Energy Content of Colliding Plane Waves using Approximate Noether   Symmetries
  This paper is devoted to study the energy content of colliding plane waves using approximate Noether symmetries. For this purpose, we use approximate Lie symmetry method of Lagrangian for differential equations. We formulate the first-order perturbed Lagrangian for colliding plane electromagnetic and gravitational waves. It is shown that in both cases, there does not exist 
M. Sharif and Saira Waheed
gr-qc