Degenerate Lagrangian
= Degenerate Lagrangian
A <Lagrangian> is degenerate when its Hessian with respect to <velocities> is singular. The <momentum>–<velocity> relation then cannot be inverted for every <velocity>. Some Euler–Lagrange equations can become algebraic constraints rather than independent second-order equations. A massless limit can produce this behaviour; disappearance of an equation at quadratic order does not establish an unconstrained mode of the exact nonlinear model.