Creating and computing functions

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    Computing Functions
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Discussion Overview

The discussion revolves around the function f(x) = x^(x^x) and the exploration of its inverse, as well as methods for computing it programmatically. Participants consider whether an explicit inverse exists and discuss potential numerical methods for approximation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the existence of an inverse function g such that g(x^(x^x)) = x.
  • Another participant expresses uncertainty regarding whether an explicit expression for x in terms of elementary functions exists.
  • Participants discuss the use of logarithms to manipulate the function, leading to a more complex expression involving ln y and ln ln y.
  • Several methods for approximating the function in code are proposed, including iterative approaches and Newton's method.
  • A participant seeks clarification on Newton's method after initially misunderstanding its name.

Areas of Agreement / Disagreement

Participants express uncertainty about the existence of an explicit inverse function, and there is no consensus on the best method for computation, as multiple approaches are suggested.

Contextual Notes

The discussion highlights limitations in determining the explicit form of the inverse function and the challenges associated with the rapid growth of x^(x^x), which may affect numerical methods.

Georgepowell
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for example:

f(x) = x^(x^x)

Firstly, is there already an inverse of this function that can be expressed in functions that exist?

And if there isn't, how would I go about computing it in code? Is there a better way than trial and error?
 
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So you're looking for a function g such that g(x^(x^x)) = x?

y = x^(x^x)
ln y = x^x ln x

ln ln y = ln (x^x ln x) = x ln x ln ln x.

Call me crazy, but I don't know whether there exists an explicit expression for x in terms of elementary functions.

To approximate the function in code, several options exist. All of them will boil down to iteratively building up a solution, though. Newton's method could probably be used, or, more simply, just start with 0000000000.00000000000 and, starting at the left, increase a 10's place digit until the x^x^x gets too big, go back to the previous version, and go to a lower power of 10, and continue until you have as much precision as you want. Since x^x^x will grow pretty insanely quickly, you shouldn't have to check much higher than 10.
 
AUMathTutor said:
Call me crazy, but I don't know whether there exists an explicit expression for x in terms of elementary functions.

That is what I thought

AUMathTutor said:
To approximate the function in code... Newton's method could probably be used.

What is Newton's method? A quick Google search hasn't given me anything. A link would be good.
 
Ahh, found the Wikipedia page. I didn't know it was actually called "Newtons method"!

I thought that was just what you called it as a general name.
 

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