整数最小值的凸最小化在 $\widetilde O(n^4)$ 时间内
Convex Minimization with Integer Minima in $\widetilde O(n^4)$ Time
AI总结:
针对整数最小值的凸最小化问题,提出结合快速LLL算法与体积中心切割平面法的新算法,将运行时间从$\widetildeΩ(n^8)$降至$O(n^4\log n)$,并改进子模函数最小化的复杂度。
摘要:
给定一个在 $\mathbb{R}^n$ 上具有整数最小化器的凸函数 $f$,我们展示了如何使用 $O(n^2 \log n)$ 次分离预言机调用和 $O(n^4 \log n)$ 时间找到 $f$ 的精确最小值。此前该问题的最佳多项式时间算法由 [Jiang, SODA 2021, JACM 2022] 给出,其预言机复杂度为 $O(n^2\log\log n/\log n)$。然而,Jiang 算法的总运行时间至少为 $\widetildeΩ(n^8)$,这是因为其使用了昂贵的子程序,如 Lenstra-Lenstra-Lovász (LLL) 算法 [Lenstra, Lenstra, Lovász, Math. Ann. 1982] 和基于随机游走的切割平面方法 [Bertsimas, Vempala, JACM 2004]。我们的显著加速是通过非平凡地结合 [Neumaier, Stehlé, ISSAC 2016] 提出的 LLL 算法的更快版本(该版本提供类似的保证)、[Vaidya, FOCS 1989] 的体积中心切割平面方法 (CPM) 以及 [Jiang, Lee, Song, Wong, STOC 2020] 给出的其快速实现而获得的。\n 对于子模函数最小化 (SFM) 这一特殊情况,我们的结果意味着该问题的一个强多项式时间算法,使用 $O(n^3 \log n)$ 次评估预言机调用和 $O(n^4 \log n)$ 次额外算术运算。我们更一般算法的预言机复杂度和算术运算次数都优于此前针对该特定问题的最佳已知运行时间算法,这些算法由 [Lee, Sidford, Wong, FOCS 2015] 和 [Dadush, Végh, Zambelli, SODA 2018, MOR 2021] 给出。
英文摘要:
Given a convex function $f$ on $\mathbb{R}^n$ with an integer minimizer, we show how to find an exact minimizer of $f$ using $O(n^2 \log n)$ calls to a separation oracle and $O(n^4 \log n)$ time. The previous best polynomial time algorithm for this problem given in [Jiang, SODA 2021, JACM 2022] achieves $O(n^2\log\log n/\log n)$ oracle complexity. However, the overall runtime of Jiang's algorithm is at least $\widetildeΩ(n^8)$, due to expensive sub-routines such as the Lenstra-Lenstra-Lovász (LLL) algorithm [Lenstra, Lenstra, Lovász, Math. Ann. 1982] and random walk based cutting plane method [Bertsimas, Vempala, JACM 2004]. Our significant speedup is obtained by a nontrivial combination of a faster version of the LLL algorithm due to [Neumaier, Stehlé, ISSAC 2016] that gives similar guarantees, the volumetric center cutting plane method (CPM) by [Vaidya, FOCS 1989] and its fast implementation given in [Jiang, Lee, Song, Wong, STOC 2020]. For the special case of submodular function minimization (SFM), our result implies a strongly polynomial time algorithm for this problem using $O(n^3 \log n)$ calls to an evaluation oracle and $O(n^4 \log n)$ additional arithmetic operations. Both the oracle complexity and the number of arithmetic operations of our more general algorithm are better than the previous best-known runtime algorithms for this specific problem given in [Lee, Sidford, Wong, FOCS 2015] and [Dadush, Végh, Zambelli, SODA 2018, MOR 2021].
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