A method and apparatus for making document information search and a magnetic disk unit to be used for realizing the method and apparatus. In the document information search method, in performing document search with respect to a desired subject key word, two stages of presearch are carried out. In a...http://www.google.de/patents/US5168533?utm_source=gb-gplus-sharePatent US5168533 - Hierarchical presearch type text search method and apparatus and magnetic disk unit used in the apparatus
