中国科学院数学与系统科学研究院期刊网
AMS
On Sullivan’s Conjecture on Cycles in 4-free and 5-free Digraphs
Hao LIANG, Jun Ming XU
Acta Mathematica Sinica . 2013, (1): 53 -64 .  DOI: 10.1007/s10114-012-1538-5