Editing Exponential function (section)

===Functional equation===
''The exponential satisfies the functional equation:''
<math display=block>\exp(x+y)= \exp(x)\cdot \exp(y).</math>
This results from the uniqueness and the fact that the function
<math> f(x)=\exp(x+y)/\exp(y)</math> satisfies the above definition. 

It can be proved that a function that satisfies this functional equation has the form {{tmath|x \mapsto \exp(cx)}} if it is either [[continuous function|continuous]] or [[monotonic function|monotonic]]. It is thus [[differentiable function|differentiable]], and equals the exponential function if its derivative at {{math|0}} is {{math|1}}.