PARTIAL DIFFERENTIAL EQUATIONSCONSTITUTIVE EQUATIONSHELMHOLTZ EQUATIONSHYPERBOLIC DIFFERENTIAL EQUATIONSMAXWELLS EQUATIONSNAVIER STOKES EQUATIONSPOISSON EQUATIONWAVE EQUATIONSPARTICLE IN CELLDIFFERENTIAL EQUATIONSRIEMANN INVARIANTS (NPT)STEKLOFF PROBLEM (NPT)TIME-LIKE VARIABLES (NPT)TRICOMI EQUATION (NPT)WEINSTEIN'S EQUATION (NPT)BLOCH EQUATION (NPT)ELLIPTIC DIFFERENTIAL EQUATIONS (NPT)FIELD EQUATION (NPT)GOLDSTEIN'S MODEL (NPT)GOURSAT PROBLEM (NPT)HADAMARD'S METHOD (NPT)HEAT EQUATION (NPT)HYPERCIRCLE METHOD (NPT)KELLOG-EVANS THEOREM (NPT)LIEBMAN METHOD (NPT)