A function is needed that has a maximum at 7Pi/4 and changes sign under the transformation phi -> phi + Pi within the range 0 < phi < 2Pi. The discussion highlights that while it's possible to find a function with a maximum at this point, ensuring it changes sign is crucial. The transformation sin(x + Pi) = -sin(x) is referenced as a method to achieve the sign change. A suggested function is sin(phi - Pi/4), which meets the criteria for both the maximum and sign change. This approach successfully addresses the requirements of the problem.