“…In Corollaries 4.3 and 4.4 we give explicit formulas of solutions of (1.2) and (1.8) in terms of irreducible representations of G. Theses formulas generalize Euler's formula cos(x) = e ix +e −ix 2 on G = R. In the last section we study stability [48] and Baker's superstability (see [5] and [6]) of the functional equations (1.1), (1.2), (1.3), (1.4), (1.5), (1.6) and (1.7). For more information concerning the stability problem we refer to [3], [5], [6], [11], [12], [22], [40], [41], [42], [43], [44], [45], [46], [47] and [48]. The results of the last sections generalize the ones obtained in [12] and [21].…”