@techreport{weko_98407_1,
   author	 = "Tokinori,Suzuki and Atsushi,Fujii",
   title	 = "A Method for Approximating Document Frequency in Top-k Document Retrieval",
   year 	 = "2014",
   institution	 = "Tokyo Institute of Technology, Tokyo Institute of Technology",
   number	 = "2",
   month	 = "jan"
}