## Abstract

A closed set of a Euclidean space is said to be Chebyshev if every point in the space has one and only one closest point in the set. Although the situation is not settled in infinite-dimensional Hilbert spaces, in 1932 Bunt showed that in Euclidean spaces a closed set is Chebyshev if and only if the set is convex. In this paper, from the more general perspective of Bregman distances, we show that if every point in the space has a unique nearest point in a closed set, then the set is convex. We provide two approaches: one is by nonsmooth analysis; the other by maximal monotone operator theory. Subdifferentiability properties of Bregman nearest distance functions are also given.

Original language | English |
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Pages (from-to) | 3-25 |

Number of pages | 23 |

Journal | Journal of Approximation Theory |

Volume | 159 |

Issue number | 1 |

DOIs | |

Publication status | Published - Jul 2009 |

## Scopus Subject Areas

- Analysis
- Numerical Analysis
- Mathematics(all)
- Applied Mathematics

## User-Defined Keywords

- Bregman distance
- Bregman projection
- Chebyshev set with respect to a Bregman distance
- Legendre function
- Maximal monotone operator
- Nearest point
- Subdifferential operators