@article{Cobzaş_2002, title={Phelps type duality results in best approximation}, volume={31}, url={https://ictp.acad.ro/jnaat/journal/article/view/2002-vol31-no1-art5}, DOI={10.33993/jnaat311-706}, abstractNote={<pre>The aim of the present paper is to show that many Phelps type</pre> <pre> duality result, relating the extension properties of various</pre> <pre> classes of functions (continuous, linear continuous, bounded</pre> <pre> bilinear, Hölder-Lipschitz) with the approximation properties</pre> <pre> of some annihilating spaces, can be derived in a unitary and</pre> <pre> simple way from a formula for the distance to the kernel of a</pre> <pre> linear operator, extending the well-known distance formula to</pre> <pre> hyperplanes in normed spaces. The case of spaces \(c_0\) and</pre> <pre> \(l^\infty\) is treated in details.</pre>}, number={1}, journal={Rev. Anal. Numér. Théor. Approx.}, author={Cobzaş, Ştefan}, year={2002}, month={Feb.}, pages={29–43} }