It is said that:
Abel summation and Euler summation are not comparable.
We were able to find examples of divergent series which are Euler summable but not Abel summable, for instance
1−2+4−8+16−…
However, we couldn't find any example of a divergent series which is Abel summable but not Euler summable.
Do you know such an example?
Thank you!
EDIT: Dear Peter, this is the definition of Euler summation:
Let ∑∞n=0an be any series. The Euler transformation of this series is defined as:
∞∑n=012n+1bn with bn:=n∑k=0(nk)ak
The series ∑∞n=0an is called Euler summable if the Euler transformation of this series
∞∑n=012n+1bn
is converges in the usual sense.
The Euler sum is then given by
∞∑n=012n+1bn.
Answer
From the Wikipedia article,
Euler summation is essentially an explicit form of analytic continuation. If a power series converges for small complex z and can be analytically continued to the open disk with diameter from −1/2 to 1 and is continuous at 1, then its value at 1 is called the Euler sum of the series a0+a1+….
Whereas Abel summation consists of taking the limit of
f(z)≡∑∞n=0anzn
as z approaches 1 from below along the real axis. For a series to be Abel-summable but not Euler-summable, it has to be that f(z) has a limit as z→1− along the real axis, but f(z) is not continuous at z=1. An example would be f(z)=exp(−z1−z), which has an essential singularity at z=1, but for which lim exists and is equal to 0.
No comments:
Post a Comment