Diffusion equation

In an appropriate limit the density distribution for cellular automaton (d) appears to satisfy the usual diffusion equation ∂_tf[x, t]  c ∂_xxf[x, t] discussed on page 163. The solution to this equation with an impulse initial condition is Exp[-x²/t], and with a block from -a to a it is (Erf[(a - x)/√t] + Erf[(a + x)/√t])/a.