Geodesic
Proximinality and co-proximinality in metric linear spaces
Annales Universitatis Mariae Curie-Skłodowska. Mathematica , Tome 69 (2015) no. 1.

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As a counterpart to best approximation, the concept of best coapproximation was introduced in normed linear spaces by C. Franchetti and M. Furi in 1972. Subsequently, this study was taken up by many researchers. In this paper, we discuss some results on the existence and uniqueness of best approximation and best coapproximation when the underlying spaces are metric linear spaces.
Keywords: Best approximation, best coapproximation, proximinal set, co-proximinal set, Chebyshev set, co-Chebyshev set 
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Narang, T. W.; Gupta, Sahil. Proximinality and co-proximinality in metric linear spaces. Annales Universitatis Mariae Curie-Skłodowska. Mathematica , Tome 69 (2015) no. 1. http://geodesic.mathdoc.fr/item/AUM_2015_69_1_a5/

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