2. The information retrieval system of claim 1 , wherein the relevance of a document is calculated on the basis of a plurality of keywords, the influence set for each keyword, and a date.
2. The information retrieval system of claim 1 , wherein the relevance of a document is calculated on the basis of a plurality of keywords, the influence set for each keyword, and a date. 3. The information retrieval system of claim 2 , wherein the date has the same influence as a keyword, and the influence can be changed in the same manner as a keyword.