The authors establish the general solution of the functional equation
deriving from quadratic and additive functions. They also prove the Hyers-Ulam-Rassias stability for the above equation in quasi Banach spaces.
The authors missed citing the paper by J. Tabor [Ann. Pol. Math. 83, No. 3, 243–255 (2004; Zbl 1101.39021)], which contains the first result about the stability of quasi-Banach spaces.