Published online by Cambridge University Press: 25 August 2021
Polyharmonic maps of order k (briefly, k-harmonic maps) are a natural generalization of harmonic and biharmonic maps. These maps are defined as the critical points of suitable higher-order functionals which extend the classical energy functional for maps between Riemannian manifolds. The main aim of this paper is to investigate the so-called unique continuation principle. More precisely, assuming that the domain is connected, we shall prove the following extensions of results known in the harmonic and biharmonic cases: (i) if a k-harmonic map is harmonic on an open subset, then it is harmonic everywhere; (ii) if two k-harmonic maps agree on an open subset, then they agree everywhere; and (iii) if, for a k-harmonic map to the n-dimensional sphere, an open subset of the domain is mapped into the equator, then all the domain is mapped into the equator.
The first author gratefully acknowledges the support of the Austrian Science Fund (FWF) through the START-project Y963-N35 of Michael Eichmair and the project P30749-N35 “Geometric variational problems from string theory”. The second author and the last author are members of the GNSAGA group of INdAM (Italy) and they are also supported by the Fondazione di Sardegna (project STAGE) and Regione Autonoma della Sardegna (Project KASBA). The third author was partially supported from the grant PN-III-P4-ID-PCE-2020-0794.