The Floquet problem is about the linear ordinary differential equation
$$ \dot{\psi} = A(t) \psi. $$
Here $A(t)= A(t+T)$ is a periodic $n\times n $ matrix. Suppose $A(t) =-i H(t)$ with $H(t)$ being hermitian. Then, we have the one-period propagation unitary operator $ U (T) $, which is determined by the equation
$$ \dot{U} = - i H(t) U(t), $$
with $U(0) = I_{n\times n}$. Apparently, the spectrum of the operator $U(T)$ is very important.
Is there any good reference on it?