partially supported by NNSFC(Grant No.11401353);TYAL of Shanxi and NSF of Shanxi Province(Grant No.2016011005);partially supported by China Scholarship Council
In 2006, Sullivan stated the conjectures: (1) every oriented graph has a vertex x such that d++ (x) ≥ d- (x); (2) every oriented graph has a vertex x such that d++ (x) + d+ (x) ≥ 2d- (x); (3) ...