Show that if  U_n and V_n are solutions of  \Delta^{2} y_{n} + p_{n} \Delta y_{n}+ q_{n}y_{n} = 0 and C_{n}= \left(\begin{array}{cc}U_{n}&Y_{n}\\ \Delta U_{n}&\Delta Y_n \end{array}\right), then  \Delta C_{n} + (p_{n} - q_{n})C_{n} = 0