Jørgen Bang-Jensen Jing Huang Anders Yeo Strongly Connected Spanning Subdigraphs with the Minimum Number of Arcs in Quasi-transitive Digraphs. 335-343 2003 16 SIAM J. Discrete Math. 2 http://epubs.siam.org/sam-bin/dbq/article/35422 db/journals/siamdm/siamdm16.html#Bang-JensenHY03