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Patrolling

Robots move forever so that no point of a fence (a segment, a circle, or the edges of a graph) stays unvisited for long.

Problem statement and measuresexpand

\(k\) robots with maximum speeds \(v_1,\dots,v_k\) move forever along a fence: a segment, a circle, or the edges of a graph. The idle time is the largest time interval during which some point is not visited. The aim is to minimise it, or, equivalently, to find the longest fence that can be patrolled with idle time 1.

Measures: Idle time, worst idle time, or the longest fence patrollable with idle time 1 (stated per variant).

Closed fence, distinct speeds

verified
Idle time: unknown

On a circle, optimal schedules can beat the simple strategies by a factor \(21/20\).

Assumed (added on the last edge are outlined)

+Fencea circle (closed fence)+Faultsno faults

Not assumed

Speedsrobots may have different maximum speedsCommunicationface-to-face: robots exchange information only when they meetRandomnessdeterministic
Formal definitions of this setting
Fence: circle
A closed fence: a circle of length \(\ell\).
Faults: no faults
Every robot detects and reports the target.
Speeds: distinct speeds
Each robot \(i\) has its own maximum speed \(v_i\), known to all.
Communication: face-to-face
Face-to-face: two robots exchange information only when they are at the same point at the same time.
Randomness: deterministic
Trajectories are fixed functions of the information received.

What is known

Idle time · worst idle time, or the longest fence patrollable with idle time 1 (stated per variant)
unknown
—
lower boundnone known
introductory—
current best
\(\tfrac{21}{20}\times\text{simple}\)

Results, in order of publication

Idle time \(\tfrac{21}{20}\times\text{simple}\)

Open questions

None recorded for this variant.

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