# Gathering with extremely restricted visibility

## TL;DR

This paper investigates the feasibility of gathering mobile processes with extremely limited visibility, where each process can only see its closest neighbor, and explores solutions and limitations in this constrained setting.

## Contribution

It introduces a new model with minimal visibility, analyzes the solvability of gathering and convergence, and proposes conditions under which these problems can be solved.

## Key findings

- Gathering is possible for up to 5 processes without additional assumptions.
- Adding an order-based rule for selecting neighbors enables gathering for any number of processes.
- The convergence problem can tolerate up to one crash failure.

## Abstract

We consider the classical problem of making mobile processes gather or converge at a same position (as performed by swarms of animals in Nature). Existing works assume that each process can see all other processes, or all processes within a certain radius. In this paper, we introduce a new model with an extremely restricted visibility: each process can only see one other process (its closest neighbor). Our goal is to see if (and to what extent) the gathering and convergence problems can be solved in this setting. We first show that, surprisingly, the problem can be solved for a small number of processes (at most 5), but not beyond. This is due to indeterminacy in the case where there are several closest neighbors for a same process. By removing this indeterminacy with an additional hypothesis (choosing the closest neighbor according to an order on the positions of processes), we then show that the problem can be solved for any number of processes. We also show that up to one crash failure can be tolerated for the convergence problem.