How to figure out Perturbed Lagrangians?

[Hybird]

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!

 

[NateD]

Thank you.A good reference on perturbation theory is the book "Classical Mechanics" by Herbert Goldstein. It is an excellent resource for learning about perturbation theory and its applications. Another good reference is the book "Quantum Mechanics" by John S. Townsend, which also covers perturbation theory.