 Solution

Put $A={\int }_0^{1}f$. ThenThe theorem on differentiation of an indefinite Lebesgue integral makes this an absolutely continuous function on $[0,1]$, withBoth $A-f$ and $Rf$ belong to the L1 space: for example, $|Rf(x)|\le |A|+\Vert f{\Vert }_{L^1}\le 2\Vert f{\Vert }_{L^1}$. The ordinary derivative of an absolutely continuous function is its weak derivative, as integration by parts against a test function shows. Therefore $\boxed{Rf\in W^{1,1}(0,1)}$, the indicated Sobolev space.