Equivalent Conditions on Periodic Feedback Stabilization for Linear Periodic Evolution Equations

This paper studies the periodic feedback stabilization for a class of linear $T$-periodic evolution equations.Several equivalent conditions on the linear periodic feedback stabilization are obtained. These conditions are related with the following subjects: the attainable subspace of the controlled evolution equation under consideration; the unstable subspace (of the evolution equation with the null control) provided by the Kato projection; the Poincar$\acute{e}$ map associated with the evolution equation with the null control; and two unique continuation properties for the dual equations on different time horizons $[0,T]$ and $[0,n_0T]$ (where $n_0$ is the sum of algebraic multiplicities of distinct unstable eigenvalues of the Poincar$\acute{e}$ map). It is also proved that a $T$-periodic controlled evolution equation is linear $T$-periodic feedback sabilizable if and only if it is linear $T$-periodic feedback sabilizable with respect to a finite dimensional subspace. Some applications to heat equations with time-periodic potentials are presented.