The improvement of the convergence properties of a series, also called convergence acceleration or accelerated convergence, such that a series reaches its limit to within some accuracy with fewer terms than required before. Convergence improvement can be effected by forming a linear combination with a series whose sum is known. Useful sums include
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(1)
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(2)
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(3)
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(4)
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Kummer's transformation takes a convergent series
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(5)
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and another convergent series
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(6)
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with known 
such that
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(7)
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Then a series with more rapid convergence to the same value is given by
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(8)
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(Abramowitz and Stegun 1972).
The Euler transform takes a convergent alternating series
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(9)
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into a series with more rapid convergence to the same value to
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(10)
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where
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(11)
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(Abramowitz and Stegun 1972; Beeler et al. 1972).
A general technique that can be used to acceleration converge of series is to expand them in a Taylor series about infinity and interchange the order of summation. In cases where a symbolic form for the Taylor series can be found, this come sometimes even allow the sum over the original variable to be done symbolically. For example, consider the case of the sum
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(12)
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(OEIS A085361) that arises in the definition of the Alladi-Grinstead constant. The summand can be expanded about infinity to get
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(13)
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(14)
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Interchanging the order of summation then gives
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(15)
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(16)
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where 
is the Riemann zeta function, which converges
much more rapidly.
A transformations of the form
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(17)
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where
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(18)
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is the 
th
partial sum of a sequence 
, can often be useful for improving series
convergence (Hamming 1986, p. 205). In particular, 
can be written
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(19)
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(20)
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The application of this transformation can be efficiently carried out using Wynn's epsilon method. Letting 
, 
, and
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(21)
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for 
,
2, ... (correcting the typo of Hamming 1986, p. 206). The values of 
are there equivalent to the results of applying
transformations to the sequence 
(Hamming 1986, p. 206).
Given a series of the form
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(22)
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where 
is an analytic at 0 and on the closed unit disk, and
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(23)
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then the series can be rearranged to
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(24)
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(25)
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(26)
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where
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(27)
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is the Maclaurin series of 
and 
is the Riemann zeta
function (Flajolet and Vardi 1996). The transformed series exhibits geometric
convergence. Similarly, if 
is analytic in 
for some positive integer 
, then
![S=sum_(n=1)^(n_0-1)f(1/n)+sum_(m=2)^inftyf_m[zeta(m)-1/(1^m)-...-1/((n_0-1)^m)],](/images/equations/ConvergenceImprovement/NumberedEquation15.svg) |
(28)
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which converges geometrically (Flajolet and Vardi 1996). Equation (28) can also be used to further accelerate the convergence of series (◇).
