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Finding a limit

by Mathoholic!
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Mathoholic!
#1
Jan27-13, 01:13 AM
P: 49
I'm familiarized with finding limits of most kinds of functions. I was struck by a problem: What if the variables of the function belong to different sets of numbers?

My point being, given the function:

f(n,q)=[itex]\frac{n}{q}[/itex]

With n belonging to the set of natural numbers and q belonging to the set of rational numbers.

How do I avaluate the following limit (if possible):

lim f(n,q) as n→∞ and q→∞

This may be a silly question but care to answer please.

Thank you
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JonDrew
#2
Jan27-13, 01:56 AM
P: 83
I am not 100% certain but I believe L'Hospital's rule still applies to multivariable functions if both the top and bottom functions are going to infinity. That would make:

lim f(n,q) as n→∞ and q→∞ = 1

only because you can take the derivative of what is upstairs and the derivative of what is downstairs with simplicity. I don't think this method would work if you had say, a function like:

f(n,q) = (n+q) / (q-n)^2

because then you would have multiple variables locked upstairs and downstairs and it would become a mess with partial derivations.
rollingstein
#3
Jan27-13, 02:05 AM
PF Gold
P: 330
That function is discontinous along one axis. Don't think L'Hospital can be used then.

Curious3141
#4
Jan27-13, 02:06 AM
HW Helper
Curious3141's Avatar
P: 2,954
Finding a limit

Quote Quote by Mathoholic! View Post
I'm familiarized with finding limits of most kinds of functions. I was struck by a problem: What if the variables of the function belong to different sets of numbers?

My point being, given the function:

f(n,q)=[itex]\frac{n}{q}[/itex]

With n belonging to the set of natural numbers and q belonging to the set of rational numbers.

How do I avaluate the following limit (if possible):

lim f(n,q) as n→∞ and q→∞

This may be a silly question but care to answer please.

Thank you
You have to define your n and q better.

As an example, let's take the limit ##\lim_{x \rightarrow \infty} \frac{\lfloor x \rfloor}{x}##, where ##x \in \mathbb{R}##

You can't simply apply L' Hopital's because the numerator is a discontinuous function.

But you can find the limit by defining the fractional part of x as ##\{x\}## and rewriting the limit as:

##\lim_{x \rightarrow \infty} \frac{x - \{x\}}{x} = 1 - \lim_{x \rightarrow \infty} \frac{\{x\}}{x} = 1##

So a lot depends on exactly what you intend the numerator and denominator to signify.
pwsnafu
#5
Jan27-13, 02:09 AM
Sci Advisor
P: 839
Quote Quote by Mathoholic! View Post
How do I avaluate the following limit (if possible):

lim f(n,q) as n→∞ and q→∞
Do you mean ##\lim_{n \rightarrow \infty}\lim_{q \rightarrow \infty} \frac{n}{q}## or ##\lim_{q \rightarrow \infty} \lim_{n \rightarrow \infty} \frac{n}{q}##?

I was struck by a problem: What if the variables of the function belong to different sets of numbers?
The general theory is that of nets.
JonDrew
#6
Jan27-13, 12:28 PM
P: 83
Quote Quote by Curious3141 View Post
You have to define your n and q better.

As an example, let's take the limit ##\lim_{x \rightarrow \infty} \frac{\lfloor x \rfloor}{x}##, where ##x \in \mathbb{R}##

You can't simply apply L' Hopital's because the numerator is a discontinuous function.

But you can find the limit by defining the fractional part of x as ##\{x\}## and rewriting the limit as:

##\lim_{x \rightarrow \infty} \frac{x - \{x\}}{x} = 1 - \lim_{x \rightarrow \infty} \frac{\{x\}}{x} = 1##

So a lot depends on exactly what you intend the numerator and denominator to signify.
Very cool! Thanks for the correction.


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