Roth’s theorem – Wikipedia

In mathematics, Roth’s theorem is a fundamental result in diophantine approximation to algebraic numbers. It is of a qualitative type, stating that a given algebraic number α {\displaystyle \alpha } \alpha may not have too many rational number approximations, that are ‘very good’. Over half a century, the meaning of very good here was refined by a number of mathematicians, starting with Joseph Liouville in 1844 and continuing with work of Axel Thue (1909), Carl Ludwig Siegel (1921), Freeman Dyson (1947), and Klaus Roth (1955).

Source: Roth’s theorem – Wikipedia