Calculus How To

Univalent Functions

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In general, a univalent function (sometimes called analytic univalent) is an analytic function that maps one input to one output (i.e. it is injective) on the complex plane. In other words, it’s a complex function that has no overlap.

More Narrow Definitions

Univalent Functions can also be defined more narrowly, as a function that maps different points to different points in the unit disk 𝔻 (Ravichandran, 2012).

In notation (Thomas, 1967), a function f(z) in a domain 𝔻 is univalent if w = f(z) takes on different values of w for different z ∈ 𝔻. In other words, f(z1) = f(z2) implies z1 = z2 for z1, z2 ∈ 𝔻. This tells you that a function f(z) defines a one to one correspondence between points of 𝔻 and a domain in the w-plane.

One example:
univalent function


  • z = a point in the complex plane,
  • a = the complex conjugate*.

φa is univalent when |a| < 1.

*A complex conjugate has a real part and an imaginary part with the same magnitude but different signs. For example, the complex conjugate of a + bi is abi (where a and b are two real numbers).

Some authors define univalent functions as being injective and meromorphic in the unit disk (e.g. Pommerenke, 1985). A meromorphic function is the ratio of two analytic functions, which are analytic with the exception of “poles“— isolated singularities.

Neighborhoods of univalent functions

An interesting feature of univalent functions is that their entire neighborhoods are also univalent functions (Pascu & Pascu, 2009). In other words, a small deviation in the function will not result in a non-univalent function.


Univalent functions can be categorized into various subclasses. For example:

  1. Regular starlike: When mapping from the unit disk Δ to the complex plane ℂ, a function is starlike if and only if
    starlike function
    (Nevanlinna, 1921).
  2. Close-to-convex: the definition is the same as regular starlike, except the function is convex if and only if:,


Nevanlinna, R. (1921). Über die konforme Abbildund Sterngebieten
Oversikt av Finska–Vetenskaps Societen Forhandlingar, 63(A) (6), pp. 48-403
Pascu,M. & Pascu, N. (2009). Neighborhoods of univalent functions. Retrieved December 19 from:
Pemmerenke, Ch. (1985). On the Integral Means of the Derivative of a Univalent Function. Journal of the London Mathematical Society, Volume s2-32, Issue 2, October 1985, Pages 254–258,
Ravichandran, V. (2012). Geometric Properties of Partial Sums of Univalent Functions. Retrieved December 19, 2019 from:
Sokół, J. (2011). A certain class of starlike functions
Study, E. (1913). Konforme Abbildung Einfachzusammenhangender Bereiche
B. C. Teubner, Leipzig und Berlin
Thomas, D. (1967). Starlike and close-to-convex functions. Retrieved December 19, 2019 from:

Stephanie Glen. "Univalent Functions" From Calculus for the rest of us!

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