18. The system as claimed in claim 17 , wherein said hot index is attained by the following formula: H = Pr n × We n × ∑ j = 0 n ⁢ TF - IDF j , in which H represents a hot index, Pr n represents a reference source gravity, We n represents a reference source weight, and n represents a year range and meets a condition of 4≦n≦10.
18. The system as claimed in claim 17 , wherein said hot index is attained by the following formula: H = Pr n × We n × ∑ j = 0 n ⁢ TF - IDF j , in which H represents a hot index, Pr n represents a reference source gravity, We n represents a reference source weight, and n represents a year range and meets a condition of 4≦n≦10. 22. The system as claimed in claim 18 , further comprising a factor estimation subsystem and a verification subsystem both connected to said processing unit, wherein said factor estimation subsystem comprises a questionnaire survey module to provide at least one questionnaire and allows said recommending subsystem to automatically adjust said reference source weight according to a result of said questionnaire, and said verification subsystem includes a usefulness term search module and a usability term search module, said usefulness term search module providing a usefulness-evaluation questionnaire for evaluating a usefulness of said recommending subsystem, said usability term search module providing a usability-evaluation questionnaire for automatically evaluating a user acceptance of said recommending subsystem.