Continuous adaptive critic designs

A continuous formulation of an adaptive critic design (ACD) is investigated. Connections to the discrete case are made, where backpropagation through time (BPTT) and realtime recurrent learning (RTRL) are prevalent. A second order actor adaptation, based on Newton's method, is established for fast actor convergence. Also, a fast critic update for concurrent actor-critic training is outlined that keeps the Bellman optimality correct to first order approximation after actor changes.