Abstract. In this paper we mainly survey results obtained in [MM3]. For example, we give an elementary proof of two versions of Koebe 1/4 theorem for analytic functions (see Theorem 1.2 and Theorem 1.4 below). We also show a version of the Koebe theorem for quasiregular harmonic functions. As an application, we show that holomorphic functions (more generally quasiregular harmonic functions) and their modulus have similar behavior in a certain sense.
Two versions of Koebe 1/4 theorem for analytic functionsThis paper can be considered as the review of some results presented in [MM3], but it also contains new results and proofs. We will use the following notation. If r > 0 and a is a complex number B(a; r) = {z ∈ C : |z − a| < r} is the open circular disc with center at a and radius r. Also we use notation ∆ r = B(0, r) and ∆ = ∆ 1 . First, we introduce a particulary interesting class of conformal mappings of the disc, the class S. We denote by S the class of holomorphic functions f in ∆ which are injective and satisfy the normalization conditions f (0) = 0 and f (0) = 1.The proof of Koebe's One-Quarter Theorem is based on the extremal property of the Grötzsch annulus, which we first need to discuss.
Grötzsch Theorem and Koebe's One-Quarter Theorem.If Ω is a double connected domain, by M (Ω) we denote the modulus of Ω; and for a given family of curves Γ by M (Γ) modulus of the family of curves Γ. If Ω is a double connected domain and E 1 and E 2 the components of ∂Ω the extremal distance d Ω (E 1 , E 2 ) between E 1 and E 2 is the modulus of Ω. Note that if Γ is the family of curves which joins the components E 1 and E 2 , then d Ω (E 1 , E 2 ) = 1/M (Γ) and if Γ is the family of curves which separates the components E 1 and E 2 then M (Ω) = M (Γ ).Let 0 < r < 1 and c be any continum that contains {0, r}; and let Γ, Γ 0 and Γ c be the families of closed curves in the unit disk that separate {0, r}, s = [0, r] 2000 Mathematics Subject Classification: Primary 30C62, 30C80, 30C55; Secondary 30H05.