Back in 1958 Hans Peter Luhn assumed in his paper "The Automatic Creation of Literature Abstracts" that "the frequency of word occurrence in an article furnishes a useful measurement of word significance" which is until now probably one of the most significant things in the Information Retrieval science and is used in all well known big and small search engines starting from Google and Yahoo to custom search solutions such as ElasticSearch and Manticore Search.

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