English
See 95906: each r-element subset of s has size r.
Русский
См. 95906: каждое r-элементное подмножество s имеет размер r.
LaTeX
$$$(\\mathrm{powersetCard}(r,s) : \\mathrm{Set}(\\mathrm{Finset}\\;\\alpha)).\\text{Sized } r.$$$
Lean4
@[simp]
theorem map_zero [∀ i j, Zero (α i j)] {β : m → n → Type w} [∀ i j, Zero (β i j)] {f : ∀ ⦃i j⦄, α i j → β i j}
(h : ∀ i j, f (0 : α i j) = 0) : (0 : DMatrix m n α).map f = 0 := by ext; simp [h]