This paper is a muster of some research production of the author when she was studying applied mathematics to achieve the master degree. Several problems about mathematical physics are discussed by qualitative methods.This paper is planned with a view to the author's research purposes. In Chapter 1, a class of even-order elliptical boundary value problems are discussed, the background of which is the physical phenomenon about the deflection of elastic beam. In Chapter 2, two parabolic initial boundary value...