Error bounds for constant step-size Q-learning

We provide a bound on the first moment of the error in the Q-function estimate resulting from fixed step-size algorithms applied to finite state-space, discounted reward Markov decision problems. Motivated by Tsitsiklis' proof for the decreasing step-size case, we decompose the Q-learning update equations into a dynamical system driven by a noise sequence and another dynamical system whose state variable is the Q-learning error, i.e., the difference between the true Q-function and the estimate. A natural persistence of excitation condition allows us to sample the system periodically and derive a simple scalar difference equation from which the convergence properties and bounds on the error of the Q-learning algorithm can be derived.