•Initial theory was SISO (Zames, Helton,
Tannenbaum)
•Nevanlinna-Pick interpolation
•Operator-theoretic methods (Sarason, Adamjan et al, Ball-
• Helton)
•Initial work handled restricted problems
(1-block and 2-block)
•Solution to 2x2-block problem
• (1984 Honeywell-ONR Workshop)