  In this master thesis I discuss how the weak three-body decay B^+ --> pi^-
pi^+ K^+ is related to the strong interactions between the final-state
particles. Current issues in hadron spectroscopy define a region of interest in
a Dalitz plot of this decay. This region is roughly characterized by energies
from 0 to 1.6 GeV of the pi^- pi^+ and the pi^- K^+ system, and energies from 3
to 5 GeV of the pi^+ K^+ system. Therefore I propose to use for the former two
systems elastic unitarity (Watson's theorem) as an approximate ansatz, and a
non-vanishing forward-peaked amplitude for the latter system. I introduce most
basic concepts in detail with the intention that this be of use to non-experts
interested in this topic.
