Calculate limit value with several variables

In summary: Also:In summary, you are calculating the entropy and pressure of a lattice gas, but you are also supposed to calculate a limit. You first calculate the limits for the entropy and pressure, and then you replace the limits for the entropy and pressure with the limit for the whole thing.
  • #1
GravityX
19
1
Homework Statement
Calculate the limit of ##P## when ##a_0 \rightarrow 0## and ##M,n \rightarrow \infty## with ##a=a_0n## and ##L=a_0*M##.
Relevant Equations
none
Hi,

I had to calculate the entropy in a task of a lattice gas and derive a formula for the pressure from it and got the following

$$P=\frac{k_b T}{a_0}\Bigl[ \ln(\frac{L}{a_0}-N(n-1)-\ln(\frac{L}{a_0}-nN) \Bigr]$$

But now I am supposed to calculate the following limit

$$\lim\limits_{a_0 \rightarrow \infty}{} \lim\limits_{M \rightarrow \infty}{} \lim\limits_{n \rightarrow \infty}{\frac{k_b T}{a_0}\Bigl[ \ln(\frac{L}{a_0}-N(n-1)-\ln(\frac{L}{a_0}-nN) \Bigr]}$$

So not the limit for ##a_0## , ##M## and ##n## but all at the same time.

Should I first calculate the limit for one, say for ##a_0## and what I got for that, the limit for ##M## or better said ##L## etc?
 
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  • #2
If the simultaneous limit exists, it doesn't matter what order you take the limits in. The eventual answer must be the same. Although some orders may be easier than others.

Where they exist, first calculate limits for components of the formula, and replace those components by their limits in the formula. That's generally valid as long as both the overall limit and the component's limit exist.

So for instance, ##\lim_{a_0\to\infty} \frac L{a_0}## is easy.
Another hint, for the expression in square brackets, use the fact that ##\log a - \log b = \log\left(\frac ab\right)## and then rewrite the fractional expression you're taking the log of as ##1 + \frac{1}{denominator}##. You'll find it easier to take limits that way.

By the way, there is no ##M## in your formulas. I presume you mean ##N##.
 
  • #3
andrewkirk said:
By the way, there is no ##M## in your formulas. I presume you mean ##N##.
GravityX said:
##a=a_0n## and ##L=a_0*M##.
Messy. First I see ##a_0\downarrow 0##, then ##a_0\uparrow \infty##. Typos ?
 
  • #4
BvU said:
Messy. First I see ##a_0\downarrow 0##, then ##a_0\uparrow \infty##. Typos ?
Also:
You have unbalanced parentheses.
 

FAQ: Calculate limit value with several variables

What is the definition of a limit with several variables?

A limit with several variables refers to the value that a function approaches as the input variables approach certain values. Formally, for a function f(x, y), the limit as (x, y) approaches (a, b) is L if for every ε > 0, there exists a δ > 0 such that if 0 < sqrt((x - a)^2 + (y - b)^2) < δ, then |f(x, y) - L| < ε.

How do you find the limit of a function with two variables?

To find the limit of a function with two variables, you can approach the point of interest along different paths (e.g., x-axis, y-axis, y = mx, y = x^2, etc.) and check if the function approaches the same value along each path. If it does, that value is the limit. If the function approaches different values along different paths, the limit does not exist.

What are common techniques for evaluating multivariable limits?

Common techniques include direct substitution, path analysis, polar coordinates transformation, and using the epsilon-delta definition. Direct substitution works if the function is continuous at the point. Path analysis involves checking the limit along various paths. Polar coordinates can simplify the evaluation by converting the problem into one involving a single variable. The epsilon-delta definition provides a formal proof of the limit.

Can L'Hôpital's rule be used for limits with several variables?

L'Hôpital's rule is generally not used for limits with several variables. Instead, it is primarily applied to single-variable functions where you have an indeterminate form like 0/0 or ∞/∞. For multivariable limits, other techniques such as path analysis, polar coordinates, and the epsilon-delta definition are typically more appropriate.

What does it mean if a multivariable limit does not exist?

If a multivariable limit does not exist, it means that the function approaches different values along different paths as the input variables approach the point of interest. This indicates that the function does not have a single, unique value that it approaches, and thus the limit is undefined at that point.

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