  We show analytically that there is anomalous diffusion when the diffusion
constant depends on the concentration as a power law with a positive exponent
or a negative exponent with absolute value less than one and the initial
condition is a delta function in the concentration. On the other hand, when the
initial concentration profile is a step, the profile spreads as the square root
of time. We verify our results numerically using particles moving
stochastically.
