* {{Citation |first=H. |last=Stephani |year=1989 |title=Differential Equations: Their Solution Using Symmetries. Edited by M. MacCallum |publisher=Cambridge University Press }}. | * {{Citation |first=H. |last=Stephani |year=1989 |title=Differential Equations: Their Solution Using Symmetries. Edited by M. MacCallum |publisher=Cambridge University Press }}. |