Tomoko Izumi Taisuke Izumi Hirotaka Ono Koichi Wada Relationship between Approximability and Request Structures in the Minimum Certificate Dispersal Problem. 56-65 2009 COCOON http://dx.doi.org/10.1007/978-3-642-02882-3_7 conf/cocoon/2009 db/conf/cocoon/cocoon2009.html#IzumiIOW09