  We show that the dark soliton of the Gross-Pitaevskii equation (GPE) that
describes the Bose-Einstein condensate (BEC) density of a system of weakly
repulsive bosons, also describes that of a system of strongly repulsive hard
core bosons at half filling. As a consequence of this, the GPE soliton gets
related to the magnetic soliton in an easy-plane ferromagnet, where it
describes the square of the in-plane magnetization of the system. These
relationships are shown to be useful in understanding various characteristics
of solitons in these distinct many-body systems.
