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I have a simple question on Sobolev space theory. Let $1\le p \le \infty$. How can one prove that every $u\in W^{1,p}(0,1)$ is equal a.e. to an absolutely continuous function and that $u'$ exists a.e. and belongs to $L^p(0,1)$?

Thank you for your assistance.

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    I mean the function can be represented by a function that is a.e. equal to an absolutely continuous function.2012-12-09

2 Answers 2

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Consider the case $p=1$. Take $u\in W^{1,1}(0,1)$ and put $v(t)=u(0)+\int_0^tu'(s)ds$, then $v\in W^{1,1}(0,1)$ and is absolutely continuous. We have $v'=u'$ a.e. so $u=v+c$ a.e.

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    @Jose27 "is absolute continuous" I still cannot see what really make $u$ equal to an absolute continuous function $v$, what was used in obtaining the absolute continuity? I saw other notes, says "fundamental theorem of calculus" why?2015-07-08
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The answer posted by Jose27 is correct. For more details and related things, see the reference here: http://www.iadm.uni-stuttgart.de/LstAnaMPhy/Weidl/fa-ws04/Suslina_Sobolevraeume.pdf, especially Theorem 5.