Geodesic
Non-Extendability of Bounded Continuous Functions
Canadian journal of mathematics, Tome 32 (1980) no. 4, pp. 867-879

Voir la notice de l'article provenant de la source Cambridge University Press

If X is a dense subspace of Y, much is known about the question of when every bounded continuous real-valued function on X extends to a continuous function on Y. Indeed, this is one of the central topics of [5]. In this paper we are interested in the opposite question: When are there continuous bounded real-valued functions on X which extend to no point of Y – X? (Of course, we cannot hope that every function on X fails to extend since the restrictions to X of continuous functions on Y extend to Y.) In this paper, we show that if Y is a compact metric space and if X is a dense subset of Y, then X admits a bounded continuous function which extends to no point of Y – X if and only if X is completely metrizable. We also show that for certain spaces Y and dense subsets X, the set of bounded functions on X which extend to a point of Y – X form a first category subset of C*(X). 
Levy, Ronnie. Non-Extendability of Bounded Continuous Functions. Canadian journal of mathematics, Tome 32 (1980) no. 4, pp. 867-879. doi: 10.4153/CJM-1980-065-9
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