Equation 10 · Part 2 · Naive, Graph, and Agentic: A Systems Comparison of RAG Architectures

Symbol x

\mathcal{Z}_k(x)

x

What this part means

the writing.

Its job in the formula

x is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Full expressionSymbol xArticle meaning

Where the article explains it

Lewis and colleagues’ original formulation makes the mechanism explicit: writing x for the query, y for the output, and z for a retrieved passage drawn from a top- k set $\mathcal{Z}_k(x)$ , the model marginalises over that set, p(y $\mid$ x) $\approx$ $\sum_{z \in \mathcal{Z}_k(x)}$ $p_\eta(z \mid x)$ \, $p_\theta(y \mid x, z)$ , with retriever parameters $\eta$ and generator parameters $\theta$ [ 1 ] .

The passage around this formula

with retriever parameters $\eta$ and generator parameters $\theta$ [ 1 ] . One retrieval call, one generation pass, and the entire system’s dependence on the corpus runs through that single set $\mathcal{Z}_k(x)$ .

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Learn the underlying idea

A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.

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Sources cited in the surrounding passage

[1] Retrieval-Augmented Generation for Knowledge-Intensive NLP Tasks ↗

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