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Hi,

On page 120, chapter 4, proposition 4.2.7 in Hubbard's Teichmuller Theory book, volume 1, he proves :

Let $U,V$ be open in $C, f:U \to V$ be a homeomorphism and the restriction of $f$ on $U \backslash l$ is $K$ quasiconformal, where $l$ is a line in $U$. Then $f$ is $K$-q.c. in $U$.

FYI: his definition 4.1.1 ( page 112 ) of a $K$-q.c. map $f:U\to V$ is :

1) $f$ is a homeomorphism.

2) Distributional/weak derivatives $f_z,f_\bar{Z}$ f_z,f_\bar{z}$of$f$exist almost everywhere and these derivatives are in$L^2_{loc}(U)$. 3)$|\frac{f_\bar{z}}{f_z}|\leq k = \frac {K-1}{K+1} $for some$K\geq1$. Now for the proof of Proposition 4.2.7, isn't it$ENOUGH$to prove/check that the derivatives$f_z, f_\bar{z}$are in$L^2(K) \forall K$compact subset of$U$,i.e. condition (2), which does not readily follow from that they are in$L^2_{loc}(K') \forall K' $compact subsets of oi$U \backslash l$. This he proves by using condition (3) stated above . But what else is there to prove, since we only care about existence of partial derivatives on a full measure set, we do NOT need to prove anything else apart from condition (2), right ? So, what does he do after proving condition (2), in page 120-121 ? Thanks and Happy Memorial Day! 1 # A quick and elementary question from Hubbard's Teichmuller Theory : Volume I Hi, On page 120, chapter 4, proposition 4.2.7 in Hubbard's Teichmuller Theory book, volume 1, he proves : Let$U,V$be open in$C, f:U \to V $be a homeomorphism and the restriction of$f$on$U \backslash l$is$K$quasiconformal, where$l$is a line in$U$. Then$f$is$K$-q.c. in$U$. FYI: his definition 4.1.1 ( page 112 ) of a$K$-q.c. map$f:U\to V$is : 1)$f$is a homeomorphism. 2) Distributional/weak derivatives$f_z,f_\bar{Z}$of$f$exist almost everywhere and these derivatives are in$L^2_{loc}(U)$. 3)$|\frac{f_\bar{z}}{f_z}|\leq k = \frac {K-1}{K+1} $for some$K\geq1$. Now for the proof of Proposition 4.2.7, isn't it$ENOUGH$to prove/check that the derivatives$f_z, f_\bar{z}$are in$L^2(K) \forall K$compact subset of$U$,i.e. condition (2), which does not readily follow from that they are in$L^2_{loc}(K') \forall K' $compact subsets of$U \backslash l\$. This he proves by using condition (3) stated above .

But what else is there to prove, since we only care about existence of partial derivatives on a full measure set, we do NOT need to prove anything else apart from condition (2), right ? So, what does he do after proving condition (2), in page 120-121 ?

Thanks !