The Diffusion Equation
Equation 3.1 has second derivative in space, therefore, the diffusion takes place in both directions and requires two boundary conditions. Also, Eq. 3.1 has a first derivative in time, the diffusion is one directional in time, in other words, the diffusion at any point depends on the previous time and no information can be transferred from the future time. Also, it requires an initial condition to solve Eq. 3.1.