proof of growth of exponential function
In this proof, we first restrict to when and are integersand only later lift this restricton.
Let be an integer, let be real, and let be aninteger.
Consider the following inequality
If , then we have
Define to be the greater of and ; when , we have
Rewrite as follows when :
By the inequality established above, each term in the product will bebounded by , hence
Since , it is also the case that , hence we havethe inequality
Combining the last two inequalities yields the following:
From this, it follows that when and are integers.
Now we lift the restriction that be an integer. Since the powerfunction is increasing, , so we have for real valuesof as well.
To lift the restriction on , let us write where is an integer and . Then we have
If , then . Since . Hence, for all real , we have
From this inequality, itfollows that for real values of as well.