for X being Banach_Algebra
for z1, z2 being Element of X st z1,z2 are_commutative holds
( exp (z1 + z2) = (exp z1) * (exp z2) & exp (z2 + z1) = (exp z2) * (exp z1) & exp (z1 + z2) = exp (z2 + z1) & exp z1, exp z2 are_commutative )