and satisfying the inequality $$ d (f (x\diamond y), f (x)* f (y))\leq\varepsilon (x, y)\quad (x,
y\in X), $$ the problem of stability in the sense of Hyers-Ulam is to construct a solution g of
the functional equation $$ g (x\diamond y)= g (x)* g (y)\quad (x, y\in X) $$ and to obtain
estimates for the pointwise distance between g and f. Applying the so-called direct method,
the stability problem for more general functional equations is also investigated.