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Logarithms of negative numbers must be complex.

But how do you find $\ln{(-2)}$ expressed in something like $x \cdot i$ where $x \in \mathbb{R}$?

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Complex logarithm is defined only modulo $2\pi i$ since $e^{x+2k\pi i} = e^x$ for any $x\in\mathbb C$ and $k\in\mathbb Z$. To find one of the possible value of logarithm you may want to represent $-x$ for $x>0$ in the form of $$-x = |x|e^{\pi i}$$ so $\operatorname{ln}(-x) = \operatorname{ln}|x|+\pi i$ modulo $2\pi i$. The complete answer is then $$\operatorname{Ln}(-x) = \operatorname{ln}|x|+(2k+1)\pi i$$ where $k\in\mathbb Z$ –  Ilya Dec 20 '11 at 22:09
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@Ilya: That looks like an answer to me. –  joriki Dec 20 '11 at 22:14
    
Me too, thanks! You can put it as an answer. –  Dimme Dec 20 '11 at 22:22

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up vote 3 down vote accepted

First note that allowing complex values, $\ln$ stops being unique (or, alternatively, it's not defined on all of $\mathbb{C}={0}$).

The key property of $\ln$ is that $e^{\ln z} = z$ for all complex numbers $z\neq 0$.

Write $\ln(z) = u(z) + iv(z)$ for 2 real valued functions $u(z)$ and $v(Z)$. Then the key property gives $$e^{u(z)}[\cos v(z) + i\sin v(z)] = z$$

In particular, we must have $|z| = e^{u(z)}$ so $u(z) = \ln(|z|)$ (this is the usual real valued $\ln$ defined on positive real numbers).

Likewise, we must have $v(z)= arg(z) + 2\pi k$ for some $k\in\mathbb{Z}$.

Putting this together gives $\ln(z) = \ln(|z|) + i(arg(z) + 2\pi k)$, valid for any nonzero complex number.

Setting $z = -2$, we have $|z| = 2$ and $arg(-2) = \pi$ so $\ln(-2) = \ln(2) + i(\pi + 2\pi k)$.

Just to verify this works, we get $$e^{\ln(-2)} = e^{\ln(2) + i(\pi + 2\pi k)} = 2e^{i\pi} = -2.$$

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Or, alternatively again, the logarithm can be unique and defined on all of $\mathbb C\setminus\{0\}$, only not continuous on all of $\mathbb C\setminus\{0\}$. –  Henning Makholm Dec 20 '11 at 22:44

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