Article

Mappings approximately preserving orthogonality in normed spaces

Nonlinear Analysis (Impact Factor: 1.64). 12/2010; 73(12):3821-3831. DOI: 10.1016/j.na.2010.08.007

ABSTRACT We answer many open questions regarding approximately orthogonality preserving mappings (in Birkhoff–James sense) in normed spaces. In particular, we show that every approximately orthogonality preserving linear mapping (in Chmieliński sense) is necessarily a scalar multiple of an ε-isometry. Thus, whenever ε-isometries are close to isometries we obtain stability. An example is given showing that approximately orthogonality preserving mappings are in general far from scalar multiples of isometries, that is, stability does not hold.

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