Proof that e is Irrational

The number e = 2.71828.. can be shown to be irrational by a very 
simple argument based on the power series expansion of the exponential
function, which gives

    1/e  =  1/0! - 1/1! + 1/2! - 1/3! + 1/4! - ...

If P(k) is the kth partial sum, we see that P(k) - P(k-1) = +-1/k!,
and so k((k-1)!)P(k-1) - k!P(k) = +-1.  It follows that placing 
each pair of consecutive partial sums on a common basis, we have 
the relations
                2/  6  <   1/e  <      3/  6
                8/ 24  <   1/e  <      9/ 24
               44/120  <   1/e  <     45/120
              264/720  <   1/e  <    265/720

and so on, where each pair of bounding numerators differs by 1, and
the denominators are m!.  The first of these relations proves that 
if 1/e is rational its denominator cannot be a divisor of 6, because
then it could be written n/6 for some integer n, and there is no 
such integer greater than 2 and less than 3.

Similarly the next relation proves that the denominator of 1/e cannot
be a divisor of 24, and the next proves that it cannot be a divisor 
of 120, and so on.  Continuing in this way, it's clear that the 
denominator of 1/e cannot be a divisor of any m! for m=2,3,4,...and 
so on to infinity.  But every integer k is a divisor of m! for all 
m >= k, so 1/e (and therefore e) cannot be a rational number.