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A ''document'' is any subset of <math>T</math>. Let<math display="block">D = \{D_1,\ ...\ ,D_n\}</math>be the set of all documents.
A ''query'' is a Boolean expression <math display="inline">Q</math> in normal form:<math display="block">Q = (W_1\ \or\ W_2\ \or\ \ldots) \and\ \ldots\ \and\ (W_i\ \or\ W_{i+1}\ \or\ \ldots)</math>where <math display="inline">W_i</math> is true for <math>D_j</math> when <math>t_i \in D_j</math>. (Equivalently, <math display="inline">Q</math> could be expressed in
We seek to find the set of documents that satisfy <math display="inline">Q</math>. This operation is called ''retrieval'' and consists of two steps, defined as follows:
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