I know that e^(2pi i)=1 so by taking the natural logarithm on both sides ln(e^(2pi i))=ln(1)=0 however, why isn't this 2pi i as expected? Is it beacuse logarithms can only provide real values?

Kendrick Finley

Kendrick Finley

Answered question

2022-10-16

What is the value of ln ( e 2 π i )
I know that
e 2 π i = 1
so by taking the natural logarithm on both sides
ln ( e 2 π i ) = ln ( 1 ) = 0
however, why isn't this 2 π i as expected? Is it beacuse logarithms can only provide real values?

Answer & Explanation

Kash Osborn

Kash Osborn

Beginner2022-10-17Added 18 answers

The exponential function is not one-to-one in complex numbers, which is an issue because exp  ( z + 2 π i ) = exp  z. As a result, it is no longer possible to define a suitable inverse for it. For example, if I only tell you a number's exponential, you won't know how many multiples of 2 π i I've added.
This might be fixed in a few different ways. One is to think of the logarithm as a relation rather than a multi-valued function, so that log  1 "=" 2 π i k for every integer k. This can be useful at times, such as when solving equations.
Another option is to define a logarithm rather than the logarithm itself, which entails selecting any imaginary component at random to provide a useful, well-defined function. You may, for example, declare you're interested only in value with imaginary part between  π and π (That the lower bound is exclusive and the upper bound is inclusive.) or 0 and 2 π, or anything else you like. Sadly, this forces your logarithm function to be discontinuous, but if you slice a line out of the complex plane from 0 down the negative reals, you can at least make the logarithm continuous everywhere other than on that line – you'll have something like imaginary part  π on one side of the line and imaginary part π on the other side. You are selecting a "branch" of the logarithm in what is referred to as a "branch cut."

Jaylyn Horne

Jaylyn Horne

Beginner2022-10-18Added 5 answers

Note that
e 2 π i = e 2 π k i 
hence, you do
ln  ( e 2 π i ) = 2 π k i ,  k  Z 
use the primary value for k = 0

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