Beltrami equation with coefficient in Sobolev and Besov spaces
  Our goal in this work is to present some function spaces on the complex plane $\C$, $X(\C)$, for which the quasiregular solutions of the Beltrami equation, $\bar\partial f (z) = \mu(z) \partial f (z)$, have first derivatives locally in $X(\C)$, provided that the Beltrami coefficient $\mu$ belongs to $X(\C)$. 
Victor Cruz, Joan Mateu, and Joan Orobitg
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