Department of Mathematics, Chungnam National University, 79 Daehangno, Yuseong-gu, Daejeon 305-764, South Korea
Copyright © 2010 Hark-Mahn Kim et al. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We investigate the general solution of the quadratic functional equation
, in the class of all functions between quasi--normed spaces, and then we prove the generalized Hyers-Ulam stability of the equation
by using direct method and fixed point method.