Sufficient and necessary conditions for conformality at a point. Part I. Geometric viewpoint

摘要

We study the local behaviour of ACL-homeomorphisms in the plane, defined in a neighbourhood of a point and satisfying the Beltrami equation with coefficient We use extremal length techniques, modules of ring domains, of quadrilaterals and of families of curves that are arcs of spirals to provide geometric sufficient and necessary conditions for conformality at a point.