Re-Defining Log: Can We Make it Intuitive?

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Discussion Overview

The discussion revolves around the redefinition of the logarithm function, particularly in the context of complex numbers. Participants explore the implications of defining the logarithm on the complex plane and the potential loss of intuitive understanding when applying such definitions.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant proposes redefining the logarithm for the complex plane as Log z = log |z| + i Arg(z), questioning whether this would work without losing intuitive values like Log(5).
  • Another participant argues that redefining the logarithm is possible, but the utility of such a definition in applications is uncertain.
  • A different participant points out that defining a logarithm on the complex plane requires a branch cut, and the choice of where to place this cut affects the availability of values like Log(5).
  • Some participants express confusion regarding the implications of the proposed definition, noting that it seems to align with conventional definitions and questioning the rationale behind the proposed changes.
  • There is a reiteration that if the branch cut is placed on the positive real line, it would exclude Log(5), raising concerns about the validity of such a definition.

Areas of Agreement / Disagreement

Participants express differing views on the implications of redefining the logarithm. There is no consensus on whether the proposed definition is valid or useful, and confusion remains regarding the conventional versus proposed definitions.

Contextual Notes

The discussion highlights the dependence on the choice of branch cuts in complex logarithm definitions and the potential loss of intuitive values, which remains unresolved.

tgt
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Would it be okay to redefine the complex definition of Log and define it for example C\[0,infinity)?

I guess then you would have Log z = log |z| + i Arg(z)

where -Pi<=Arg(z)<Pi

Everything would work fine?

But then you can't have Log 5 for example which would be very counter unintuitive.
 
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Why can't you have Log 5? I see no problems with that. You can redefine anything you want - how useful it ends up being in its applications is another question though.
 
Gib Z said:
Why can't you have Log 5? I see no problems with that. You can redefine anything you want - how useful it ends up being in its applications is another question though.

To have a Log function defined on the complex plane, you need a branch cut somewhere. Exactly where is arbitary right? So what happens if you choose the positive real line? You'd lose Log (r) for r in the positive reals, including Log(5).
 
Gib Z said:
Assuming log(x) is the logarithim function of the reals, and Log (x) is the complex function you wish to define, your definition gives Log (5) = log |5| + i arg ( 5 + 0i) = log 5 + i*0 = log 5.

I must admit I am somewhat confused as to what is actually happening here - what you gave in the original post is the conventional branch anyway - http://en.wikipedia.org/wiki/Complex_logarithm#Log.28z.29_as_the_inverse_of_the_exponential_function

It should be assuming log(|x|) is the logarithm of the reals. I'm just saying since the branch cut can be applied anywhere, what happens if we apply it on the real line? Then we don't have Log(5). Then that wouldn't be a good definition would it?
 

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