Demostrar que si x,y,ε ∈ ℝ tales que 0 < ε ≤ 1, |x| < ε y |y| < ε, entonces |x*y| < ε.
Para ello, completar la siguiente teoría de Lean:
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import data.real.basic tactic example : ∀ {x y ε : ℝ}, 0 < ε → ε ≤ 1 → |x| < ε → |y| < ε → |x * y| < ε := sorry |
Soluciones con Lean
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import data.real.basic tactic -- 1ª demostración -- =============== example : ∀ {x y ε : ℝ}, 0 < ε → ε ≤ 1 → |x| < ε → |y| < ε → |x * y| < ε := begin intros x y ε he1 he2 hx hy, by_cases h : (|x| = 0), { calc |x * y| = |x| * |y| : abs_mul x y ... = 0 * |y| : by rw h ... = 0 : zero_mul (abs y) ... < ε : he1 }, { have h1 : 0 < |x|, { have h2 : 0 ≤ |x| := abs_nonneg x, show 0 < |x|, exact lt_of_le_of_ne h2 (ne.symm h) }, calc |x * y| = |x| * |y| : abs_mul x y ... < |x| * ε : (mul_lt_mul_left h1).mpr hy ... < ε * ε : (mul_lt_mul_right he1).mpr hx ... ≤ 1 * ε : (mul_le_mul_right he1).mpr he2 ... = ε : one_mul ε }, end -- 2ª demostración -- =============== example : ∀ {x y ε : ℝ}, 0 < ε → ε ≤ 1 → |x| < ε → |y| < ε → |x * y| < ε := begin intros x y ε he1 he2 hx hy, by_cases h : (|x| = 0), { calc |x * y| = |x| * |y| : by apply abs_mul ... = 0 * |y| : by rw h ... = 0 : by apply zero_mul ... < ε : by apply he1 }, { have h1 : 0 < |x|, { have h2 : 0 ≤ |x|, apply abs_nonneg, exact lt_of_le_of_ne h2 (ne.symm h) }, calc |x * y| = |x| * |y| : by rw abs_mul ... < |x| * ε : by apply (mul_lt_mul_left h1).mpr hy ... < ε * ε : by apply (mul_lt_mul_right he1).mpr hx ... ≤ 1 * ε : by apply (mul_le_mul_right he1).mpr he2 ... = ε : by rw [one_mul] }, end -- 3ª demostración -- =============== example : ∀ {x y ε : ℝ}, 0 < ε → ε ≤ 1 → |x| < ε → |y| < ε → |x * y| < ε := begin intros x y ε he1 he2 hx hy, by_cases (|x| = 0), { by finish }, { have : 0 < |x|, by finish, calc |x * y| = |x| * |y| : by rw abs_mul ... < |x| * ε : by finish ... ≤ 1 * ε : by nlinarith ... = ε : by finish }, end |
Se puede interactuar con la prueba anterior en esta sesión con Lean.
Referencias
