Hey! OP here Great question - h' in Equation 1a refers to the derivative of h with respect to time (t). This is a differential equation which we can solve mathematically when we have x in order to get a closed-form solution for h. We would then plug in that h (the hidden state) into equation 1b.

In our case, we don't actually wait for a closed-form solution but instead compute the discrete representation (Equation 2)

Hope that helps!