Help with serie summation

[yassermp]

Has anyone heard about a way to find the sum of a serie of this form:
s=\sum_i{\exp(a+b\sqrt(i))}

[tiny-tim]

Has anyone heard about a way to find the sum of a serie of this form:
s=\sum_i{\exp(a+b\sqrt(i))}

Hi yassermp! Welcome to PF! :smile:

Why would you want to sum such a series? :confused:

Have you noticed you can take the "a" outside the ∑, and write it:

s = e^a\,\sum_n{e^{b\sqrt{n}}.

[yassermp]

Hi yassermp! Welcome to PF! :smile:

Why would you want to sum such a series? :confused:

Have you noticed you can take the "a" outside the ∑, and write it:

s = e^a\,\sum_n{e^{b\sqrt{n}}.

Hi Tiny Tim, i see what you say, you are totally right. Essencially, that kind of sum arises when you try to sum contributions of several spherical waves, from scattering centers located at r_j=\sqrt{y^2+(z-z_j)^2}, with z_j=jh, j=1...n. The original sum is:
s=\sum_j{e^{ikr_j}/r_j
Very often some approximations are used here, but i would like to obtain an exact analytical expression (no matter what complicated it could be). Id really thank any usefull sugestion(I know this is not an easy one). I tried a bit with some Fourier transform but i think it takes to an endless road.
Thks

[tiny-tim]

… i would like to obtain an exact analytical expression (no matter what complicated it could be). Id really thank any usefull sugestion(I know this is not an easy one). I tried a bit with some Fourier transform but i think it takes to an endless road.
Thks

Hi yassermp!

Sorry … but I can't help you there. :blushing:

(btw, not a good idea to use i as an index when you're dealing with complex numbers! :smile:)