  We discuss the differences between several partial-wave analysis formalisms
used in the construction of three-body decay amplitudes involving fermions.
Specifically, we consider the decay Lambda_b -> psi p K- , where the hidden
charm pentaquark signal has been reported. We analyze the analytical properties
of the amplitudes and separate kinematical and dynamical singularities. The
result is an amplitude with the minimal energy dependence compatible with the
S-matrix principles.
