Rational approximation of famous numbers
Introduction
The Dirichlet's theorem shows that, for each irrational number , the inequality
has infinitely many solutions. On the other hand, sometimes it is useful to know that
cannot be approximated by rationals too well, or, more precisely, that
is not a Liouvillian number, i.e., that for some power
, the inequality
holds for all sufficiently large denominators
. So, how does one show that a number is not Liouvillian? The answer is given by the following
Main theorem
Suppose that there exist ,
and a sequence of rational numbers
such that for all sufficiently large
,
and
. Then, for every
, the inequality
has only finitely many solutions.
The exact formulation of the main theorem in this article is fitted to the Beukers proof of the non-Liouvillian character of but the general spirit of all such theorems is the same: roughly speaking, they tell you that in order to show that
cannot be approximated by rationals too well, one needs to find plenty of good but not too good rational approximations of
.
Proof of the Main Theorem
Choose the least such that
, i.e., that
. Note that for such choice of
, we have
. There are two possible cases:
Case 1: . Then
if
is large enough.
Case 2: . Then
if
is large enough (recall that
).
Applications
- Beuker's proof that pi is not Liouvillian
- Proof that e is not Liouvillian
- Proof that ln 2 is not Liouvillian