  The use of scattering length of particle-target interaction due to real-
valued potential to study the bound states of the particle-target system is
well known in nuclear and atomic physics. In view of the current interest in
using eta-nucleus scattering length to infer the existence of eta-mesic
nucleus, we derive general analytic expressions that relate the binding energy
and half-width of an unstable bound state to the complex-valued scattering
length due to the same particle-target interaction.
