We consider the H-infinity control problem for continuous time systems
with sampled observation studied by Sun, Nagpal and Khargonekar. We e
stablish two Redheffer's lemmas for systems with hybrid signals. We th
en give the characterization of all suboptimal controllers which was n
ot provided by Sun et al. We also introduce a chain of three systems w
ith jumps generated through the output estimation problem, and solve t
he H-infinity problems in this chain.