From this inequality it is easy to see that
−ε < − ε
1 +εexp(−a) < exp(x)−exp(a) < ε.
We have proved that for any ε > 0 and any real number a there exists δ =
log(1 +εexp(−a)) such that if |x−a| < δ then |exp(x)−exp(a)| < ε, that is,
limx→a exp(x) = exp(a).
In view of Property 8, the equation logx = y has unique solution on (0,∞)
for any real number y. The solution of the equation logx = 1 is denoted by e.
This is the base of natural logarithms.