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Concept
Dirichlet's approximation theorem
Summary
In number theory, Dirichlet's theorem on Diophantine approximation, also called Dirichlet's approximation theorem, states that for any real numbers and , with , there exist integers and such that and
Here represents the integer part of .
This is a fundamental result in Diophantine approximation, showing that any real number has a sequence of good rational approximations: in fact an immediate consequence is that for a given irrational α, the inequality
is satisfied by infinitely many integers p and q. This shows that any irrational number has irrationality measure at least 2. This corollary also shows that the Thue–Siegel–Roth theorem, a result in the other direction, provides essentially the tightest possible bound, in the sense that the bound on rational approximation of algebraic numbers cannot be improved by increasing the exponent beyond 2. The Thue–Siegel–Roth theorem uses advanced techniques of number theory, but many simpler numbers such as the golden ratio can be much more easily verified to be inapproximable beyond exponent 2. This exponent is referred to as the irrationality measure.
The simultaneous version of the Dirichlet's approximation theorem states that given real numbers and a natural number then there are integers such that
This theorem is a consequence of the pigeonhole principle. Peter Gustav Lejeune Dirichlet who proved the result used the same principle in other contexts (for example, the Pell equation) and by naming the principle (in German) popularized its use, though its status in textbook terms comes later. The method extends to simultaneous approximation.
Proof outline: Let be an irrational number and be an integer. For every we can write such that is an integer and .
One can divide the interval into smaller intervals of measure . Now, we have numbers and intervals. Therefore, by the pigeonhole principle, at least two of them are in the same interval. We can call those such that . Now:
Dividing both sides by will result in:
And we proved the theorem.
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