 Solution

Using $(Rf{)}^{\prime }={\int }_0^{1}f-f$ and integration by parts, with $\varphi$ a test function, givesFor each $k\ge 1$, apply the same identity to $R^{k}f$ and the test function ${\varphi }^{(k)}$. Since ${\int }_0^1{\varphi }^{(k)}=0$, it givesStarting from $k=1$ and iterating provesThe intermediate means disappear because derivatives of a test function have zero integral; no assumption that $R^{k}f$ has zero mean is needed.

For the bound, writeThe triangle inequality and Fubini's theorem implyThus $\boxed{c=\frac{1}{2}\text{ is valid}}$, and every iterate $R^{k}f$ is in the L1 space.