English
A technical version: from Legendre's approximation, there exists n with a rational equal to ξ.convergent n when certain coprimality and inequality conditions hold.
Русский
Техническая версия Лежандра: при выполнении условий на пары чисел и копротности существует n, для которого q = ξ.convergent n.
LaTeX
$$$$\\exists n\\; (u/v)=\\xi.convergent n \\quad\\text{under suitable Legendre conditions.}$$$$
Lean4
/-- Define the technical condition to be used as assumption in the inductive proof. -/
def Ass (ξ : ℝ) (u v : ℤ) : Prop :=
IsCoprime u v ∧
(v = 1 → (-(1 / 2) : ℝ) < ξ - u) ∧
|ξ - u / v| <
((v : ℝ) * (2 * v - 1))⁻¹
-- ### Auxiliary lemmas
-- This saves a few lines below, as it is frequently needed.