Salim Y. El Rouayheb Alexander Sprintson Costas N. Georghiades On the Index Coding Problem and its Relation to Network Coding and Matroid Theory http://arxiv.org/abs/0810.0068 2008 CoRR abs/0810.0068 db/journals/corr/corr0810.html#abs-0810-0068 informal publication