I see this proof on http://www.math.uiowa.edu/~lwang/cccalderon.pdf and I couldn't understand what he did. If $||f||_{L^p(B_{4})} = \delta$ is small and the measure $|\{ x \in B_1; M(|D^2u|^2)>N_1^2 \}| \leq \epsilon|B_1| $. Then, we have:
$$\sum_{i=1}^{+ \infty} (N_{1})^{ip}|\{M(|f|^2) > \delta^2 (N_{1})^{2i}\}| \leq \frac{pN^{p}}{\delta^{p}(N-1)}||f||_{{L^p}(B_{1})}^{p}$$
Someone could help me? Here M denotes the maximal function of Hardy Littlewood.
