Unraveling a Strange Integral: Computing \int^\infty_0{x}^{-x}dx

Click For Summary

Discussion Overview

The discussion focuses on the evaluation of the integral \(\int^\infty_0{x}^{-x}dx\), exploring methods of computation, potential closed forms, and approximations. Participants share their attempts and thoughts on the integral's properties, including its convergence and representation.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant expresses curiosity about the integral and notes that computational tools suggest it is approximately 1.995456.
  • Another participant asserts that the integral likely does not have a closed form and can only be approximated, expressing confidence that it cannot be represented with purely elementary functions.
  • A similar viewpoint is reiterated, emphasizing uncertainty about whether the integral can be represented with a power series.
  • One participant presents a limit-based approach to compute the integral, providing a specific expression and noting that it yields accurate results for certain values of \(a\) and \(n\).
  • A later reply suggests considering the residue theorem and mentions the use of an indefinite exponential integral on the positive half-line as a potential method for evaluation.

Areas of Agreement / Disagreement

Participants generally agree that the integral does not have a closed form, but there are differing opinions on whether it can be approximated or represented in other forms, such as power series. The discussion remains unresolved regarding the best method for computation.

Contextual Notes

Some participants express uncertainty about the convergence of the integral and the behavior of their computational methods, indicating potential limitations in their approaches.

gamesguru
Messages
84
Reaction score
2
I'm just curious to find how what this integral is, but more importantly how it's computed. I've looked at the method that the sophomore's dream integrals are computed with and they do not seem to work here, I get to a step where I'm evaluating a sum from x=0 to x=\infty.
Here's the integral, and I've tried it with CAS and it oddly appears to be ~1.995456≈2:
\int^\infty_0{x}^{-x}dx
Thanks in advanced.
 
Last edited:
Physics news on Phys.org
well, if you are trying to exactly compute it, i do not think it has a closed form. It can only be approximatet.
 
sutupidmath said:
well, if you are trying to exactly compute it, i do not think it has a closed form. It can only be approximatet.
I should have said this above, but I'm fairly confident that an answer that can be expressed with purely elementary functions and constants does not exist. However, I'm not yet certain whether or not it can be represented with a power series, just as the sophomore's dream integrals can be.
 
Well, I found the answer to be the following limit:
\int^\infty_0{x}^{-x}dx=\lim_{a\rightarrow\infty}a\sum_{n=0}^\infty (-a)^n \sum_{k=0}^n (-1)^k \frac{(\log a)^{n-k}}{(n-k)!(n+1)^{k+1}}\approx 1.99545595\approx2
When I plug this in for large a in CAS, I get weird behavior, but for a approximately 5 and summing from n=0 to n\approx 100 it gives accurate and fairly precise answers.
 
Did you try the residue theorem ...an indefinte exponential integral on the positive halfline ...
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 8 ·
Replies
8
Views
4K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 17 ·
Replies
17
Views
5K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 5 ·
Replies
5
Views
4K