How can I find the Proper starting point in FindRoot command?

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Discussion Overview

The discussion revolves around the challenges of finding an appropriate starting point for the FindRoot[] command in Mathematica, particularly when dealing with equations that involve Bessel functions. Participants explore methods for identifying roots without a known starting point and discuss the implications of various numerical methods.

Discussion Character

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

Main Points Raised

  • One participant expresses difficulty in finding the proper starting value for the FindRoot[] command, indicating a lack of knowledge about how to identify roots without a starting point.
  • Another participant explains that methods like Newton's method require a starting point and emphasize that these algorithms focus on local searches, which means they can only find one root at a time.
  • A participant shares an example of using FindRoot with Bessel functions but mentions that their chosen starting point did not yield the correct solution and inquires about alternative commands for solving such equations.
  • There is a suggestion to check the Mathematica manual for guidance, along with the idea of generating random starting points, although skepticism is expressed about their usefulness.
  • One participant encourages making educated guesses for starting points, suggesting that iterative methods may not converge if the guesses are not close enough to the actual root.

Areas of Agreement / Disagreement

Participants generally agree that a starting point is necessary for the FindRoot[] command and that methods like Newton's method rely on this requirement. However, there is no consensus on the best approach to determine a suitable starting point or alternative methods for finding roots.

Contextual Notes

Participants mention the limitations of relying on starting points and the potential for multiple roots in equations, which complicates the search for solutions. The discussion does not resolve the question of how to effectively find starting points for the specific case of Bessel functions.

Mona_r
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Hi

I have some problems in finding the proper starting value in FindRoot[] command.
Cause I don't know exactly what the point is,I can't get the right answer! :cry:
How can I find roots without having the starting point?

Thanks
 
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Well, what is the FindRoot command? What are you referring too?

Anyways, I assume you are talking about a popular, well known analytical algorithm like Newton's method. For these methods, you NEED a starting point. The crucial idea behind these algorithms is that you start somewhere, and using information relating to the gradient of the function, you hone in on a root (if possible - consider f(x) = x^(1/3)). It is like finding a maximum or minimum of a function. These algorithms take a local search approach, so there is no way to take on a global perspective.

Thus, if your function has several roots, you can only find one of them at a time, and you can only find a specific root given that you starting point is sufficiently close to it.
 
It's something like this:
Table[FindRoot[(a function of variable x containing Bessel functions==0),{x,k[]}],{i,1,N}]
I gave one starting point but I couldn't get the proper solution!
I just want to know if there is another command for calculating these kind of equations or not?

thanks a lot
 
Well, what software are you using? It would help if you specify the environment in which you are working.
 
Mathematica 6
 
Well, you could check the Mathematica manual. You could automatically have it give itself a random starting point with some code, but I highly doubt that is what you want and I highly doubt that will be useful. I also would think that if Mathematica had such a function (I don't doubt one exists), it would simply give arbitrary starting positions anyways.

Like I said, the FindRoot function utilizes an analytic approach that REQUIRES a starting point. Look up Newton's method and the like to see how they work - they all depend on a good starting point.
 
Make an educated guess! You should be able to at least approximate a root for an equation. Try it. While I don't know precisely which algorithm is used, it is probably an iterative method and if it does not converge to a solution it does not converge. So 'make a guess' and if it does not give an answer- make another guess!
 
Thanks dears
I'm going to try these suggestions.
Good luck :-)
 

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