Methods of Mathematical Physics Vol. - 2: Partial Differential Equations - Courant, R.; Hilbert, David (New York John Wiley & Sons, Inc., 1962)
General Theory of Partial Differential Equations of First Order Differential Equations of Higher Order Potential Theory and Elliptic Differential Equations Hyperbolic Differential Equations in Two Independent Variables Hyperbolic Differential Equations in More Than Two Independent Variables