Finding Solutions to a Cubic Equation with Multiple Roots

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Discussion Overview

The discussion revolves around finding solutions to a specific cubic equation with multiple roots, focusing on identifying the smallest real positive solution. Participants explore various methods for solving the equation, including numerical algorithms and algebraic techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in finding the smallest real positive solution to the cubic equation, noting that typical solvers yield only one root of a large magnitude.
  • Another participant suggests that the Newton-Raphson algorithm may not be effective due to the derivative being zero at the desired root, which complicates finding a good initial guess.
  • A different approach is proposed involving the rational root theorem, with a reference to Wolfram Alpha as a potential tool for finding the root.
  • One participant indicates that solving the derivative of the function can identify double roots, suggesting a method to find the roots without changing the solving technique.
  • A substitution method is introduced, where substituting x = 10^-3y simplifies the equation and reveals a double root at a specific value.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method for finding the roots, with multiple approaches and techniques being discussed. The effectiveness of the Newton-Raphson algorithm is contested, and various methods are proposed without agreement on a single solution.

Contextual Notes

The discussion highlights the challenges associated with solving cubic equations that may have multiple roots, particularly when numerical methods fail. There are unresolved assumptions regarding the behavior of the function and the applicability of different solving techniques.

Big-Daddy
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I'm finding it hard to find the solutions to this cubic equation:

1/2 x^3 - 2.025647693*10^14 x^2 + 8.102590772*10^11 x - 8.102590772*10^8 = 0

I'm looking for the smallest real positive solution but no matter what solver I use I keep getting only one root (the one of order of magnitude 10^14). There should be a root of the order of 10-3 but I don't know how to find it, nor why my usually trusty solvers are giving only 1 root.
 
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You are probably using the Newton-Rhapson algorithm. This algorithm is used to solve ##f(x)=0## for various functions. It works by taking an initial guess and then recursively refine the guess.

Not all guesses work however. So the question is to find out what the best guess is and whether there are good guesses at all!

We have the following: http://en.wikipedia.org/wiki/Newton-Rhapson#Analysis

So if ##\alpha## is your root and if ##f^\prime(\alpha)## is nonzero, then there are good initial guesses. However, this is exactly what goes wrong here! The root ##\alpha## that you seek does have ##f^\prime(\alpha)=0##. This is why no initial guess will work and why you didn't find the solution.

There are other ways to find the solution though. It is hardly practical in many cases, but the rational root theorem works here. Either way, wolfram alpha could find the root: http://www.wolframalpha.com/input/?...x^2+++8.102590772*10^11+x+-+8.102590772*10^8+
and it's a very pretty root too!
 
^Yes
one thing you can do without changing to a different method is to solve
f'(x)=0
3/2 x^2 - 2*2.025647693*10^14 x + 8.102590772*10^11 = 0
if f'(x)=0 and f(x)=0 then x is a double root of f
 
If you substitute x = 10-3y into the equation, you get:

y2-4y+4 = 0. This neglects the cubic term, which is going to be minute compared to the other terms. So there is a double root at y = 2, or, equivalently, at x = 2 x 10-3.

Chet
 

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