A domain Ω is a QED domain if its QED constant M(Ω) is finite. QED domains were introduced by Gehring and Martio [GM] as a useful class of domains in the study of quasiconformal mappings. In this paper we will concentrate on Jordan domains whose QED constants are finite. The QED constant is called a conformal invariant because it is invariant under Mobius transformations (or conformal mappings of the extended plane C). It is determined by the geometry of a domain and measures how far a domain is from being a disk.
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