A maximum principle for $n$-metaharmonic functions
S.N. Chow, D. R. Dunninger · Proceedings of the American Mathematical Society · 1974
A class of $n$-metaharmonic functions is shown to satisfy the inequality, $|u(x)| \leqq k|u({x_0})|$, where $x$ is an arbitrary point in a domain $\bar D,{x_0}$ is some fixed point on the boundary of $D$, and $k$ is a constant.