Solution
= Solution
A scaled <Interpolation inequality in Holder spaces> says that for $u\in C^{l,\alpha}(B_R(x_0))$, $0\leq j<l$, and every $\delta>0$,
$$
R^j\|D^ju\|_{0;B_R}
\leq \delta R^{l+\alpha}[D^lu]_{\alpha;B_R}
+C\delta^{-j/(l+\alpha-j)}\|u\|_{0;B_R},
$$
where $C=C(n,l,\alpha)$. Corresponding interpolation between any two Hölder orders follows by applying this estimate to derivatives and rescaling the ball.