On the characterization of polyharmonic functions through iterated means

We introduce an infinite family of mean-value formulas (exact and asymptotic) given in terms of linear combinations of iterated means. We prove that the mean-value formulas in this family characterize real-valued polyharmonic functions of finite order, and that a simple algebraic condition partitions the family into equivalence classes according to the order of polyharmonicity. Our key results include strong converses to the mean-value properties – locally integrable functions satisfying a mean-value property in the family are polyharmonic – and a regularity result – locally integrable functions satisfying a mean-value property in the family, whether exact or asymptotic, are smooth.