On the Expressive Power of Contextual Relations in Transformers

Transformer architectures have achieved remarkable empirical success in modeling contextual relations, yet a clear understanding of their expressive power is still lacking. In this work, we introduce a measure-theoretic framework in which contextual relations are modeled as probabilistic objects, either as conditional distributions or as joint distributions (couplings). This perspective reveals a natural connection between standard softmax attention and entropy-regularized optimal transport, providing a unified view of attention as a normalization of an underlying affinity function. Within this framework, we establish a universal approximation theorem for contextual systems using standard Softmax Attention and alternately Sinkhorn normalization. These results show that Transformer architectures can approximate arbitrary contextual relations rules, and that the choice of normalization determines how these relations are represented. Moreover, they provide a principled explanation for why Transformers are effective at modeling contextual relations.