Tuesday, December 22, 2015

Divergent series which is Abel summable but not Euler summable



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
12+48+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:=nk=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 z1 along the real axis, but f(z) is not continuous at z=1. An example would be f(z)=exp(z1z), which has an essential singularity at z=1, but for which lim exists and is equal to 0.


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