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

In summary, the conversation is about whether there is a closed form for the infinite series of x to the power of n squared, for values of x between 0 and 1. One person doubts that there is a closed form, while another suggests using an elliptic theta function. However, this function is also defined through infinite series and does not have a closed form for x=1/2. The possibility of deriving a closed form through a combinatorial argument is mentioned.
  • #1
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:

[itex]\sum_n x^{n^2}[/itex]

for [itex]0 < x < 1[/itex]
 
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  • #2
I strongly doubt it.
 
  • #3
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...
 
  • #4
WA does not even have a closed form for x=1/2. Does not look good.
 
  • #5
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.
 
  • #6
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...
 

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

1. What is a "closed form" for an infinite sum?

A closed form for an infinite sum is a mathematical expression that gives an exact value for the sum, without the need for an infinite number of terms. It is often desirable to find a closed form for infinite sums, as it allows for easier calculations and deeper understanding of the sum's behavior.

2. How do you find a closed form for an infinite sum?

Finding a closed form for an infinite sum often involves using known mathematical formulas and techniques, such as geometric series, telescoping series, and the binomial theorem. It also requires a thorough understanding of the properties of the infinite sum and the underlying patterns in its terms.

3. Can all infinite sums have a closed form?

No, not all infinite sums have a closed form. Some infinite sums are considered "unsolvable" or do not have a recognizable pattern in their terms. In these cases, it may not be possible to find a closed form for the sum.

4. What are the benefits of finding a closed form for an infinite sum?

There are several benefits to finding a closed form for an infinite sum. It allows for easier and more efficient calculations compared to using an infinite number of terms. It also provides a deeper understanding of the sum's behavior and can lead to new insights and discoveries in mathematics.

5. Can a closed form for an infinite sum change over time?

Yes, a closed form for an infinite sum can change over time as new mathematical techniques and discoveries are made. What was once considered "unsolvable" may now have a closed form, or a previously known closed form may be simplified or improved upon. It is an ongoing and evolving field of study in mathematics.

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