GeNGA: A Generalization of Natural Gradient Ascent with Positive and Negative Convergence Results

Natural gradient ascent (NGA) is a popular optimization method that uses a positive definite metric tensor. In many applications the metric tensor is only guaranteed to be positive semidefinite (e.g., when using the Fisher information matrix as the metric tensor), in which case NGA is not applicable. In our first contribution, we derive generalized natural gradient ascent (GeNGA), a generalization of NGA which allows for positive semidefinite non-smooth metric tensors. In our second contribution we show that, in standard settings, GeNGA and NGA can both be divergent. We then establish sufficient conditions to ensure that both achieve various forms of convergence. In our third contribution we show how several reinforcement learning methods that use NGA without positive definite metric tensors can be adapted to properly use GeNGA.