Published online by Cambridge University Press: 15 August 2002
A control system of the second order in time with control $u=u(t)\in L^2([0,T];U)$ is considered. If thesystem is controllable in a strong sense anduT is the controlsteering the system to the rest at timeT,then the L 2–norm of uT decreases as $1/\sqrt T$
while the $L^1([0,T];U)$
–norm of uT is approximately constant.The proof is based on the moment approachand properties of the relevant exponential family. Results areapplied to the wave equation with boundary or distributed controls.