English
For natural a,b, a*gcdA(a,b) ≡ gcd(a,b) (mod b).
Русский
Для натуральных a,b имеет место a · gcdA(a,b) ≡ gcd(a,b) (mod b).
LaTeX
$$$(a : \\mathbb{Z}) \\cdot \\mathrm{gcd_A}(a,b) \\equiv \\mathrm{gcd}(a,b) [ZMOD b].$$$
Lean4
theorem gcd_a_modEq (a b : ℕ) : (a : ℤ) * Nat.gcdA a b ≡ Nat.gcd a b [ZMOD b] :=
by
rw [← add_zero ((a : ℤ) * _), Nat.gcd_eq_gcd_ab]
exact (dvd_mul_right _ _).zero_modEq_int.add_left _