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arXiv 1407.6328cs.DS
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约束单调函数极大化与超模度

Constrained Monotone Function Maximization and the Supermodular Degree

Moran Feldman, Rani Izsak

摘要:

约束单调集函数的最大化问题有许多实际应用,并推广了许多组合问题。不幸的是,它通常是不可能的,以最大化一个单调集函数到一个可接受的近似比,即使受到简单的约束。一个高度研究的方法,以应付这种困难是限制集函数。对集合函数施加这样的限制的一个突出缺点是,对于偏离限制的集合函数,即使是轻微的偏离,也不意味着任何结果。Feige和Izsak研究的一种更灵活的方法是设计一种近似算法,其近似比取决于实例的复杂度,如通过某种复杂度度量所测量的。具体来说,他们引入了一种称为超模块度的复杂性度量,测量与子模块化的偏差,并设计了一种用于福利最大化问题的算法,其近似比取决于该度量。在这项工作中,我们给出了第一个(据我们所知)算法最大化任意单调集函数,受k-可扩展系统。例如,这类约束捕获k-拟阵的交集(注意,单个拟阵约束足以捕获福利最大化问题)。我们的近似比随着集合函数和k的复杂性而优雅地恶化。我们的工作可以被看作是推广了Fisher,Nemhauser和Wolsey的经典结果,最大化一个子模集函数的k-可扩展系统,和Feige和Izsak的结果的福利最大化问题。此外,当我们的算法被应用到每一个这些简单的情况下,它获得了相同的近似比的各自的原始工作。

英文摘要:

The problem of maximizing a constrained monotone set function has many practical applications and generalizes many combinatorial problems. Unfortunately, it is generally not possible to maximize a monotone set function up to an acceptable approximation ratio, even subject to simple constraints. One highly studied approach to cope with this hardness is to restrict the set function. An outstanding disadvantage of imposing such a restriction on the set function is that no result is implied for set functions deviating from the restriction, even slightly. A more flexible approach, studied by Feige and Izsak, is to design an approximation algorithm whose approximation ratio depends on the complexity of the instance, as measured by some complexity measure. Specifically, they introduced a complexity measure called supermodular degree, measuring deviation from submodularity, and designed an algorithm for the welfare maximization problem with an approximation ratio that depends on this measure. In this work, we give the first (to the best of our knowledge) algorithm for maximizing an arbitrary monotone set function, subject to a k-extendible system. This class of constraints captures, for example, the intersection of k-matroids (note that a single matroid constraint is sufficient to capture the welfare maximization problem). Our approximation ratio deteriorates gracefully with the complexity of the set function and k. Our work can be seen as generalizing both the classic result of Fisher, Nemhauser and Wolsey, for maximizing a submodular set function subject to a k-extendible system, and the result of Feige and Izsak for the welfare maximization problem. Moreover, when our algorithm is applied to each one of these simpler cases, it obtains the same approximation ratio as of the respective original work.

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