Before revision:  Among them, a popular method, named proximal policy optimization (PPO), has been introduced  . PPO clips density ratio of the latest and baseline policies with a threshold, while its minimization target is unclear.
After revision:  Among them, a popular method, named proximal policy optimization (PPO), has been introduced for this purpose . PPO clips density ratio of the latest and baseline policies with a threshold, while its minimization target is unclear.