaystylex-x2+x4-x8+ … \\[1em] &=&\displaystylex-\left[(x2)-(x2)2+(x2)4- … \right]\\[1em] &=&\displaystylex-F(x2). \end{array}
F(x)=\Psi(x)+\Phi(x)
\Phi(x)=
| |||||
| \sum | \left(log | ||||
| n=0 |
| 1x\right) | |
| n |
Given any function φ(x) such that φ(0) = 1, and the derivative of φ is integrable over (0, +∞), then the generalized φ-sum of Grandi's series exists and is equal to 1⁄2:
S\varphi=\lim\delta\downarrow0
| infty(-1) | |
| \sum | |
| k=0 |
k\varphi(\deltak)=
| 12. | |
The Cesaro or Abel sum is recovered by letting φ be a triangular or exponential function, respectively. If φ is additionally assumed to be continuously differentiable, then the claim can be proved by applying the mean value theorem and converting the sum into an integral. Briefly:
\begin{array}{rcl} S\varphi&=&\displaystyle\lim\delta\downarrow0
| infty\left[\varphi(2k\delta) | |
| \sum | |
| k=0 |
-\varphi(2k\delta+\delta)\right]\\[1em] &=&\displaystyle\lim\delta\downarrow0
| infty\varphi'(2k\delta+c | |
| \sum | |
| k)(-\delta) |
\\[1em] &=&\displaystyle-
| 12\int | |
| 0 |
infty\varphi'(x)dx=-
| 12\varphi(x)| | |
| 0 |
infty=
| 12. \end{array} | |
The Borel sum of Grandi's series is again 1⁄2, since
| 1-x+ | x2 | - |
| 2! |
| x3 | + | |
| 3! |
| x4 | |
| 4! |
- … =e-x
| infty | |
| \int | |
| 0 |
e-xe-x
| infty | |
| dx=\int | |
| 0 |
e-2xdx=
| 12. | |
The series can also be summed by generalized (B, r) methods.[13]
The entries in Grandi's series can be paired to the eigenvalues of an infinite-dimensional operator on Hilbert space. Giving the series this interpretation gives rise to the idea of spectral asymmetry, which occurs widely in physics. The value that the series sums to depends on the asymptotic behaviour of the eigenvalues of the operator. Thus, for example, let
\{\omegan\}
\sumnsgn(\omegan)
where
sgn(\omegan)=\pm1
\limt\to\sumnsgn(\omegan)
| -t|\omegan| | |
| e |
which, for many interesting cases, is finite for non-zero t, and converges to a finite value in the limit.
The integral function method with pn = exp (-cn2) and c > 0.[14]
The moment constant method with
d\chi=
| -k(logx)2 | |
| e |
x-1dx
The geometric series in
(x-1)
| 1 | |
| x |
=1-(x-1)+(x-1)2-(x-1)3+(x-1)4-...
is convergent for
|x-1|<1
x=2
| 1 | |
| 2 |
=1-1+1-1+1-...
However,
x=2
|x-1|<1
. G. H. Hardy . Divergent Series . 1949 . Clarendon Press . .