Stability of the entropy equation

In this paper we prove that the so--called entropy equation, i.e., $$H\left(x, y, z\right)=H\left(x+y, 0, z\right)+H\left(x, y, 0\right)$$ is stable in the sense of Hyers and Ulam on the positive cone of $\mathbb{R}^{3}$, assuming that the function $H$ is approximatively symmetric in each variable and approximatively homogeneous of degree $\alpha$, where $\alpha$ is an arbitrarily fixed real number.