Thursday, 9 January 2014

real analysis - Convergence of two unusual "nested" sums

I was contemplating convergent sums, trying to think of very unusual or unorthodox sums that might be treatable recursively. Eventually, the following sum occurred to me:



ξ=1+12+34+78+89+45+910+1011+23+56+1112+1213+67+1314+1415+



There is no especial motivation, but I endeavored to determine if I could show at least that it was bounded. I was not sure how to treat it however; it seemed like it might be possible to put it into some form of continued fraction, however I confess I am not familiar enough with continued fractions to see how this might be done (if it is possible). I calculated the first "convergents", or terms of the sequence {ξn}nN0 as I imagined it, if I were to write it as



ξ0=1,ξ1=1+1223=1+34=1.75,ξ2=1+12+344523+5667=1+12+151623+3536=1+231659361.87712,ξ3=1+12+34+788945+910101123+56+1112121367+13141415=1.8887.



which reinforced my thought that it was potentially a convergent sum. If anyone has any advice as to what methods may be used to establish this, or any other insights, they would be a great assistance in sating my curiosity.



The perceived intractability of the aforementioned sum lead me to consider the "visually similar" but more simple sum



1+1223+34455667+788991010111112121313141415+



One can notice that the fraction ratios are of the form p1ppp+1=p21p2=11p2, so that I could express this sum as



1+(1122)+11421162+(1182)(11142)(11102)(11122)+(11162)(11222)(11262)(11282)(11182)(11202)(11242)(11302)+



Unfortunately, I do not see a very obvious way to generalize this to a sum perhaps roughly of the form
n2Z+2ni=0( something... )


which might be tackled more effectively.



If anyone can offer any assistance, or is familiar with similar problems, I am very interested in learning more regarding properties, especially convergence, of these types of "nested" sums.

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