Infinite Sum of Powers: Is There a Closed Form for the Series?

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Discussion Overview

The discussion revolves around the question of whether there is a closed form for the infinite series \(\sum_n x^{n^2}\) for values of \(x\) in the range \(0 < x < 1\). The topic touches on mathematical concepts related to series, functions, and potentially combinatorial arguments.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the existence of a closed form for the series.
  • Another participant mentions that Wolfram Alpha expresses the series in terms of an elliptic theta function, indicating a lack of familiarity with this function.
  • A different participant notes that Wolfram Alpha does not provide a closed form for the specific case of \(x = 1/2\), suggesting challenges in finding a closed form.
  • One participant speculates that deriving a closed form might involve a clever combinatorial argument, proposing the idea of viewing the series as a generating function.

Areas of Agreement / Disagreement

Participants generally express skepticism about the existence of a closed form for the series, but there is no consensus on the matter, and multiple competing views remain regarding the potential approaches to the problem.

Contextual Notes

Participants reference the need for specific conditions and definitions related to elliptic theta functions and generating functions, which may influence the discussion's direction and understanding.

stevendaryl
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This isn't quite a calculus question, but it didn't seem right for any of the other mathematics forums, either.

Does anybody if there is a closed form for the following infinite series:

\sum_n x^{n^2}

for 0 &lt; x &lt; 1
 
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I strongly doubt it.
 
Wolfram alpha gives it in terms of an elliptic theta function...no idea what that is:

http://www.wolframalpha.com/input/?i=1+x+x^4+x^9+...

Looking at the wikipedia page, it looks like these functions are defined through infinite series such as the one seen in the OP...
 
WA does not even have a closed form for x=1/2. Does not look good.
 
I just realized, you have to include the ...'s at the end of that URL or else it doesn't work just by clicking on the link.
 
If you could derive one, my best guess of how would be by some clever combinatorial argument, viewing it as a generating function. Just a guess...
 

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