差分

:$$z^n=\displaystyle\left[\sum_{k=0}^{\lfloor (n-1)/2\rfloor}\binom{n-k-1}{k}r^{n-2k-1}l^{k}\right]z-+\left[\sum_{k=0}^{\lfloor (n-2)/2\rfloor}\binom{n-k-2}{k}r^{n-2k-2}l^{k}\right]l$$