Derivation of CN scheme
To derive the Crank-Nicolson equation,
we start with the forward Euler scheme and Taylor expand
u(x,t+\Delta t) ,
u(x+\Delta x, t) and
u(x-\Delta x,t)
\begin{align}
u(x+\Delta x,t)&=u(x,t)+\frac{\partial u(x,t)}{\partial x} \Delta x+\frac{\partial^2 u(x,t)}{2\partial x^2}\Delta x^2+\mathcal{O}(\Delta x^3),
\tag{10}\\ \nonumber
u(x-\Delta x,t)&=u(x,t)-\frac{\partial u(x,t)}{\partial x}\Delta x+\frac{\partial^2 u(x,t)}{2\partial x^2} \Delta x^2+\mathcal{O}(\Delta x^3),\\ \nonumber
u(x,t+\Delta t)&=u(x,t)+\frac{\partial u(x,t)}{\partial t}\Delta t+ \mathcal{O}(\Delta t^2).
\tag{11}
\end{align}