Floquet's theorem: for a periodically driven Hamiltonian H(t+T)=H(t), solutions take the form |ψ(t)⟩ = e^{−iε_αt}|u_α(t)⟩ where ε_α are quasienergies (analogous to crystal momentum) and |u_α(t)⟩ are T-periodic Floquet modes. Avoided crossings in the quasienergy spectrum reveal resonance and chaos.