Mapping in normed linear spaces and characterization of orthogonality problem of best approximations in 2-norm.
Multiplicative characterization of Hilbert spaces and other interesting classes of Banach spaces.
For a Banach space X, we show how the existence of a norm-one element u in X and a norm-one continuous bilinear mapping f: X x X --> X satisfying f(x,u) = f(u,x) = x for all x in X, together with some more intrinsic conditions, can be utilized to characterize X as a member of some relevant subclass of the class of Banach spaces.