\begin{eqnarray}
R^k_j &=& L^{(1)}u(x_j,t_k)-[Lu]^k_j\\
&=& -\tau[\frac{1}{12r}-\frac{1}{2} ]\left ( \frac{\partial^2 \widetilde{u}}{\partial t^2}\right ) ^k_j+O(\tau^2+h^2)\\
&=& O(\tau+h^2)
\end{eqnarray}
\begin{eqnarray}
\begin{aligned}
R^k_j &= L^{(1)}u(x_j,t_k)-[Lu]^k_j\\
&= -\tau[\frac{1}{12r}-\frac{1}{2} ]\left ( \frac{\partial^2 \widetilde{u}}{\partial t^2}\right ) ^k_j+O(\tau^2+h^2)\\
&= O(\tau+h^2)
\end{aligned}
\end{eqnarray}
此时需要去掉一个&
\begin{eqnarray}
R^k_j &=& L^{(2)}u(x_j,t_k)-[Lu]^k_j\\
&=& -\tau[\frac{1}{12r}+\frac{1}{2} ]\left ( \frac{\partial^2 \widetilde{u}}{\partial t^2}\right ) ^k_j+O(\tau^2+h^2)\\
&=& O(\tau+h^2)
\end{eqnarray}
\begin{eqnarray}
\begin{aligned}
R^k_j &=& L^{(2)}u(x_j,t_k)-[Lu]^k_j\\
&=& -\tau[\frac{1}{12r}+\frac{1}{2} ]\left ( \frac{\partial^2 \widetilde{u}}{\partial t^2}\right ) ^k_j+O(\tau^2+h^2)\\
&=& O(\tau+h^2)
\end{aligned}
\end{eqnarray}