A proof by graphical construction of the no-pumping theorem of stochastic pumps

A stochastic pump is a Markov model of a mesoscopic system evolving under the control of externally varied parameters. In the model, the system makes random transitions among a network of states. For such models, a "no-pumping theorem" has been obtained, which identifies minimal conditions for generating directed motion or currents. We provide a derivation of this result using a simple graphical construction on the network of states.