Equation 19 · From BM25 to Agentic Retrieval: A History of Retrieval-Augmented Generation
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By the time Lewis and colleagues published in 2020, learned dense retrieval and large pretrained generators both already existed as separate lines of work. The paper’s specific technical contribution was neither — it was making the whole pipeline trainable end to end through a single differentiable objective, treating the retrieved passage as a latent variable to be marginalized over rather than a fixed input handed to a frozen reader [ 8 ] . Writing x for the input, y for the output, z for a retrieved passage, and and for the retriever’s and generator’s parameters respectively, the RAG-Sequence variant approximates
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