List orientation number of graphs

The orientation number of a graph G , denoted by o ( G ) , is the minimum integer k such that there exists an orientation D with d D + ( v ) ≤ k for every v ∈ V ( G ) , where d D + ( v ) is the out-degree of v in the directed graph G with respect to the orientation D . In this paper, we define its list-analog as the list-orientation number o ℓ ( G ) , and conjecture that o ℓ ( G ) ≤ o ( G ) + 2 for every graph G . We give partial solutions and tight constructions for this conjecture for some fundamental classes of graphs, such as bipartite graphs, graphs with o ( G ) ≤ 2 , and planar graphs. In addition, we extend some of the results from unsigned graphs to signed ones.