  A systematic procedure to synthesize interval observers for nonlinear
discrete-time systems is proposed. The feedback gains and other matrices are
found from the solutions to semidefinite feasibility programs. Two cases are
considered: (1) the interval observer is in the same coordinate frame as the
given system, and (2) the interval observer uses a coordinate transformation.
The conditions where coordinate transformations are necessary are detailed.
Numerical examples are provided to showcase the effectiveness of the interval
observers and demonstrate their application to sampled-data systems.
