摘要： The prensent paper is a resume in the qualitative researches of the ultrahyperbolic equation,it involves a new proof of Asgeirsson's mean value theorem,extensions of mean value theorem,non-analyticity of the generalized potential solution,extensionality of solution,boundary problems and the elementary solution. Especially,we stggest to study ultrahyperbolic equation by means of the elementary solution,which is a natural way.
