ber eine Randwertaufgabe der Differentialgleichung $$r^2 \frac{{\partial ^2 f}}{{\partial r^2 }} + r^2 \frac{{\partial f}}{{\partial r}} + q^2 \frac{{\partial ^2 f}}{{\partial \varphi ^2 }} = r^2 k\frac{{\partial f}}{{\partial t}}$$