<t>Amin, I assume that you are trying to model the probability distribution for the two-dimensional process p(S,V,t). You have to specify four boundaries. I suggest the following ones:1) p_{S}(S,V,t)|_{S=0}=0,2) p_{V}(S,V,t)|_{V=0}=0,These are reflecting boundaries - the diffusion is reflected at ze...