I hope this is the right area to ask this, but does anyone know of a good link which describes perturbation theory? Or even a good book? I have a lagrangian that is a function of the vector potential and I need to figure out the perturbed lagrangian by perturbing the vector potential. That is, I need to figure out Delta of Lagrangian of Vector Potential. Or how to I figure out: dL = L(A+dA) - L(A) = ??? Say L = 2A + K Would it work the same way differentials are taken? How do you figure out: L(A+dA) ????? Clearly I'm missing something here!!!