Digraph analogues for the Nine Dragon Tree Conjecture

Abstract

The fractional arboricity of a digraph $D$, denoted by $\gamma(D)$, is defined as $\gamma(D)= \max_{H \subseteq D, |V(H)| >1} \frac {|A(H)|} {|V(H)|-1}$. Frank in [Covering branching, Acta Scientiarum Mathematicarum (Szeged) 41 (1979), 77-81] proved that a digraph $D$ decomposes into $k$ branchings, if and only if $\Delta^{-}(D) \leq k$ and $\gamma(D) \leq k$. In this paper, we study digraph analogues for the Nine Dragon Tree Conjecture. We conjecture that, for positive integers $k$ and $d$, if $D$ is a digraph with $\gamma(D) \leq k + \frac{d-k}{d+1}$ and $\Delta^{-}(D) \leq k+1$, then $D$ decomposes into $k + 1$ branchings $B_{1}, \ldots, B_{k}, B_{k+1}$ with $\Delta^{+}(B_{k+1}) \leq d$. This conjecture, if true, is a refinement of Frank's characterization. A series of acyclic bipartite digraphs is also presented to show the bound of $\gamma(D)$ given in the conjecture is best possible. We prove our conjecture for the cases $d \leq k$. As more evidence to support our conjecture, we prove that if $D$ is a digraph with the maximum average degree $mad(D)$ $\leq$ $2k + \frac{2(d-k)}{d+1}$ and $\Delta^{-}(D) \leq k+1$, then $D$ decomposes into $k + 1$ pseudo-branchings $C_{1}, \ldots, C_{k}, C_{k+1}$ with $\Delta^{+}(C_{k+1}) \leq d$.