# Applying numerical schemes like Runge Kutta to hyperbolic PDEs where the partial derivative of one variable is a function of another variable

If anyone can help with how to begin in this case, it would be a great help.

Imagine if we have a 1D hyperbolic system of equations like

$$\frac{\partial U}{\partial t}=-i\omega\zeta$$ and $$\frac{\partial \zeta}{\partial t}=-i\omega U$$.

Now, if we want to apply a runge kutta method, say second order (RK2), then I know RK2 becomes,

$$k_1=hf(t_n,y_n)$$
$$k_2=hf(t_n+h,y_n+k_1)$$
$$y_{n+1}=y_n +(k_1+k_2)/2$$

Can someone please help me with how do we implement the RK2 or RK3 in this case such that the expression for $$U_{n+1}$$ also contains $$\zeta$$ terms? All of Runge kutta methods are mostly explained in books like $$\frac{dU}{dt}=f(t,u)$$ but here it's rather $$\frac{\partial U}{\partial t}=f(t,\zeta)$$ and I don't know how to proceed with this case.

Any help would be incredibly appreciated.

You are solving a system. Let $$W = (U,\xi)^T$$ and $$f(t,W)=(-i\omega\xi, -i\omega U)^T$$ and apply your method of choice (RK2, RK3...) to $$\frac{dW}{dt} = f(t,W)$$.