Univalent function

Short description: Mathematical concept

For other uses, see Univalent.

In mathematics, in the branch of complex analysis, a holomorphic function on an open subset of the complex plane is called univalent if it is injective.

Examples

The function $f:z\mapsto 2z+z^2$ is univalent in the open unit disc, as $f(z)=f(w)$ implies that $f(z)-f(w)=(z-w)(z+w+2)=0$. As the second factor is non-zero in the open unit disc, $z=w$ so $f$ is injective.

Basic properties

One can prove that if $G$ and $\mathrm{\Omega }$ are two open connected sets in the complex plane, and

$f:G\to \mathrm{\Omega }$

is a univalent function such that $f(G)=\mathrm{\Omega }$ (that is, $f$ is surjective), then the derivative of $f$ is never zero, $f$ is invertible, and its inverse $f^{-1}$ is also holomorphic. More, one has by the chain rule

$(f^{-1}{)}^{\prime }(f(z))=\frac{1}{f^{\prime }(z)}$

for all $z$ in $G.$

Comparison with real functions

For real analytic functions, unlike for complex analytic (that is, holomorphic) functions, these statements fail to hold. For example, consider the function

$f:(-1,1)\to (-1,1)$

given by $f(x)=x^3$. This function is clearly injective, but its derivative is 0 at $x=0$, and its inverse is not analytic, or even differentiable, on the whole interval $(-1,1)$. Consequently, if we enlarge the domain to an open subset $G$ of the complex plane, it must fail to be injective; and this is the case, since (for example) $f(\epsilon \omega )=f(\epsilon )$ (where $\omega$ is a primitive cube root of unity and $\epsilon$ is a positive real number smaller than the radius of $G$ as a neighbourhood of $0$).

See also

• De Branges's theorem
• Koebe quarter theorem – Statement in complex analysis
• Riemann mapping theorem – Mathematical theorem
• Nevanlinna's criterion – Criterion in complex analysis

References

• Conway, John B. (1995). "Conformal Equivalence for Simply Connected Regions". Functions of One Complex Variable II. Graduate Texts in Mathematics. 159. doi:10.1007/978-1-4612-0817-4. ISBN 978-1-4612-6911-3. https://books.google.com/books?id=yV74BwAAQBAJ&pg=PA32. 
• "Univalent Functions". Sources in the Development of Mathematics. 2011. pp. 907–928. doi:10.1017/CBO9780511844195.041. ISBN 9780521114707. https://doi.org/10.1017/CBO9780511844195.041. 
• Duren, P. L. (1983). Univalent Functions. Springer New York, NY. p. XIV, 384. ISBN 978-1-4419-2816-0. 
• Gong, Sheng (1998). Convex and Starlike Mappings in Several Complex Variables. doi:10.1007/978-94-011-5206-8. ISBN 978-94-010-6191-9. 
• Jarnicki, Marek; Pflug, Peter (2006). "A remark on separate holomorphy". Studia Mathematica 174 (3): 309–317. doi:10.4064/SM174-3-5. 
• Nehari, Zeev (1975). Conformal mapping. New York: Dover Publications. p. 146. ISBN 0-486-61137-X. OCLC 1504503. 

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