  We consider the first order periodic systems perturbed by a $2N\ts 2N$
matrix-valued periodic potential on the real line. The spectrum of this
operator is absolutely continuous and consists of intervals separated by gaps.
We define the Lyapunov function, which is analytic on an associated N-sheeted
Riemann surface. On each sheet the Lyapunov function has the standard
properties of the Lyapunov function for the scalar case. The Lyapunov function
has branch points, which we call resonances. We prove the existence of real or
complex resonances. We determine the asymptotics of the periodic, anti-periodic
spectrum and of the resonances at high energy (in terms of the Fourier
coefficients of the potential). We show that there exist two types of gaps: i)
stable gaps, i.e., the endpoints are periodic and anti-periodic eigenvalues,
ii) unstable (resonance) gaps, i.e., the endpoints are resonances (real branch
points). Moreover, we determine various new trace formulae for potentials and
the Lyapunov exponent.
