Help Understand Denominator Separation from Zero in Math Equations

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In summary, the author is discussing the issues that arise when trying to solve a set of equations numerically, specifically when the denominator of the first equation becomes very small. This can lead to inaccurate results and the author is seeking a solution to this problem. One approach is to use a different method, such as Newton's method, but this may also have its own issues.
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williamshipman
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Please help me to understand: "the denominator is numerically separated from zero"

Hi everyone,

I've come across this statement in a dissertation I'm reading and I don't have a clue as to what the author is speaking about. Can anyone give me an explanation. For reference, the equations are

[tex]\lambda_0 = \frac{C_T}{2 \eta \sqrt{\mu^2 + (\lambda_0 - mu_z)^2}}[/tex]

and

[tex]C_T^{ideal} = \frac{a\sigma}{2}\left(\theta_0\left(\frac{1}{3}+\frac{\mu^2}{2}\right)+\frac{\mu_z-\lambda_0}{2}\right)[/tex]

[tex]C_T[/tex] equals [tex]C_T^{ideal}[/tex] unless [tex]C_T^{ideal}[/tex] is larger than [tex]C_T^{max}[/tex] or smaller than [tex]C_T^{min}[/tex], in which case it saturates at the applicable limit.

The objective is to numerically solve the above equations for [tex]\lambda_0[/tex] and then calculate [tex]C_T[/tex]. Problems occur when [tex]\mu[/tex] is close to zero and [tex]\mu_z[/tex] is close to [tex]\lambda_0[/tex] as the denominator of the first equation gets very small. Trying to solve this using Newton's method fails because the iterations don't converge to an answer. I'm hoping that if I understand what "numerically separating the denominator (of the first equation) from zero" means, then I might make some progress.

Thanks for the help.
 
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  • #2


Where does the author use that phrase "numerically separating..." or are you paraphrasing? I'm not enough of an expert to provide details, but it appears that the author is saying a denominator too small leads to round off error in the computation that will give inaccurate results. It is an important but purely practical matter.

I am also confused how one would calculate λ0 numerically without knowing CT, and then calculate CT.
 
  • #3


A numerically awkward difference (i.e, a difference between two numbers too close to have a barrier against loss of significant digits), can be made less awkward in various ways, for example by this strategy:

Let x be less than, but very close to 1.

Then, we have:
[tex]1-x=\frac{1-x^{2}}{1+x}=\frac{1-x^{4}}{(1+x)(1+x^{2})}[/tex]
and so on.
Note that the retained difference within the expression will be an improvement relative to the first order, original difference 1-x
 
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  • #4


Thanks for the replies.

Tedjn: The quote comes straight from the author's work. You are right to say that a small denominator gives incorrect results, at least with the current iteration scheme. To solve the equations numerically, you can use something like Newton's method. One can substitute the equation for [tex]C_T[/tex] into the first equation, giving you one equation to solve. Its possible to get an initial guess of [tex]\lambda_0[/tex], so the equations can be solved. Right now, I'm using Matlab's fsolve function, but I'd like something faster. So I tried the Newton's method iteration formula from the author's work, but with the noted problems. And I did check the derivation.

arildno: Thanks for this idea. So if I use this method to work out [tex]\lambda_0-\mu_z[/tex] in the denominator of the first equation, is that then numerically separating the denominator from zero? I'll give this a go and see what happens.

Merry Christmas and thanks for helping me even though it is holiday time.
 
  • #5


A bit of caution, though:
Newton's methon doesn't always work, as you probably know. It depends upon the local curvature (check it out).

Now, if you're unlucky, no amount of twiddling with that difference would help you, because your problem isn't really numerical, but rather with Newton's method as such.
 

FAQ: Help Understand Denominator Separation from Zero in Math Equations

What is a denominator and why is it important in math equations?

A denominator is the bottom number in a fraction that represents the total number of equal parts that make up a whole. It is important in math equations because it gives context to the numerator and helps determine the size or value of the fraction.

Why is it impossible to divide by zero?

Dividing by zero is impossible because it violates the basic rules of arithmetic. When dividing, we are essentially asking what number multiplied by the divisor will result in the dividend. However, there is no number that, when multiplied by zero, will result in a non-zero number, making it impossible to divide by zero.

What happens when a denominator is equal to zero in a math equation?

When a denominator is equal to zero in a math equation, the fraction becomes undefined. This means that the value of the fraction cannot be determined and the equation becomes invalid.

How can we avoid denominator separation from zero in math equations?

The easiest way to avoid denominator separation from zero is to check the denominators of all fractions in a math equation before solving it. If any denominator is equal to zero, the equation cannot be solved and must be rewritten or reevaluated.

Can we ever divide by zero in a math equation?

No, we cannot divide by zero in a math equation. It is mathematically undefined and goes against the basic principles of arithmetic. However, in some advanced mathematical concepts, such as calculus, we can approach dividing by zero using limits, but it is not the same as actually dividing by zero.

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