On Factors with Prescribed Degrees in Bipartite Graphs

We establish a new criterion for a bigraph to have a subgraph with prescribed degree conditions. We show that the bigraph $G[X,Y]$ has a spanning subgraph $F$ such that $g(x)\leq deg_F(x) \leq f(x)$ for $x\in X$ and $deg_F(y) \leq f(y)$ for $y\in Y$ if and only if $\sum\nolimits_{b\in B} f(b)\geq \sum\nolimits_{a\in A} \max \big\{0, g(a) - deg_{G-B}(a)\big\}$ for $A\subseteq X, B\subseteq Y$. Using Folkman-Fulkerson's Theorem, Cymer and Kano found a different criterion for the existence of such a subgraph (Graphs Combin. 32 (2016), 2315--2322). Our proof is self-contained and relies on alternating path technique. As an application, we prove the following extension of Hall's theorem. A bigraph $G[X,Y]$ in which each edge has multiplcity at least $m$ has a subgraph $F$ with $g(x)\leq deg_F(x)\leq f(x)\leq deg(x)$ for $x\in X$, $deg_F(y)\leq m$ for $y\in Y$ if and only if $\sum_{y\in N_G(S)}f(y)\geq \sum_{x\in S}g(x)$ for $S\subseteq X$.