Lagrangian formalism

• How Does Hamilton’s Principle Work with Semiholonomic Constraints?
  For nonintegrable velocity constraints, distinguish Hamilton’s principle on constrained paths from the d’Alembert variation used for ideal constraints.
• Can You Determine Whether a Theory Is Unitary from Its Lagrangian?
  A Lagrangian can reveal warning signs such as ghosts, but deciding unitarity also requires the quantization, physical states, and quantum consistency checks.