# The comparison of two Hardy-Littlewood maximal function

The uncentered Hardy-Littlewood maximal function of $f$ is $$\tilde{M}(f)=\sup\limits_{|y-x|<\delta}\frac{1}{|B(y,\delta)|}\int_{B(y,\delta)}|f|$$ If we denote the original Hardy-Littlewood maximal function as $M(f)$. In Grafakos's book, he states that $\tilde{M}(f)\leq 2^nM(f)$. I don't know how to prove this.

If $|y-x|<\delta$ and $|z-y|<\delta$ then $|z-x|<2\delta$. Therefore $B(y,\delta)\subset B(x,2\delta)$, hence $$\frac{1}{|B(y,\delta)|}\int_{B(y,\delta)}|f|\leq \frac{|B(x,2\delta)|}{|B(y,\delta)|}\frac{1}{|B(x,2\delta)|}\int_{B(x,2\delta)}|f|$$ $$=2^n\frac{1}{|B(x,2\delta)|}\int_{B(x,2\delta)}|f|\leq 2^nMf(x)$$ and the result follows by taking the supremum.