無監督強化學習藉由探索環境來學習環境知識，可以應用先前的知識有效
率的學習下游任務。在眾多方法中，有一派作法額外學習技能來抽象化學得的
知識，藉此幫助代理學習任務。在無監督學習時，如果我們可以學習各種技能
來做出多樣化的行為，就意味著在學習下游任務時，可以挑選適合的技能來幫
助我們快速的達成目標任務。而如何探索環境以學習多樣化的行為便是無監
督技能學習中的關鍵因素。近期的一篇研究，APS [1]提出了使用介於”基於模
型”和”無模型”的後繼特徵框架: (successor feature framework)，結合狀態熵作為無監督技能學習最大互訊息的下界，不需要額外的模型就可以捕捉環境的狀態
轉換，使得探索過的資訊不會被遺忘，可快速應用技能至下游任務。但其技能
並無完善的多樣化學習，因此在下游任務中的表現不佳。因此我們提出了一種
最大互資訊無監督技能學習方法，使用噪聲對比估計作為學習技能與狀態特徵
的目標函數；在此之上我們發現，理想中，使用噪聲對比估計恰好符合SF為線
性的限制，而即使存在著微小的噪聲依然可以在高維度的環境表現得很好。實
驗結果顯示，在運動模型與機械手臂操作的任務中，我們的效能比最先進的無
監督技能學習方法CIC還要好7%，且與同樣使用SF框架的APS相比，效能增幅
高達83%

Learning diverse primitive skills to efficiently solve novel tasks is the essence of
unsupervised skill discovery. One general strategy is to specify a learning objective that maximizes the mutual information (MI) between skill and states so that
the learned skills are distinct and can collectively explore large parts of the state
space, facilitating the learning of novel downstream tasks. Unfortunately, as the
above learning objective is intractable, most previous works had to resort to strong
assumptions or approximation to make it tractable. This results in poor state coverage by the discovered skills due to insufficient exploration of the environment.
A recent work, Active Pretraining with Successor Features (APS) [1], proposes a
novel lower bound of MI by combining state entropy maximization [2] with variational successor features [3] to increase state exploration and speed up learning
for downstream tasks. Although APS increases environmental exploration, the
proposed variational approximation falls short in learning distinguishable skills,
leading to poor diversity and short-sighted behavior. To address this limitation,
we propose a successor-feature-based contrastive learning approach for unsupervised skill discovery by utilizing noise contrastive estimation as the lower bound
of the intractable conditional entropy for learning distinctive skills and state features. With the learned representation, the behavioral diversity is enhanced by
reaching individually distinct regions. The proposed approach is evaluated along
with other baselines on locomotion and manipulation tasks on the Unsupervised
Reinforcement Learning Benchmark (URLB) [4]. Experimental results show that
our method is superior to previous unsupervised skill discovery methods, such as
CIC, with an average 7% improvement. Furthermore, when compared to APS,
which also shares the SF framework, our method results in an average of 83%
boost in performance.

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