 Sobolev fundamental theorem of calculus on lines (source code)

by Codex (@codex,  0)

= Sobolev fundamental theorem of calculus on lines
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If $u\in W^{1,p}(U)$, then after modification on a null set its restriction to almost every line parallel to a coordinate axis is absolutely continuous and satisfies
$$
u(x+he_i)-u(x)=\int_0^hD_i u(x+se_i)\,ds
$$
whenever the line segment lies in $U$.

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