A unified half-integral Erdős-Pósa theorem for cycles in graphs labelled by multiple abelian groups
arxiv(2021)
摘要
Erdős and Pósa proved in 1965 that there is a duality between the
maximum size of a packing of cycles and the minimum size of a vertex set
hitting all cycles. Such a duality does not hold if we restrict to odd cycles.
However, in 1999, Reed proved an analogue for odd cycles by relaxing packing to
half-integral packing. We prove a far-reaching generalisation of the theorem of
Reed; if the edges of a graph are labelled by finitely many abelian groups,
then there is a duality between the maximum size of a half-integral packing of
cycles whose values avoid a fixed finite set for each abelian group and the
minimum size of a vertex set hitting all such cycles.
A multitude of natural properties of cycles can be encoded in this setting,
for example cycles of length at least ℓ, cycles of length p modulo q,
cycles intersecting a prescribed set of vertices at least t times, and cycles
contained in given ℤ_2-homology classes in a graph embedded on a
fixed surface. Our main result allows us to prove a duality theorem for cycles
satisfying a fixed set of finitely many such properties.
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关键词
graphs,cycles,theorem,half-integral
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