English
Auxiliary lemma expressing the swap relationship in terms of left/right membership conditions and multiplication.
Русский
Вспомог lemma, выражающий соотношение смены координат через условия принадлежности слева и справа и умножение.
LaTeX
$$Auxiliary relation: (x.snd ∈ s ∧ x.fst ∈ t ∧ x.snd * x.fst = a) ↔ x ∈ mulAntidiagonal t s a.$$
Lean4
@[to_additive (attr := simp)]
theorem swap_mem_mulAntidiagonal_aux [CommMagma α] {s t : Set α} {a : α} {x : α × α} :
x.snd ∈ s ∧ x.fst ∈ t ∧ x.snd * x.fst = a ↔ x ∈ Set.mulAntidiagonal t s a := by simp [mul_comm, and_left_comm]