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In the expression ∣{d∈D:t∈d}∣ {}: denotes a set. d∈D: means "document d is in the set D" (i.e., d is one of the documents in the corpus D). t∈d means "term t is in document d" (i.e., the term t appears in the document d). : can be read as "such that". So, {d∈D:t∈d} describes the set of all documents d in the corpus D such that the term t appears in d. In plain English, it represents the set of a..