Given the differential equation
$ A_{1}U_{xx}+A_{2}U_{yy}+A_{3}U_{xy}+A_{4}U_{x}+A_{5}U_{y}=U $
what does the value of $(A_{3})^{2}-A_{2}A_{1}$ mean whether its equal, less or larger than zero? I remember reading that this equation determines something about the PDE.