J Austral Math Soc Ser A 52 pp401--418, 1992.

Euclidean Linear Invariance and Uniform Local Convexity

Wancang Ma and David Minda

Abstract

Let S(p) be the family of holomorphic functions f defined on the unit disk D, normalized by f(0) = f¢(0) - 1 = 0 and univalent in every hyperbolic disk of radius r. Let C(r) be the subfamily consisting of those functions which are convex univalent in every hyperbolic disk of radius r. For r = ¥ these become the classical families S and C of normalized univalent and convex functions, respectively. These families are linearly invariant in the sense of Pommerenke; a natural problem is to calculate the order of these linearly invariant families. More precisely, we give a geometric proof that C(r) is the universal linearly invariant family of all normalized locally schlicht functions of order at most coth(2r). This gives a purely geometric interpretation for the order of a linearly invariant family. In a related matter, we characterize those locally schlicht functions which map each hyperbolically k-convex subset of D onto a euclidean concex set. Finally, we give upper and lower bounds on the order of the linearly invariant family S(r) and prove that this class is not equal to the universal linearly invariant family of any order.

1991 AMS Subject Classification: 30C99, 30C45, 30C50, 30D45

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Authors

Wancang Ma

David Minda

Department of Mathematical Sciences, University of Cincinnati, Cincinnati, Ohio 45221-0025, U.S.A.