Stability of the Drygas Functional Equation on Restricted Domain

Brzdȩk J., Chudziak J., Páles Zs.: A fixed point approach to stability of functional equations. Nonlinear Anal. 74, 6728–6732 (2011) Czerwik S.: On the stability of the quadratic mapping in normed spaces. Abh. Math. Sem. Univ. Hamburg 62, 59–64 (1992) Drygas H.: Quasi-inner products and their applications. In: Gupta, A.K. (ed.) Advances in Multivariate Statistical Analysis, pp. 13–30. Reidel Publishing Company, Boston (1987) Ebanks B.R., Kannappan Pl., Sahoo P.K.: A common generalization of functional equations characterizing normed and quasi-inner-product spaces. Can. Math. Bull. 35, 321–327 (1992) Forti G.L., Sikorska J.: Variations on the Drygas equations and its stability. Nonlinear Anal. 74, 343–350 (2011) Jung S.-M., Sahoo P.K.: Stability of a functional equation of Drygas. Aequ. Math. 64, 263–273 (2002) Sikorska J.: On a direct method for proving the Hyers–Ulam stability of functional equations. J. Math. Anal. Appl. 372, 99–109 (2010) Smajdor W.: On set-valued solutions of a functional equation of Drygas. Aequ. Math. 77, 89–97 (2009) Yang D.: Remarks on the stability of Drygas’ equation and the Pexider-quadratic equation. Aequ. Math. 68, 108–116 (2004)