English
The leftRel relation on a product s × s' is the product of the leftRel relations on s and s'.
Русский
Отношение leftRel на произведении s × s' равно произведению leftRel на s и на s'.
LaTeX
$$$\text{leftRel}(s \times s') = \text{leftRel}(s) \times \text{leftRel}(s')$$$
Lean4
@[to_additive leftRel_prod]
theorem leftRel_prod {β : Type*} [Group β] (s' : Subgroup β) : leftRel (s.prod s') = (leftRel s).prod (leftRel s') :=
by
refine Setoid.ext fun x y ↦ ?_
rw [Setoid.prod_apply]
simp_rw [leftRel_apply]
rfl