On Seymour's and Sullivan's Second Neighbourhood Conjectures

Jiangdong Ai, Stefanie Gerke, Gregory Gutin +3

For a vertex $x$ of a digraph, $d^{+}(x)$ ($d^{-}(x)$, resp.) is the number of vertices at distance 1 from (to, resp.) $x$ and $d^{++}(x)$ is the number of vertices at distance 2 from…