Using the fixed point theorem [12, Theorem 1] in (2,?)-Banach spaces, we
prove the generalized hyperstability results of the bi-Jensen functional
equation 4f(x + z/2; y + w/2) = f (x,y) + f (x,w) + f (z,y) + f
(y,w). Our main results state that, under some weak natural assumptions,
functions satisfying the equation approximately (in some sense) must be
actually solutions to it. The method we use here can be applied to various
similar equations in many variables.
On the hyperstability of Jensen functional equation in 2-Banach spaces
