Cheng Huang Minghua Chen Jin Li Pyramid Codes: Flexible Schemes to Trade Space for Access Efficiency in Reliable Data Storage Systems. 79-86 2007 conf/nca/2007 NCA http://doi.ieeecomputersociety.org/10.1109/NCA.2007.37 db/conf/nca/nca2007.html#HuangCL07