Mixed variational formulation

Hello everyone;

As you may know, for deriving governing equations of a system(mechanical,electrical,etc.) we can use different methods for example for a specific problem like vibration of simple mass and spring system, we can use weather Newton's second law or Lagrange method or Hamilton's principle. During studying of a book which was about the theory of piezoelectricity, at first I faced with variational formulation that the auther had been used Hamilton's principle; I could get that part easily. Next I faced with this title "Mixed variational formulation" that sounded pretty new to me.

would you please tell me what does it mean?

Thank you;