Differential equations models

A man wanders around under a street lamp and looks at the ground. A cop approaches and asks the man what he’s doing. Looking for my keys, replies the man. Where’d you lose them? asks the cop. Back there, says the man, pointing to a dark alley. Then why are you looking under the street lamp? asks the cop. Because the light’s better here, replies the man. The iterative equations of Ch. 2 work well in some settings, and are plausible models for systems that consist of agents which can occupy only a finite number of states: the number of