Exploring the Relationship between -ln(-∞) and ln(∞)

In summary, we discussed the concept of finding the natural log of a negative number. It is not defined for real numbers, unless we involve imaginary numbers. The graph of the natural log function is similar to the square root function, and it has no value for negative numbers. Imaginary numbers will be useful in differential equations, but for now, we can forget about them.
  • #1
DrCrowbar
15
0
For instance, say I have

-ln(-∞)​

Does the negative sign on the natural log cancel with the negative sign on the infinity?

Is this true?
-ln(-∞) = ln(∞)​

Thank you

-Drc
 
Physics news on Phys.org
  • #2
Hi DrCrowbar! :smile:

There is no such thing as the ln of a negative number. :wink:

(unless we're allowed complex numbers, in which case eg ln(-1) = πi)
 
  • #3
Hi Tim!

Ah... clumsy me. I knew that. I really did. Well, used to... :wink:

So if you have ln(-∞), is that essentially ∞ or -∞?

I'm finding the solution for a calculus III improper integrals question. I guess it doesn't matter, though, because either way it diverges (a possible solution).

Thanks again

-Drc
 
  • #4
DrCrowbar said:
Hi Tim!

Ah... clumsy me. I knew that. I really did. Well, used to... :wink:

So if you have ln(-∞), is that essentially ∞ or -∞?
No, it's simply not defined, as tiny-tim said. I'm assuming you're working with real numbers.
tiny-tim said:
Hi DrCrowbar! :smile:

There is no such thing as the ln of a negative number. :wink:

(unless we're allowed complex numbers, in which case eg ln(-1) = πi)
 
  • #5
DrCrowbar said:
So if you have ln(-∞), is that essentially ∞ or -∞?

No, that's ∞i :wink:
 
  • #6
Ah, ok.

So ln(-#) is the same as -ln(#)... That makes sense, actually. I had forgotten what the graph of the natural log function looks like.

Thanks guys. It's the first time I've asked a math question online and actually received a correct answer!

-Drc
 
  • #7
DrCrowbar said:
So ln(-#) is the same as -ln(#)

no it isn't!

ln(-#) is an imaginary number (something times i)

if we're only allowed to use real numbers, then ln(-#) doesn't exist!
 
  • #8
Oh, ok.

So you can have a negative natural log function (-ln|cscx+cotx| for instance) but you cannot have the natural log of a negative number unless you involve imaginary numbers.

I haven't really seen much of imaginary numbers, but I hear they're used a bit in D.E.
 
Last edited:
  • #9
yes, they'll be useful later

for now, forget about them

the graph of ln(x) is like the graph of √x …

it simply has no value for x < 0
 
  • #10
tiny-tim said:
No, that's ∞i :wink:

What?? In what context is [itex]ln(-\infty)=\infty i[/itex]? What does [itex]infty i[/itex] even mean?
 
  • #11
oops! :redface:

i should have said ln(-∞) = πi + ∞ :rolleyes:
 

1. Does -ln(-∞) equal ln(∞)?

No, they are not equal. -ln(-∞) is undefined, while ln(∞) is equal to infinity.

2. Why is -ln(-∞) undefined?

-ln(-∞) is undefined because the natural logarithm function, ln(x), is only defined for positive real numbers. Since -∞ is not a positive real number, the function does not have a valid output.

3. Can you explain the concept of infinity in terms of logarithms?

Infinity is not a number, but rather a concept of something that is unbounded or without limit. In terms of logarithms, infinity represents the value of a function that continues to increase without bound. For example, ln(x) approaches infinity as x approaches infinity.

4. How is logarithm related to exponentiation?

The logarithm function is the inverse of the exponentiation function. This means that if y = a^x, then x = loga(y). In other words, logarithms tell us what power (or exponent) we need to raise a base number to in order to get a given result.

5. Why is it important to understand the concept of infinity in mathematics?

Infinity is a fundamental concept in mathematics, used in many different areas such as calculus, number theory, and geometry. It allows us to work with and understand quantities that are unbounded or without limit, and helps us solve complex problems and make predictions about the behavior of functions and equations.

Similar threads

  • Calculus and Beyond Homework Help
Replies
5
Views
1K
  • General Math
2
Replies
44
Views
3K
  • Calculus and Beyond Homework Help
Replies
3
Views
825
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
5
Views
1K
  • Calculus and Beyond Homework Help
Replies
5
Views
2K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
2K
  • Calculus and Beyond Homework Help
Replies
10
Views
2K
Back
Top