# What is the proper way of introducing a pair of invertible complex functions $\exp$ and $\log$?

I need to introduce a pair of invertible complex functions $$\exp$$ and $$\log$$ with the following properties:

$$A$$ being a branch (or strip?) of $$\mathbb{C} \backslash \{0\}$$:

$$\forall a \in A \quad \exp(\log(a)) = \log(\exp(a)) = a$$

$$\forall a \in \mathbb{R}^* \quad \log(-|a|) = \log(|a|) + i\pi$$

What is the proper way of introducing these two functions? I am especially concerned about the proper enunciation of the functions’ domains and codomains.

Furthermore, is there a proper way of introducing the “continuation” of the first property to a subset containing 0?

I am somewhat familiar with complex logarithms, but I still struggle with the proper definition of domains and codomains using branches (strips?). I would like the definitions to be as precise and unambiguous as can be.

• I am afraid that your functions cannot coincide with the usual exponential and logarithm. Because exponential is not injective on $\mathbb C\setminus\{0\}$, and so it cannot have an inverse and the first property cannot hold. What you can do is considering the restriction of the exponential to a strip $\{z=x+iy\ :\ y\in [a, a+2\pi)\}$. This function is a bijection onto $\mathbb C\setminus\{0\}$. – Giuseppe Negro Jan 9 at 16:16
• @GiuseppeNegro I believe this is what I am looking for. Would you mind writing the full definitions in the form $f : A \longrightarrow B$ for both $\exp$ and $\log$ as an answer to the question? Thanks. – ismael Jan 9 at 16:19
• I disagree with the downvote, which, moreover, is not accompanied by a motivating comment. I'll see if I can convert my comment into an answer. – Giuseppe Negro Jan 9 at 16:50
• @GiuseppeNegro Thank you! I edited the original question in order to clarify things. – ismael Jan 9 at 16:51
• This is a great case of definition question. – ismael Jan 10 at 15:32

For the first property to hold, the exponential must be restricted to a subset where it is injective. Now, the exponential is injective only on "horizontal strips" $$A_\lambda:=\{x+iy\ :\ y\in [\lambda, \lambda+2\pi)\},$$ where $$\lambda\in \mathbb R$$. This is a consequence of Euler's formula $$e^{x+iy}=e^x(\cos y + i \sin y).$$ For each fixed $$\lambda$$, since $$\exp$$ is a bijection of $$A_\lambda$$ onto $$\mathbb C\setminus\{0\}$$, there is an inverse function $$\log_\lambda \colon \mathbb C\setminus\{0\} \to A_\lambda.$$ WARNING: This function is discontinuous on the half-line $$\{re^{i\lambda}\ :\ r\ge 0\}.$$
For all $$\lambda\in\mathbb R$$, it holds that $$\log_\lambda(-z)=\log_\lambda(z)\pm i\pi,$$ where the sign is chosen in such a way that $$\log_\lambda(z)\pm i\pi$$ stays in $$A_\lambda$$.