To show that a scheme is consistent we define a smooth function $\phi$, and consider the following:
$$\frac{\phi_m^{n+1}-\phi_m^{n-1}}{2k}+a\left(\frac{\phi_{m+1}^{n}-\phi_{m-1}^{n}}{2h}\right)=0$$
I'm stuck on showing how to demonstrate this. I understand that we use Taylor series, then we find the difference between $\phi u-\phi_{k,h}=0$ as $(h,k)\to 0$.