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Alexandre Eremenko
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For the equivalence of definitions of quasiconformal maps the reference is J. Heinonen, Lectures on analysis on metric spaces, Springer 2001. Notice that the $K$ in the definiton you cite is not the same $K$ as in the Ahlfors definitions. So your definition of quasiconformality is equivalent to the usual one, but with a different $K$.

Alexandre Eremenko
  • 91.8k
  • 9
  • 259
  • 429