Generalization of Menger's Edge Theorem to Four Vertices
Menger's Edge Theorem asserts that there exist $k$ pairwise edge-disjoint paths between two vertices in an undirected graph if and only if a deletion of any $k-1$ or less edges does not disconnect these two vertices. Alternatively, there exist $k$ pairwise summand-disjoint formal sums of edges with coefficients in $\mathbb{F}_2$, each one of which is mapped by the boundary map to the sum of vertices $A$ and $B$, if and only if after a deletion of any $k-1$ or less edges there still exist a formal sum of edges with coefficients in $\mathbb{F}_2$ which is mapped by the boundary map to $A+B$. We extend this result to four vertices $A,B,C,D$. We prove that in an undirected graph, in which all the vertices different from $A,B,C,D$ have even degrees, the following two statements are equivalent: There exist $k$ pairwise summand-disjoint formal sums of edges with coefficients in $\mathbb{F}_2$, each one of which is mapped by the boundary map to $A+B+C+D$; After a deletion of any $k-1$ or less edges there still exists a formal sum of edges with coefficients in $\mathbb{F}_2$ which is mapped by the boundary map to $A+B+C+D$. Equivalently, if after a deletion of any $k-1$ or less edges, the four vertices $A,B,C,D$ can be split into two pairs of vertices, and the two vertices in each pair then can be connected by a path so that these two paths are edge-disjoint, then the four vertices $A,B,C,D$ can be split $k$ times into two pairs of vertices and the two vertices in each one of these $2k$ pairs can then be connected by a path in such a way that all these $2k$ paths are pairwise edge-disjoint.