4 The exponential function.
By Properties 5 and 6 of previous section, the logarithmic function log defined
by (3.8) is strictly increasing and continuous. On the other hand, by virtue of
Property 8, its range is the set of real numbers. Thus, log : (0,∞) → (−∞,∞)
is one to one and onto. The inverse function of log is denoted by exp so that
exp : (−∞,∞) → (0,∞) is strictly increasing, one to one and onto. Moreover,
exp(log(x)) = x for any x > 0 and log(exp(x)) = x for any real x. (4.1)
Observe that
exp(x1 +x2) = exp(x1) exp(x2) for any real numbers x1 and x2. (4.2)
Indeed, let y1 = exp(x1) and y2 = exp(x2) so that x1 = logy1 and x2 = logy2.
We have
exp(x1 +x2) = exp(logy1 + logy2) = exp(log(y1y2)) = y1y2 = exp(x1) exp(x2).The logarithmic function as a limit 4517
In particular, exp(x) exp(−x) = exp 0 = 1. This gives exp(−x)=1/exp(x).
Then
exp(x1 −x2) = exp (x1) exp(−x2) = exp(x1)
exp(x2)
. (4.3)