1. The problem statement, all variables and given/known data It's problem 2.b) on this page: http://www.phys.washington.edu/users/schick/322A/322-08ps3.pdf 3. The attempt at a solution So, what it looks like to me, since the only terms in the actual vector potential are all multiplied, Lambda would have to be a constant, so that when you take the gradient of it, it becomes 0 and doesn't change A'. So basically A=A' and Lambda = some constant vector. If Lambda wasn't constant, then taking the gradient would give some additional terms added to A, whereas no addition takes place.