Is it possible for a holomorphic function on a connected domain to be conformal but not injective? Also, is it possible for a holomorphic function to be injective but not conformal?


Example. The exponential function is conformal but not injective: its derivative is nowhere vanishing (so conformal) but $\exp 0 = \exp 2 \pi i$ (so not injective).

Proposition. The derivative of an injective holomorphic function $f : D \to \mathbb{C}$, where $D$ is an open subset in $\mathbb{C}$, is nowhere vanishing. In particular, $f$ is conformal.

Proof. We prove the a stronger statement, which will imply the contrapositive of the above proposition. Let $z_0 \in D$. By a change of coordinates if necessary, we may assume that $f(z_0) = 0$ and $z_0 = 0$ without loss of generality. Then, there is a positive integer $n$ and a holomorphic function $h : D \to \mathbb{C}$ such that $f(z) = z^n h(z)$ for all $z$ in $D$ and $h(0) \ne 0$. So in particular there is an open neighbourhood $U$ such that $0 \in U \subseteq D$ and $h$ is nowhere vanishing on $U$. That implies that there is a holomorphic function $g : U \to \mathbb{C}$ such that $g(z)^n = h(z)$ for all $z$ in $U$. (Take a holomorphic branch of the map $w \mapsto w^{1/n}$.) So $f(z) = (z \, g(z))^n$ for all $z$ in $U$.

Note that $z \mapsto z \, g(z)$ has non-zero derivative at $0$ (since $g(0) \ne 0$), so by the inverse function theorem, if $U$ is small enough, $z \mapsto z \, g(z)$ is an invertible holomorphic function on $U$. This implies $f$ is an $n$-to-$1$ function on $U \setminus \{ 0 \}$. But if $f'(0) = 0$, by considering the power series of $f$, we must have $n \ge 2$.

We conclude that a holomorphic function $f : D \to \mathbb{C}$ has derivative vanishing at $z_0$ if and only if it fails to be locally injective at $z_0$.

(This is probably my favourite result in complex analysis.)

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    $\begingroup$ I happen to stumble across this now. Is there a simple way to determine if a given conformal map is injective (aside from trying to find or not find two points, etc)? Perhaps a clever use of the inverse function theorem or something? $\endgroup$ – davidlowryduda Dec 9 '11 at 7:37
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    $\begingroup$ @mixedmath: There is no a priori reason to expect anything nice to happen. The inverse function theorem is a result about local behaviour, and conformality is a local property, while injectivity is a global property. $\endgroup$ – Zhen Lin Dec 9 '11 at 8:12

It is not possible for a holomorphic function to be injective but not conformal; for a holomorphic f to be injective, its derivative must be non-zero, and conformality of f is precisely equivalent to having $f'(z)\neq 0$.

On the other hand, it is possible for f to be conformal but not injective: $expz:=e^z$ is one example.Conformality follows from the fact that f(z) is analytic , and $\frac{de^z}{dz}=e^z$, and $e^z\neq 0$; (use the identity $e^ze^{-z}=1$ to show $e^z\neq 0$)

EDIT: Conformality follows from the Cauchy-Riemann equations , which show that the effect of f'(z) is that of stretching and rotating. If f'(z) is not zero, then any tangent vector forming an angle $\theta$ will be rotated, under f'(z) by an agle $\theta$, so that two vectors $v,w$ forming an angle $\gamma$ between them, will each be rotated by the same amount, with the effect that the net difference in the angles of the vectors v,w, and their respective images f(v),f(w) is $\theta -\theta=0$

  • $\begingroup$ Yes! I forgot that there was such a easy example! Thank you! $\endgroup$ – Braindead Aug 1 '11 at 16:42
  • $\begingroup$ @Braindead: A simpler example of a conformal but not injective function is $z \mapsto z^2$ in a domain that does not include the origin. $\endgroup$ – lhf Aug 1 '11 at 16:43
  • $\begingroup$ @lhf: but my point in using $e^z$ was that its "non-injectivity" is extreme, in that it is $\infty\rightarrow 1$ $\endgroup$ – gary Aug 1 '11 at 16:48
  • $\begingroup$ This is strange; my answer had been accepted, and somehow it was unaccepted. I had never seen this before. $\endgroup$ – gary Aug 1 '11 at 17:56
  • $\begingroup$ @gary - After I accepted your answer, Zhen Lin posted another answer, which I accepted because it completely answered my question by reminding me that all holomorphic functions look like $z\to z^n$ locally. I mean, it feels like you are taking this personally for some reason judging from a series of edits you've made, but I didn't mean any offense. I definitely upvoted your reponse. $\endgroup$ – Braindead Aug 1 '11 at 18:10

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