Can Logarithms Be Defined for Negative Numbers in Advanced Mathematics?

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Discussion Overview

The discussion centers on the question of whether logarithms can be defined for negative numbers within the context of advanced mathematics, particularly in relation to calculus and complex analysis. Participants explore the implications of defining logarithms for negative values and the mathematical frameworks that allow for such definitions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that in precalculus, logarithms are typically defined only for positive numbers and questions the justification for extending this definition to all numbers.
  • Another participant references Euler's identity and suggests that it allows for the evaluation of the natural logarithm at negative values, providing examples such as ln(-1) = iπ.
  • A different participant agrees that logarithms can be defined for complex numbers, highlighting that the logarithm becomes a multivalued function in this context, which complicates the familiar rules of logarithms.
  • One post points out that the discussion may be a duplicate of a previous thread, indicating ongoing interest in the topic.

Areas of Agreement / Disagreement

Participants express differing views on the definition of logarithms for negative numbers. While some support the idea that logarithms can be extended to complex numbers, others emphasize the traditional restriction to positive numbers in precalculus. The discussion remains unresolved regarding the broader implications and acceptance of these definitions.

Contextual Notes

The discussion touches on the complexities of defining logarithms in the context of complex analysis and the implications of multivalued functions, which may not align with traditional logarithmic rules. There are also references to specific mathematical identities that facilitate these definitions.

physicsdreams
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I recently had a test (precalc) where we had to solve log(x)-log(x+4)=2 for x.

The answer comes out negative.
I understand that in precalc we are defining the logarithms for just positive numbers, but-

Is it ever justified to define a logarithm for all numbers, both negative and positive?
(higher levels of math?)

Thanks
 
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well in calculus while going over series, my professor introduced us to euler's famous identity/formula e^(ipi)+1=0, which is a pretty cool thing, you should look it up on google if you've never seen it
anyways my professor then went on that with this formula we are able to evaluate the natural logarithm at negative values
for example e^(ipi)=-1 taking the natural log of both sides you get ipi=ln(-1)
and you can do this for other negative values as well
ln(-5)=ipi+ln5
since ln(-5)=ln(-1)+ln(5)
if you'd like to learn more, your best bet would be complex analysis i believe
 
Hi physicsdreams! :smile:

Just like the square root of a negative number is defined for complex numbers, the logarithm of negative numbers, or rather of complex numbers in general, is defined.

You can find some info and pictures here:
http://mathworld.wolfram.com/NaturalLogarithm.html
Wikipedia also has a good article, but that may be more than you're bargaining for.

However, this is rather tricky, since the logarithm for complex numbers is not a normal function any more - it is a multivalued function.
In particular this means that the rules for logarithms that you're familiar with, do not work anymore.
 

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