# Where is the mistake?

I know that log function is defined only for positive reals but please help me understand where i go wrong in the following proof : TO PROVE : $\ln\big(i) = \frac{i\pi}{2}$ PROOF : $\ln(i) = \ln(e^{\frac{i\pi}{2}})$ but we know that $\ln(e^x) = x$ and hence $\ln(i) = \ln(e^{\frac{i\pi}{2}}) = \frac{i\pi}{2}$ Hence proved . Is it correct ? Although , i am sure it is wrong but which step is precisely not allowed here? Note by Ujjwal Mani Tripathi
4 years, 3 months ago

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## Comments

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The sequence of steps that you took essentially work, but you have to be clear if you are working with "equality of multi-valued functions" or "evaluate at primitive branch".

To be clear, the multi-valued function $\ln i$ is indeed equal to $\frac{ i \pi }{2} + 2k \pi i$.

Staff - 4 years, 3 months ago

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Calvin, please, I want to be your friend, but you don't make me easy it... $\frac{i \pi}{2} + 2k\pi \neq i\cdot(\frac{\pi}{2} + 2k \pi)$

- 4 years, 3 months ago

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Ah yes, fixed the typo. thanks.

Staff - 4 years, 3 months ago

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Don't worry, can we be friends?

- 4 years, 3 months ago

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Of course!

Staff - 4 years, 3 months ago

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Thank you very much.

- 4 years, 3 months ago

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Calvin, I would write great wikis, but I have some problems:

a) Where can I start?

b) I can't do everything, I'm not God/Godness or...

c) For example, suppose I want to write a wiki on numerical analysis, before doing this, would I have to define what a vectorial space is, would I have to give a topology or abstract algebra course first, would I have to establish some prior knowledge of mathematical analysis?...

d) In short, what would I have to do?

- 4 years, 3 months ago

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1. Here is a list of wikis that we're working on. If there are any that you're interested in helping out with, ping #wikis.
If you're working on a new wiki and need help with developing the outline, let me know.

2. Don't worry about it. Focus on what you're good at / what you're interested in.

3. Do not worry about ensuring that foundational material exists when developing out a page. We can fill that in subsequently. and use "red links" to indicate that wiki pages are undeveloped. E.g. In the Gram Schmidt wiki, we would assume knowledge of vector space, and link out to it.

4. Where do you want to start?

Staff - 4 years, 3 months ago

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Good answer, let's start at the beginning, there is already a story about this, a teacher from Tai-chi taught it to me. Suppose I want to talk about arithmetic theory: where do I have to start? Defining Peano's axioms?... Now suppose I want to talk about set theory, would I have to give a theory about what a class is?

P.S.- Imagine that I am nobody, neither a mathematician nor a physicist ..

- 4 years, 3 months ago

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Look, we can make one thing, I like so much topology, and here I don't see Topology. What about if I start talking about Topology, with more or less a notion of a "set", I'll "definite it", I don't want Russell's paradox again... do you remember what happened...? and later I continue talking about Topology, and when I need help, I can email you or stop... Remember, we are free if we respect ourselves... what do you think about this?... I have other question: where is Mateo Matija...? I can't get see him... It's important for me...

- 4 years, 3 months ago

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Hello my friend again

- 4 years, 3 months ago

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it's not the same the exponentation or logharythmic functions with the complex numbers, that the exponentation or logharytmic functions with real numbers. Look at this : $i = e^{ \frac {i \pi}{2}} =e^{ \frac {5i \pi}{2}}$...

- 4 years, 3 months ago

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thanks for the help :)

- 4 years, 3 months ago

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If you need something about me, you can ask to me, and if I can help you, I'll help you.

- 4 years, 3 months ago

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thank you ...

- 4 years, 3 months ago

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thank you for you,of course

- 4 years, 3 months ago

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this discussion has turned out to be fun......all are becoming friends :)

- 4 years, 3 months ago

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thank you again friend... I like that we are all friends

- 4 years, 3 months ago

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What makes you think that $\ln i \neq \frac{ i \pi } { 2}$?

Also, be very careful with complex exponentiation and logarithms.

Staff - 4 years, 3 months ago

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@Guillermo Templado sir , this doubt arose from your solution to imaginary^imaginary . Can you please help me?

- 4 years, 3 months ago

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Yes, I'm going on there...

- 4 years, 3 months ago

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