A sharp lower bound for the nodal volume of harmonic functions

Abstract: Let $u$ be a non-zero real-valued harmonic function in $B_4\subset\mathbb{R}<sup>n$ with $n\geq3$ and $u(0)=0$. We prove that $$\mathcal{H}^{{n-1}\bigl({u(x)=0}\cap} B_2\bigr)\ge C_n\mathcal{N},$$ where $C_n$ is a positive constant depending only on $n$, and $\mathcal{N}$ is the doubling index defined by $$\mathcal{N}=\log_2\frac{\sup_{B_1}|u|}{\sup_{B_{\frac{1}{2}}}|u|}.$$ The linear dependence on $\mathcal{N}$ is optimal. This estimate confirms a folklore conjecture on the nodal volume of harmonic functions.