The car-sharing problem, proposed by Luo, Erlebach and Xu in 2018, mainly focuses on an online model in which there are two locations: 0 and 1, and $k$ total cars. Each request which specifies its pick-up time and pick-up location (among 0 and 1, and the other is the drop-off location) is released in each stage a fixed amount of time before its specified start (i.e. pick-up) time. The time between the booking (i.e. released) time and the start time is enough to move empty cars between 0 and 1 for relocation if they are not used in that stage. The model, called $k$S2L-F, assumes that requests in each stage arrive sequentially regardless of the same booking time and the decision (accept or reject) must be made immediately. The goal is to accept as many requests as possible. In spite of only two locations, the analysis does not seem easy and the (tight) competitive ratio (CR) is only known to be 2.0 for $k=2$ and 1.5 for a restricted $k$, i.e., a multiple of three. In this paper, we remove all the holes of unknown CR; namely we prove that the CR is $\frac{2k}{k + \lfloor k/3 \rfloor}$ for all $k\geq 2$. Furthermore, if the algorithm can delay its decision until all requests have come in each stage, the CR is improved to roughly 4/3. We can take this advantage even more, precisely we can achieve a CR of $\frac{2+R}{3}$ if the number of requests in each stage is at most $Rk$, $1 \le R \le 2$, where we do not have to know the value of $R$ in advance. Finally we demonstrate that randomization also helps to get (slightly) better CR's.
翻译:由Luo、Erlebach和Xu在2018年提议的汽车共享问题 { 主要是侧重于一个在线模式,其中空车在0和1个位置之间,总汽车费用为$1。每个指定其接车时间和接车地点的请求(0和1之间,另一个是降车地点)在每个阶段都发布固定时间,在指定开始(即接车)时间之前。订车(即释放)时间和起始时间之间的时间足够在零和1之间移动,如果在该阶段不使用,则用于搬迁的空车在0和1之间。该模型称为$S2L-F,假定每个阶段的请求都是按顺序发出的,而不管相同的订车时间和接车地点(在0和1之间)和接车地点(接受或拒绝地点),目标是尽可能接受许多请求。尽管只有两个地点,但分析似乎不简单,而(未来)竞争比率(CR)只有2.0美元和1.5的优势,如果在这个阶段里我们只能拿到了每笔美元要求,也就是说每张。