Equitable factorizations of highly edge-connected graphs: complete characterizations

In this paper, we show that every highly edge-connected graph $G$, under a necessary and sufficient degree condition, can be edge-decomposed into $k$ factors $G_1,\ldots, G_k$ such that for each vertex $v\in V(G_i)$ with $1\le i\le k$, $|d_{G_i}(v)-d_G(v)/k|<1$. This characterization covers graphs having at least $k-1$ vertices with degree not divisible by $k$. In addition, we investigate almost equitable factorizations in arbitrary edge-connected graphs. Next, we establish a simpler criterion for the existence of factorizations $G_1,\ldots, G_k$ satisfying $d_{G_i}(v)\ge \lfloor d_G(v)/k\rfloor$ for all vertices $v$ (reps. $d_{G_i}(v)\le \lceil d_G(v)/k\rceil$). As an application, we come up with a criterion to determine whether a highly edge-connected graph with $\delta(G)\ge \delta_1+\cdots+ \delta_m$ (resp. $\Delta(G)\le \Delta_1+\cdots+ \Delta_m$) can be edge-decomposed into factors $G_1,\ldots, G_m$ satisfying $\delta(G_i)\ge \delta_i$ (resp. $\Delta(G_i)\le \Delta_i$) for all $i$ with $1\le i \le m$, provided that $\delta_1+\cdots+ \delta_m$ is divisible by an odd number $p$ and $\delta_i\ge p-1\ge 2$ (resp. $\Delta_1+\cdots+ \Delta_m$ is divisible by $p$ and $\Delta_i\ge p-1\ge 2$). For graphs of even order, we replace an odd-edge-connectivity condition. In particular, for the special case $m=2$, we refine the needed odd-edge-connectivity further by giving a sufficient odd-edge-connectivity condition for a graph $G$ to have a partial parity factor $F$ such that for each vertex $v$ with a given parity constraint, $| d_{F}(v)-\varepsilon d_G(v)|< 2$, and for all other vertices $v$, $| d_{F}(v)-\varepsilon d_G(v)|\le 1$, where $\varepsilon$ is a real number and $0< \varepsilon < 1$. Finally we introduce another application on the existence of almost even factorizations of odd-edge-connected graphs.