Let UHDk (resp. UHDk) be
the class of Riemannian n-manifolds (n ≧ 2) on which there exist k non-proportional
HD-minimal (resp. HD-minimal) functions. The purpose of the present paper is to
construct a Riemannian n-manifold n ≧ 3 which carries a unique (up to constant
factors) HD-minimal function but no HD-minimal functions. Thus the inclusion
relation
is strict for n ≧ 3. By welding k copies of this Riemannian n-manifold, it is then
established that the inclusion relation
is strict for all k ≧ 1 and n ≧ 3. The problem still remains open for
n = 2.