Lonely runner conjecture

Example of Lonely runner conjecture with 6 runners

In number theory, and especially the study of diophantine approximation, the lonely runner conjecture is a conjecture originally due to J. M. Wills in 1967. Applications of the conjecture are widespread in mathematics; they include view obstruction problems^[1] and calculating the chromatic number of distance graphs and circulant graphs.^[2] The conjecture was given its picturesque name by L. Goddyn in 1998.^[3]

The conjecture

Consider k runners on a circular track of unit length. At t = 0, all runners are at the same position and start to run; the runners' speeds are pairwise distinct. A runner is said to be lonely at time t if he is at a distance of at least 1/k from every other runner at time t. The lonely runner conjecture states that each runner is lonely at some time.

A convenient reformulation of the problem is to assume that the runners have integer speeds,^[4] not all divisible by the same prime; the runner to be lonely has zero speed. The conjecture then states that for any set D of k − 1 positive integers with gcd 1,

$\exists t\in \mathbb{R}\quad \forall d\in D\quad ||td|| \geq \frac{1}{k},$

where ||x|| denotes the distance of real number x to the nearest integer.

Known results

Unsolved problem in mathematics: Can the Lonely runner conjecture be proved for k≥8? (more unsolved problems in mathematics)

k	year proved	proved by	notes
1	-	-	trivial: t = 0; any t
2	-	-	trivial: t = 1 / (2 * (v1-v0))
3	-	-	Any proof for k>3 also proves k=3
4	1972	Betke and Wills;[5] Cusick[6]	-
5	1984	Cusick and Pomerance;[7] Bienia et al.[3]	-
6	2001	Bohman, Holzman, Kleitman;[8] Renault[9]	-
7	2008	Barajas and Serra[2]	-

Dubickas^[10] shows that for a sufficiently large number of runners for speeds $v_1 < v_2 < ... < v_k$ the lonely runner conjecture is true if $\frac{v_{i+1}}{v_i} \geq 1 + \frac{33\log(k)}{k}$.

Notes

1. T. W. Cusick (1973). "View-Obstruction problems". Aequationes Math. 9 (2–3): 165–170. doi:10.1007/BF01832623.
2. J. Barajas and O. Serra (2008). "The lonely runner with seven runners". The Electronic Journal of Combinatorics 15: R48.
3. W. Bienia; et al. (1998). "Flows, view obstructions, and the lonely runner problem". Journal of combinatorial theory series B 72: 1–9. doi:10.1006/jctb.1997.1770.
4. This reduction is proved, for example, in section 4 of the paper by Bohman, Holzman, Kleitman
5. Betke, U.; Wills, J. M. (1972). "Untere Schranken für zwei diophantische Approximations-Funktionen". Monatshefte für Mathematik 76 (3): 214. doi:10.1007/BF01322924.
6. T. W. Cusick (1974). "View-obstruction problems in n-dimensional geometry". Journal of Combinatorial Theory, Series A 16 (1): 1–11. doi:10.1016/0097-3165(74)90066-1.
7. Cusick, T.W.; Pomerance, Carl (1984). "View-obstruction problems, III". Journal of Number Theory 19 (2): 131–139. doi:10.1016/0022-314X(84)90097-0.
8. Bohman, T.; Holzman, R.; Kleitman, D. (2001), "Six lonely runners", Electronic Journal of Combinatorics 8 (2)
9. Renault, J. (2004). "View-obstruction: A shorter proof for 6 lonely runners". Discrete Mathematics 287: 93–101. doi:10.1016/j.disc.2004.06.008.
10. Dubickas, A. (2011). "The lonely runner problem for many runners". Glasnik Matematicki 46: 25–30. doi:10.3336/gm.46.1.05.

External links

• article in the Open Problem Garden