In mathematics, specifically the area of Diophantine approximation, the Davenport–Schmidt theorem tells us how well a certain kind of real number can be approximated by another kind. Specifically it tells us that we can get a good approximation to irrational numbers that are not quadratic by using either quadratic irrationals or simply rational numbers. It is named after Harold Davenport and Wolfgang M. Schmidt.
Statement Given a number α which is either rational or a quadratic irrational, we can find unique integers x, y, and z such that x, y, and z are not all zero, the first non-zero one among them is positive, they are relatively prime, and we have
x α 2 + y α + z = 0. {\displaystyle x\alpha ^{2}+y\alpha +z=0.}
If α is a quadratic irrational we can take x, y, and z to be the coefficients of its minimal polynomial. If α is rational we will have x = 0. With these integers uniquely determined for each such α we can define the height of α to be
H ( α ) = max { | x | , | y | , | z | } . {\displaystyle H(\alpha )=\max\{|x|,|y|,|z|\}.}
The theorem then says that for any real number ξ which is neither rational nor a quadratic irrational, we can find infinitely many real numbers α which are rational or quadratic irrationals and which satisfy
| ξ − α | < C H ( α ) − 3 max ( 1 , ξ 2 ) , {\displaystyle |\xi -\alpha |<CH(\alpha )^{-3}\max(1,\xi ^{2}),}
where C is any real number satisfying C > 160/9. While the theorem is related to Roth's theorem, its real use lies in the fact that it is effective, in the sense that the constant C can be worked out for any given ξ.
Notes
References Wolfgang M. Schmidt. Diophantine approximation. Lecture Notes in Mathematics 785. Springer. (1980 [1996 with minor corrections]) Wolfgang M. Schmidt.Diophantine approximations and Diophantine equations, Lecture Notes in Mathematics, Springer Verlag 2000
External links "Davenport-Schmidt theorem". PlanetMath.