Collaborative exploration
\(k\) agents start together at the root of an unknown graph and must visit all its nodes; how much faster than one agent can a team be?
Problem statement and measuresexpand
A team of \(k\) distinguishable agents starts at a node \(s\) (the root) of an unknown connected graph with \(n\) nodes; \(D\) is the largest distance from \(s\). In synchronous rounds every agent traverses at most one edge. When an agent reaches a node it sees the incident edges. The team must visit every node (some papers also require all agents to return to \(s\); the bounds coincide asymptotically).
Time is the number of rounds (in the asynchronous model: the total number of moves). The offline optimum is \(\Theta(n/k + D)\), so an algorithm that explores every tree in \(\mathrm{CR}(k)\cdot(n/k+D)\) rounds is called \(\mathrm{CR}(k)\)-competitive; a single depth-first search is \(O(k)\)-competitive. Memory is bits per agent; most of this literature lets it be unbounded.
Unless a variant says otherwise, nodes are labelled (an agent recognises a node it has seen), which is the standard assumption of collective tree exploration; on trees it does not matter for lower bounds Disser et al. 2020. With anonymous nodes and \(k = n\), the problem is solved by rooted dispersion.
Measures: Time, rounds until every node is visited (asynchronous: total moves); Memory, bits per agent.
Trees, no communication
verifiedWithout any communication, \(k\) agents are no faster than one.
Assumed (added on the last edge are outlined)
Not assumed
Formal definitions of this setting
- Graph dynamics: static graph
- \(G_t=G\) for all \(t\): the edge set never changes.
- Edge direction: undirected
- \(G=(V,E)\) with \(E\subseteq\binom V2\); every edge can be traversed in both directions.
- Node identifiers: labelled nodes
- Each node \(v\) carries a distinct identifier that agents at \(v\) can read; hence an agent can recognise a node it has visited.
- Randomness: deterministic
- The next state and move of an agent are a function of its current state and what it observes.
- Communication: no communication
- Agents exchange no messages; an agent at \(v\) observes only the number of agents at \(v\) (and, where stated, whether that number changed).
- Node memory: no whiteboards
- Nodes have no storage: no information can be left at a node except by an agent staying there.
- Timing: synchronous
- Time proceeds in rounds \(t=1,2,\dots\); in every round every agent communicates, computes and moves (at most one edge) simultaneously.
- Team size: any \(k\)
- Any number \(k\ge 2\) of agents.
- Topology: tree
- \(G\) is connected and acyclic: \(|E|=n-1\).
What is known
- each agent knows only what it explored itself
- some trees
Results, in order of publication
The baseline that the lower bound matches: one agent explores the whole tree by depth-first search, in \(2(n-1)\) moves, while the others wait; this is within a factor \(O(k)\) of the optimum \(\Theta(n/k+D)\) Fraigniaud et al. 2006.
Open questions
None recorded for this variant.
What carries over from neighbouring settings
Read off the order of assumptions, not checked case by case: results may carry extra assumptions (shown on their pages), and asynchronous time is measured in epochs.
Algorithms from weaker settings work here
No catalogued weaker setting.
Lower bounds from stronger settings hold here
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