# Logarithm question

1. Feb 16, 2005

### Kamataat

Is it true that that $ln(-x)$ is defined for $x \in R$ such that $x < 0$?

- Kamataat

2. Feb 16, 2005

### Muzza

Well, yes... If x < 0 then -x > 0, and surely the natural logarithm is defined for all positive numbers.

I believe it can be extended to the negative reals as well, by using Euler's formula. For example, e^(ln(3) + ipi) = e^ln(3) * e^(ipi) = -3, so one might say that ln(-3) = ln(3) + ipi. I assume there are some technical difficulties in actually making such an extension rigorous, since (for example) ln(3) + 3ipi is also a possible "candidate" for being the natural logarithm of -3.

3. Feb 16, 2005

### dextercioby

Complex logarithm is multivalued indeed.But that's another story.

Daniel.

4. Feb 16, 2005

### Kamataat

I thought it was so (I mean, it's pretty obvious), but I had this weird doubt (sometimes that happens when I study maths), so had to ask. Thanks again, Muzza.

- Kamataat