Symbol k

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…specifies is exactly the ancestor of what every later retriever, sparse or dense, still does: score a term by how often it occurs locally and how rare it is globally. They define the inverse document frequency of a term k , for a collection of n documents in which k appears in $d_k$ of them, as $(\mathrm{IDF})_k = \lceil \log_2 n \rceil - \lceil \log_2 d_k \rceil + 1$. and combine it multiplicatively with raw term frequency so that a term scores highest when it occurs often in one document but rarely across the collection. Evaluated on three test collections in…

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