L//2-INSTABILITY CRITERIA FOR INTERCONNECTED SYSTEMS.

Various conditions are presented for a feedback interconnection of subsystems to be $L_2 $-unstable. These results are of two types: (i) “small gain” type results that involve the formulation of a test matrix, and (ii) “passivity” type results that can be applied directly to the various subsystem and interconnection operators. The application of these results is illustrated through examples.