Collective fast delivery by energy-efficient agents

TL;DR

This paper studies energy-efficient strategies for multiple mobile agents delivering an item in a graph, balancing delivery time and energy consumption, with polynomial-time solutions and hardness results for various optimization criteria.

Contribution

It provides polynomial-time algorithms for minimizing delivery time, analyzes complexity for lexicographic and combined objectives, and introduces a 3-approximation algorithm for energy-efficient delivery.

Findings

Minimizing delivery time is polynomial-time solvable.

Lexicographic (T,E) minimization is polynomial for uniform velocities, NP-hard otherwise.

A 3-approximation algorithm exists for combined energy and time optimization.

Abstract

We consider k mobile agents initially located at distinct nodes of an undirected graph (on n nodes, with edge lengths) that have to deliver a single item from a given source node s to a given target node t. The agents can move along the edges of the graph, starting at time 0 with respect to the following: Each agent i has a weight w_i that defines the rate of energy consumption while travelling a distance in the graph, and a velocity v_i with which it can move. We are interested in schedules (operating the k agents) that result in a small delivery time T (time when the package arrives at t), and small total energy consumption E. Concretely, we ask for a schedule that: either (i) Minimizes T, (ii) Minimizes lexicographically (T,E) (prioritizing fast delivery), or (iii) Minimizes epsilon*T + (1-epsilon)*E, for a given epsilon, 0<epsilon<1. We show that (i) is solvable in polynomial time, and show that (ii) is polynomial-time solvable for uniform velocities and solvable in time O(n + k log k) for arbitrary velocities on paths, but in general is NP-hard even on planar graphs. As a corollary of our hardness result, (iii) is NP-hard, too. We show that there is a 3-approximation algorithm for (iii) using a single agent.