# Packing T-joins

**Conjecture**There exists a fixed constant (probably suffices) so that every graft with minimum -cut size at least contains a -join packing of size at least .

**Definitions:** A *graft* consists of a graph together with a distinguished set of even cardinality. A -*cut* is an edge-cut of with the property that is odd. A -*join* is a set with the property that a vertex of has odd degree if and only if it is in . A -join *packing* is a set of pairwise disjoint T-joins.

It is an easy fact that every -join and every -cut intersect in an odd number of elements. It follows easily from this that the maximum size of a -join packing is always less than or equal to the minimum size of a -cut. There is a simple example of a graft with with minimum -cut size which contains only disjoint T-joins. The above conjecture asserts that this is essentially the worst case. DeVos and Seymour [DS] have obtained a partial result toward the above conjecture, proving that every graft with minimum -cut size contains a -join packing of size at least the floor of .

**Definition:** We say that a graft is an -*graph* if is -regular, , and every -cut of G has size at least .

**Conjecture (Rizzi)**If is an -graph, then contains a -join packing of size at least .

In an -graph, every perfect matching is a -join, so the above conjecture is true with room to spare for -graphs which are -edge-colorable. Indeed, Seymour had earlier conjectured that every -graph contains disjoint perfect matchings. This however was disproved by Rizzi [R] who constructed for every an -graph in which every two perfect matchings intersect. Rizzi suggested the above problem as a possible fix for Seymour's conjecture. DeVos and Seymour have proved that every -graph has a -join packing of size at least the floor of .

**Definition:** Let be a graph and let be the set of vertices of of odd degree. A -join of is defined to be a *postman set*.

Note that when is the set of vertices of odd degree, a cocycle of is a -cut if and only if it has odd size. Rizzi has shown that the following conjecture is equivalent to the above conjecture in the special case when is odd.

**Conjecture (The packing postman sets conjecture (Rizzi))**If every odd edge-cut of has size then the edges of may be partitioned into postman sets.

The Petersen graph (or more generally any non -edge-colorable -graph) shows that the above conjecture would be false with the weaker assumption that every odd edge-cut has size . The following conjecture asserts that odd edge-cut size is enough (for the same conclusion) if we assume in addition that G has no Petersen minor.

**Conjecture (Conforti, Johnson)**If has no Petersen minor and every odd edge-cut of has size then the edges of may be partitioned into postman sets.

Gerard Cornuejols [C] has kindly offered $5000 for a solution to this conjecture. However, it will be tough to find a quick proof since this conjecture does imply the 4-color theorem. Robertson, Seymour, Sanders, and Thomas [RSST] have proved the above conjecture for cubic graphs. Conforti and Johnson [CJ] proved it under the added hypothesis that G has no 4-wheel minor.

## Bibliography

[CJ] M. Conforti and E.L. Johnson, Two min-max theorems for graphs noncontractible to a four wheel, preprint.

[C] G. Cornuejols, Combinatorial Optimization, packing and covering, SIAM, Philadelphia (2001).

[R] R. Rizzi, Indecomposable r-Graphs and Some Other Counterexamples, J. Graph Theory 32 (1999) 1-15. MathSciNet

[RSST] N. Robertson, D.P. Sanders, P.D. Seymour, and R. Thomas, A New Proof of the Four-Color Theorem, Electron. Res. Announc., Am. Math. Soc. 02, no 1 (1996) 17-25.

[S] P.D. Seymour, Some Unsolved Problems on One-Factorizations of Graphs, in Graph Theory and Related Topics, edited by J.A. Bondy and U.S.R. Murty, Academic Press, New York 1979) 367-368.

* indicates original appearance(s) of problem.