The second variation at a bang-bang extremal. - Andrei A. Agrachev; Gianna Stefani; PierLuigi Zezza (ECC, 2001)

Strong Optimality for a Bang-Bang Trajectory. - Andrei A. Agrachev; Gianna Stefani; PierLuigi Zezza (SIAM J. Control. Optim., 2002)